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Neuroradiology
Common Neurologic Problems
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Neuroradiology Patricia]. Luttgen, DVM, MS, * Robert D. Pechman, DVM , MS,t and Sandee M. Hartsfield, DVM:j:
The majority of clinical cases presenting with neurologic dysfunction will require some form of neuroradiography during the diagnostic process. Specialized x-ray equipment is not necessary to produce high-quality radiographs of the skull or spine, but attention to detail during radiography is required to yield the best results. High MAS, low KvP radiographic techniques produce radiographs of high contrast and detail and should be used for neuroradiographic procedures. Careful collimation of the x-ray beam will help reduce scatter radiation and will increase radiographic contrast and detail. A grid, although not absolutely essential, should be used to further improve radiographic quality. Par-speed x-ray films and screens will proVide adequate detail for neuroradiographic procedures. ANESTHESIA AND RESTRAINT Neuroradiographic procedures usually require general anesthesia in animals .6 Proper positioning is essential for excellent radiographs. Anesthesia is needed to provide the muscle relaxation necessary for skull and spinal radiographs of diagnostic quality, subarachnoid puncture for myelography, and other special neuroradiographic procedures. Because of the central nervous system (CNS) effects of contrast media, myelograms are not performed without general anesthesia.
*Diplomate,
American College of Veterinary Internal Medicine (Neurology); Associate Professor, Neurology and Neurosurgery, Department of Small Animal Medicine and Surgery, Texas A & M University College of Veterinary Medicine, College Station, Texas tDiplomate, American College of Veterinary Radiology; Associate Professor of Veterinary Radiology, Veterinary Clinical Sciences, Louisiana State University School of Veterinary Medicine, Baton Rouge, Louisiana :t:Diplomate, American College of Veterinary Anesthesiologists; Professor and Chief of AnestheSiology, Department of Small Animal Medicine and Surg.. ry, Texas A & M University College of Veterinary Medicine, College Station, Texas Veterinary Clinics of North America: Small Animal Practice-Vol. 18, No.3, May 1988
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Preanesthetic Evaluation The approach to general anesthesia for neuroradiographics should include the patient's historical, physical, and laboratory data. This information will allow proper selection of anesthetic drugs and techniques based on the needs of each individual patient. A complete discussion of the selection of drugs and techniques for general anesthesia in patients with CNS disease is beyond the scope of this article, but a few recommendations are in order. First, historical data should be taken to elucidate any unusual responses to previous anesthetics or tranquilizers. Such responses may eliminate some drugs or suggest a more extensive evaluation of hepatic function. A history of seizures will contraindicate certain drugs (for example, ketamine, phenothiazine tranquilizers, enflurane, and methohexital) from the anesthetic regimen. 6 If an animal has received anticonvulsant therapy (phenobarbital, for example), tolerance to barbiturates may have developed. 6 Conversely, hepatic function may be impaired by chronic administration of anticonvulsants. 3 If hepatic disease is present, anesthetic drugs should be selected that do not require hepatic function for clinical recovery of the patient. Second, the physical examination should emphasize cardiopulmonary evaluation. Murmurs, arrhythmias, or abnormal respiratory sounds should signal a detailed evaluation (for example, electrocardiogram and thoracic radiographs) prior to selecting an anesthetic regimen. In addition, the impact of CNS disease on cardiovascular and respiratory functions should be considered in monitoring and managing neurologic patients. Third, a minimum database including a complete blood count, a biochemical profile with electrolytes, a urinalysis, and, if deemed necessary for the individual case, a blood gas analysis should be evaluated before selection of anesthetic drugs and supportive techniques. Particular attention should be paid to hepatic and renal functions. Also, plasma glucose levels should be evaluated prior to anesthesia because hypoglycemia can affect recovery. General Anesthesia With certain exceptions (for example, compressive lesions in the brain), routine anesthetic regimens can be used with confidence for dogs or cats showing only neurologic disease. Premedications can include the anticholinergic glycopyrrolate (0.01 mg per kg intravenously or subcutaneously), which crosses the blood-brain barrier to only a limited extent, II and tranquilizers such as diazepam (0.2 to 0.4 mg per kg intramuscularly), which is anticonvulsant in nature, or lenperone '(0.2 to 0.4 mg per kg intramuscularly or intravenously). Lenperone may not lower the threshold for seizures. 13 However, the butyrophenones have variable effects on seizure threshold, I and their use in periodically seizing animals remains somewhat equivocal. For anesthesia, injection of thiobarbiturates (for example, thiopental 8 to 14 mg per kg intravenously) for induction and administration of inhalation anesthetics (halothane, isoflurane, or methoxyflurane in oxygen) for maintenance of anesthesia are appropriate. Alternate regimens may be indicated in debilitated animals or animals with hepatic, renal, cardiac, or respiratory dysfunction . The practitioner is referred to anesthesiology texts for a complete discussion of other possible anesthetic regimens. 6. II
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Table 1. Effects of Various Anesthetics and Related Drugs on Cerebral Blood Flow* DRUG
CBF
DEGREE OF EFFECT
Thiopental Morphine Droperidol Midazolam Diazepam Ketamine Etomidate Lidocaine Halothane Enllurane Isollurane Methoxyllurane Nitrous oxide
Decrease Decrease Decrease Decrease Decrease Increase Decrease Decrease Increase Increase Increase Increase Increase
Large Small Small Moderate Small Large Large Moderate Large Moderate Small Moderate Small
*Based on data collected from both animal and human subjects. 5. 6.17
Positioning is important for neuroradiology and may require extreme cervical flexion in some animals. Such manipulation may impair air movement through the trachea or endotracheal tube. Also, some radiographic views require removal or adjustment of the endotracheal tube. When these manipulations are necessary, constant attention to the airway and to ventilation is important. In the neurologic patient, one should consider the effects of drugs and procedures on cerebral blood flow (CBF) and consequently on intracranial pressure (ICP), especially if compressive lesions are suspected (Table 1). Inhalation anesthetics and ketamine increase CBF and potentially ICP, whereas narcotics and thiobarbiturates decrease CBF. 5 . 6. 17 Increases in ICP or intraspinal pressure can decrease perfusion or cause cerebral or cerebellar herniation. 6 Carbon dioxide retention, hypoxia, positive pressure ventilation, and endotracheal intubation may affect CBF and ICP. 9 Thus, drugs and techniques to minimize these changes should be employed when ICP appears to be critical. In particular, controlled ventilation to a PaC0 2 of about 28 to 30 mmHg may reduce the magnitude of the increase in ICP associated with inhalation anesthetics. 6 Radiographic Contrast Studies Contrast studies in neuroradiology are commonly performed and require general anesthesia. Various media are used, and injection of radiopaque materials like methiodal, metrizamide, and iohexol may be associated with postmyelographic seizures as well as significant cardiopulmonary effects. The potential for cardiovascular effects such as arrhythmias and elevations in heart rate and blood pressure creates the need for good, continuous cardiopulmonary monitoring during contrast studies. 12 Likewise, the propensity for postmyelographic seizures requires careful patient management in the period immediately following anesthesia. The incidence of cardiovascular and respiratory (apnea) complications appears to be of more significance with methiodal than newer agents. The prevalence of postmyelographic seizures seems to be greater with methiodal,12 less with metrizamide,4. 20 and much less with iohexol. 14
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Several studies have been done to evaluate methods to control seizures associated with myelography. Two studies have shown that the use of 5 per cent dextrose during maintenance of anesthesia reduces the incidence of postanesthetic seizures in dogs given metrizamide. 7 , 23 The mechanisms postulated were diuresis to hasten contrast clearance23 and increased CNS glucose to overcome contrast inhibition of neuronal metabolism. 7 Another study reported reduced postmyelographic convulsions using pentobarbital (5 mg per kg) for premedication and methoxyflurane for maintenance of anesthesia. S A common anesthetic regimen for myelography includes an optional anticholinergic, no phenothiazine tranquilizers, induction with a thiobarbiturate, inhalation maintenance, and use of anticonvulsants (diazepam, pentobarbital and/or phenobarbital) in the postanesthetic period if they are required. IS Others routinely include preanesthetic dexamethasone (0.5 to l.0 mg per kg intravenously). In addition, it is appropriate to maintain anesthesia for at least 45 minutes after the time of contrast injection, to position the patient's head higher than the median plane of the body, to continue fluid administration (5 to 10 ml per kg per hour) throughout the anesthetic period, and to prevent hypothermia during anesthesia and recovery. Finally, other potential complications related to neuroradiography include the interactions of contrast media with anesthetics. Methiodal is known to displace thiopental from protein and can alter the amount of active barbiturate,2 possibly increasing depth or duration of anesthesia. Air contrast studies (ventriculograms) should not be done with nitrous oxide in the anesthetic regimen because of the potential for increasing the volume or pressure of the gas within the CNS, which leads to significant tissue damage. 16 Management in Critical Patients In traumatized or other critical patients, neuroradiographic screening for gross abnormalities may be needed before the patient's overall condition can be stabilized. Patients with increased ICP are poor candidates for anesthesia. lo In these animals, general anesthesia is risky and should be avoided, even if the quality of radiographs or proper positioning must be sacrificed. However, such radiographs are usually adequate to diagnose and prognose obvious fractures, subhixations, and luxations. Some patients that are considered to be greater risks for general anesthesia may be sedated or tranquilized to make them more cooperative. Oxymorphone (0.1 to 0.2 mg per kg intravenously), diazepam (0.1 to 0.2 mg per kg intravenously), and butorphanol (0.2 to 0.4 fig per kg intravenously) are possible choices. Diazepam has been recommended in patients with head trauma because of its minimal cardiopulmonary effects.1O The advantages of these drugs are that they are either of relatively short duration or potentially reversible. Opioids can be antagonized by naloxone, and antagonists for benzodiazepines are presently being evaluated. When opioids or other respiratory depressants are used, ventilation must be carefully monitored as long as the patient is under the influ"ence of the drug. In conclusion, management of patients to facilitate neuroradiography should be done using sound principles of anesthesia, including preanesthetic
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preparation prior to induction, careful selection of drugs, maintenance of a patent airway, regular monitoring, and institution of supportive measures as needed. Drug selection should be based on evaluation of the various body systems, with special attention paid to monitoring and supporting cardiopulmonary function. The recovery period must be considered a crucial time for monitoring the patient, especially following neuroradiographic contrast studies that can promote seizures. Drugs for controlling seizures should be readily available and used as soon as hyperactivity is recognized (for example, muscle fasciculations, extensor rigidity, or seizures).
PATIENT POSITIONING Proper patient positioning is critical to obtain radiographs of good diagnostic quality of the skull and spine. The skull and vertebral column are complex bony structures. Positioning that is less than perfect may result in radiographs with confusing shadows that severely impair accurate interpretation. General anesthesia will permit the best possible positioning of the patient for neuroradiologic procedures. Protective aprons and gloves should be worn by any person manually restraining the patient during the examination. However, mechanical restraint of the patient is safer and is easily achieved if the patient is adequately sedated or is under general anesthesia. Excellent references are available that detail the use of restraint devices for neuroradiologic examinations. 15, 22 Figures 1 through 11 illustrate proper and improper positioning for radiographs of the skull and spine. Skull Films
Routine Skull Series. A routine skull series consists of lateral and either ventrodorsal (VD) or dorsoventral (DV) views. This series is valuable for general screening of conditions such as hydrocephalus, cranial trauma, sinusitis with possible extension through the cribriform plate, opaque neoplastic masses, and conditions causing lysis of bone. It is extremely important that these views be symmetric for proper interpretation. Foam rubber can be used to elevate the nose and cervical spine so that the lateral skull profile is parallel to the table surface (Fig. 1). For the VD view, the animal's head should be positioned perfectly straight with the dorsum of the nose parallel to the table so that the x-ray beam passes equally through both halves (Fig. 2). To accomplish this, the chest and neck must also be held in a straight position or the head will be rotated (Fig. 7A). If a DV view is preferred (easier to position without assistants), the animal should be placed in sternal position with the forelimbs drawn caudally. The head needs to be perfectly straight for the x-ray beam to pass equally through both halves (Fig. 3). Tympanic Bullae. Evaluation of the tympanic bullae is indicated in animals displaying vestibular signs, Horner's syndrome, facial nerve paralysis, and/or otitis media-interna. The tympanic bullae can be more clearly viewed on specially positioned radiographs of the head. The "open-mouth" view (Fig. 4), with the x-ray beam directed at the back of the oropharynx, affords excellent simultaneous visualization of the "bullae with minimal overlying bone. This is the best view for a direct comparison of bullae Text continued on page 523
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Figure 1. Proper positioning of the head for lateral skull films. Note the foam rubber used to elevate the nose and cervical spine, so that the lateral skull profile is parallel to the surface of the table.
Figure 2. Proper positioning of the head for ventrodorsal skull films .
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Figure 3. Proper positioning of the head for dorsoventral skull films viewed from the top (:\) and side (B).
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c Figure 4. Proper positioning of the head for an open-mouth rostrocaudal view of the tympanic bullae seen from the side (A) and top (8). The x-ray beam should be directed at the open mouth toward the back of the oropharynx (C). Note that the endotracheal tube has been momentarily removed for the exposures.
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Figure 5. A view of the back of a dog lying in lateral recumbency (A) demonstrates the scoliotic position (B) the spinal column assumes if padding is not used to keep the entire spine parallel to the tabletop. If padding is not placed under the sternum and between the limbs, axial rotation occurs as the sternum and upper limbs roll down toward the tabletop (C).
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Figure 6. Proper positioning of the head, neck, and shoulders for lateral cervical radiographs (A). Foam should be placed under the nose, head, and neck (B and C), so that the cervical spine from the occiput to the thoracic vertebrae is straight and perfectly parallel to the tabletop. Tucking the nose under slightly while applying gentle cranial traction on the ears further assists in straightening the cervical spine (D).
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.-----~---------'~ B Figure 7. Proper ventrodorsal positioning of the cervical area requires the sternum to be held directly over the thoracic spinal column with forelimbs drawn back while placing gentle traction on the head or ears to straighten the cervical spine (A and B).
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Figure 8. Proper lateral positioning for thoracic, lumbar, sacral, and coccygeal vertebrae requires foam padding to prevent scoliotic sagging and axial rotation (A, B, and C) . The cervical spine should also be supported to maintain proper positioning of the upper thoracic vertebrae.
Figure 9. Proper ventrodorsal positionIng for thoracic, lumbar, and sacral views requires the sternum and midline of the abdomen to be held directly over the spinal column to avoid axial rotation. The forelimbs should be drawn cranially and the hindlimbs caudally.
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Figure 10. Schematic drawings of myelographic changes seen in extradural (A) , intradural extramedullary (B), and intramedullary (C) spinal cord lesions.
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Figure 11. Lateral (A), dorsoventral (B), and open-mouth rostrocaudal (C and D [closeup view of C]) views of the skull of a normal dog. The dorsoventral and rostrocaudal projections are especially helpful because the skull is a symmetric structure and the two sides may be compared to determine the presence of disease.
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Figure 12. Lateral radiograph of the skull of a dog with multiple fractures of the calvarium (black arrows). There is considerable soft tissue swelling surrounding the skull (white arrows). Additional views could be made to thoroughly evaluate the skull for other fractures and to determine the extent of the evident fracture.
Figure 13. Dorsoventral (A) and open-mouth rostrocaudal (B) radiographs of the skull of a dog with clinical signs of otitis media. The left petrous temporal bone is increased in opacity and the left osseous bulla is thickened and increased in opacity (black arrows) (compare with Fig. liB and lie). Mineral opacity in the left external ear canal (A) indicates chronic otitis extern a (white arrow).
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Figure 14. A lateral radiograph of a young dog with hydrocephalus. The skull is domeshaped and the bones of the calvarium are thinned. This puppy had an "open fontanelle" at the frontoparietal suture (white arrow). The inner surface of the calvarium lacks the prominent gyral markings found in normal dogs (compare with Fig. 1lA). Additionally, the first cervical vertebra (C 1) is hypoplastic, and atlantoaxial subluxation is present.
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Figure 15. Lateral radiographs of two dogs with neoplastic lesions of the skull. A, A 9year-old German Shepherd with an aggressive adenocarcinoma involving the left frontal sinus. Bone lysis and proliferation is present as well as superimposed soft tissue swelling (arrows). B, An 8-year-old Poodle with seizures. There is nearly complete destruction of the dorsal portion of the ethmoturbinates (small arrow) and the cribriform plate (large arrow), with increased soft tissue opacity in the same region. Nasal adenocarcinoma with extension to the brain through the cribriform plate was diagnosed at necropsy.
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Figure 16. Lateral radiographs of the cervical spine (A), thoracolumbar spine (8), and lumbar spine (C) of a normal dog. Complete radiographic evaluation should also include ventrodorsal views of each region. Intervertebral disc spaces are clear and normal in width. Superimposed rib shadows can make interpretation of the caudal thoracic spine difficult and great care should be exercised.
Figure 17. Degenerative joint disease of the dorsal articular facets of the spine (A) and spondylosis deformans (8) often may be seen, particularly in large breeds of dogs. The severity of either disease can differ widely among affected dogs. Spondylosis deformans is unlikely to produce clinical signs of disease, but degenerative joint disease of the articular facets can produce pain and reduced mobility. Spondylosis deformans must not be confused with discospondylitis.
Figure 18. A lateral radiograph of the caudal lumbar spine of a dog with discospondylitis. There is bone lysis of the opposing vertebral endplates and ventral bony bridging of L6-L7. Bone lysis of the vertebral endplates is the critical radiographic abnormality in this disease (compare with Fig. 178).
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Figure 19. Spinal fractures occur often in traumatized patients. A, Transverse fracture of the second cervical vertebra with dorsal displacement and overriding occurred when this dog fell from a table. B, Oblique compression fracture of the third lumbar vertebra occurred when this dog was hit by an automobile. All spinal fractures are not this apparent. C, In young dogs, a physis of the vertebral body may fracture and displace only slightly (arrow) . These fractures can be very difficult to identifY unless one searches for them carefully. In any patient with a spinal fracture, a complete neurologic examination should be performed to assess the integrity of the spinal cord.
Figure 20. Lateral radiograph of the cervicothoracic spine of a 7-year-old dog with osteosarcoma of the first thoracic vertebra (Tl). Bone lysis and proliferation are present and the vertebral body is compressed due to pathologic fracture. Neoplasia of the spine is not common. However, both primary and metastatic neoplasia may involve the spine.
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Figure 21. Lateral radiograph of the cervical spine of a quadriparetic Poodle. There is atlantoaxial subluxation (white arrows) and absence of the dens (solid black arrow). Note the complete absence of an intervertebral disc space (open black arrow) between the second and third cervical vertebrae. Congenital anomalies of the spine are not uncommon.
Figure 22 See legend on opposite page
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Figure 23. Lateral radiograph of a myelogram in a Dachshund with intervertebral disc herniation at L3-U . The myelogram illustrates the appearance of an extradural lesion (arrows). There is dorsal displacement of the ventral contrast column and compression of the dorsal contrast column. Mineralized disc material is seen on the floor of the neural canal.
Figure 24. Lateral view of a myelogram in a dog with left hindleg weakness. The myelogram depicts an intradural-extramedullary mass that is outlined by the contrast medium (arrows). The "golf-tee" sign at the cranial end of the mass is characteristic of intraduralextramedullary lesions.
Figure 22. Survey radiographic signs of intervertebral disc herniation include narrowing or wedging of the intervertebral space, reduced size and increased opacity of the intervertebral foramen, decreased width of the dorsal vertebral articulation, and mineralized (opaque) material in the neural canal. A, Mineralized intervertebral disc material is clearly seen in the neural canal (arrow) at L3-U and the disc space is narrowed. B, The L2-L3 disc space is narrowed and slightly wedged, the intervertebral foramen is decreased in size and increased in opacity (solid arrow), and the width of the dorsal articulation is reduced (open arrow). If the radiographic changes do not correspond with the findings derived from neurologic examination, a myelogram should be performed.
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Figure 25. Intramedullary spinal cord lesions display evidence of an expanding spinal cord mass on all radiographic views (arrows). Thoracic and lumbar ventrodorsal views may be difficult to interpret because of superimposed thoracic and abdominal structures such as the sternum and intestine. A, Lateral view of the myelogram of a dog with a T12-T13 intramedullary astrocytoma. B, Ventrodorsal view of a dog with an L2-L3 intramedullary ependymoma. (Two different dogs are shown for better photographic detail.)
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anatomy and density. Intentionally obliqued lateral views of the skull can also be performed to place each bulla directly into the x-ray beam without overlying bone. This view is helpful in evaluating the thickness of the bulla wall and in determining the presence of fluid in the middle ear, but does not allow simultaneous comparison of bullae. Nasal Sinuses. Occasionally, an inflammatory or neoplastic process in the nasal sinuses will cause neurologic signs by direct extension through the cribriform plate. In addition to the routine lateral and VD views, the frontal sinuses can be evaluated by directing the x-ray beam through them in an rostrocaudal direction while the patient is lying with the occipital bone on the table, which is similar to the position for "open-mouth" views. Direct evaluation of sinus density and symmetry can then be made. Spinal Films A routine spinal series consists of lateral and VD views . The extent of the spinal column to be radiographed will depend on the neuroanatomic localization of the suspected lesion or lesions. The most common cause of undiagnostic spinal films is poor positioning (Fig. 5). It is imperative that time be taken to position the animal properly so that the portion of the spine in the x-ray beam is perfectly straight and parallel to the tabletop. Cervical and Cervicothoracic Films. Lateral Views . For diagnostic lateral cervical radiographs, the cervical spine must be positioned straight and parallel to the tabletop (no rotation down the long axis of the spine; no scoliosis). When an animal is lying in lateral recumbency, the neck falls into a bowed (scoliotic) position off the elevated shoulders and head, and the upper cervical area rotates as the nose drops to the tabletop (see Fig. 5). To position the cervical and upper thoracic spinal column correctly, foam rubber will be needed to elevate the nose, head, neck, and sternum so that the cervical column is parallel to the tabletop from the occiput to the thoracic area (Figs. 6A to C). If technical help is available, one person can tuck the elevated nose under slightly while pulling gently cranially on the ears . Another person then holds the thoracic spinal column in position while drawing the forelimbs caudally, thus moving the shoulders out of the way of the caudal cervical area (Fig. 6D) . In most animals, two lateral cervical exposures should be made: one centered over the C2-C3 area (upper cervical) and one centered over the C5-C6 area (lower cervical). In very small animals, one view aimed at the C3-C4 area may suffice. Additional "coned-down" views may be desirable after examination of the initial films. In addition, a view centered over the C7-T2 area can be made without repositioning. Ventrodorsal Views . For VD views, the animal should be placed in dorsal recumbency. The cervical spine should be positioned perfectly straight without rotation. The head should be left resting on the occipital bone with the nose angled upward. Applying gentle traction on the ears will help straighten the spine. The forelimbs should be drawn caudally while the sternum is aligned directly over the thoracic vertebral column in a line perpendicular to the tabletop (Fig. 7). If technical assistance is not available, it will be ne"cessary to stabilize the head and chest with tape and a tray or sand bags. The head should be
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taped pulling the nose down as little as possible to avoid lifting the upper cervical vertebrae. The forelimbs should be drawn back and tape applied across the chest. Foam rubber will be needed under the neck to eliminate kyphosis created by pulling the nose down with the tape. Traction to straighten the spine is not possible in this method of positioning and often scoliosis will be evident on the films. It is possible to have the upper cervical spinal column parallel to the tabletop in the VD position. However, the caudal cervical vertebrae make a natural ventral curvature into the thoracic area, making it difficult to position the cervicothoracic vertebrae parallel to the tabletop. Some obliquity may be present in VD caudal cervical views, especially in large, deep-chested dogs. This can be corrected most easily by tilting the head of the radiographic unit to direct the x-ray beam perpendicularly through the spinal column of the cervicothoracic views. If this is not possible, repositioning the animal after taking the upper cervical view may be necessary. Views of the Dens. Evaluation of the dens of C2 is important in suspected cases of atlantoaxial subluxation or dens fracture/luxation. The dens is best visualized by the open-mouth view previously described for the tympanic bullae (Fig. 4). Thoracolumbar, Lumbosacral, and Sacrococcygeal Films. The same principles of positioning discussed for the cervical and upper thoracic spine apply when taking radiographs of the thoracic, lumbar, sacral, and coccygeal spinal segments; the spine should be straight and as parallel to the tabletop as possible. Lateral Views. In lateral recumbency without padding, the lumbar spine normally falls into a bowed position. The tail drops to the table at an angle (Fig. 5A and B). In the thorax, the sternal area is narrower than the epaxial area. Without padding under the sternum, the vertebral column is rotated on its long axis. The weight of the upper limbs dropping across the chest and hindquarters also causes vertebral rotation (Fig. 5C). Foam rubber padding should be placed under the lumbar spine and hips as necessary to align the thoracic, lumbar, and sacral vertebrae in a straight line parallel to the tabletop (Fig. 8A, B and C) . To prevent axial rotation, a wedge of foam should be placed under the lower chest wall to place the sternum in the same plane above the tabletop as the vertebral column. Foam should also separate the two forelimbs and two hindlimbs so that the body/limb relationship is more nearly like that of the standing animal with limbs perpendicular to the body. If the coccygeal vertebrae are of interest, foam may be placed under the tail to elevate them into line with the spinal column. With this padding in place, a series of lateral radiographs may be taken along the thoracic, lumbar, and sacral spine 'with no technical assistance. If desired, technicians can apply mild traction to further straighten the spinal column by pulling the forelimbs and head cranially and the hindlimbs and tail caudally. They should be careful to hold the limbs perpendicular to the body to avoid axial rotation. The number of lateral views taken will depend on the nature of the neurologic problem, the size of the animal, and how well the site can be localized on neurologic examination. In external tr~uma victims, "screening" radiographs of large numbers of vertebrae may be advisable to quickly evaluate the entire thoracic, lumbar, and sacral spinal column and to judge
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the safety of performing VO views. Once the extent of damage and location is known, "coned-down" views (three to five vertebrae per field) centered over the affected area(s) can be made for better detail. If thoracolumbar disc disease is suspected, coned-down views centered over T12-T13 and L2-L3 are usually adequate to confirm the diagnosis. Ventrodorsal Views . The animal should be positioned in dorsal recumbency for the VO views. When technical assistance is available, the forelimbs should be held cranially and the hindlimbs caudally with gentle traction to help straighten the spine. The sternum and midline of the abdomen should be aligned directly over the vertebral column to prevent axial rotation (Fig. 9). Without technical assistance, a tray or sand bags, in combination with tape across the chest and abdomen, will be necessary to hold the animal in dorsal recumbency. The forelimbs can be taped forward or allowed to fall as long as the paws are not lying across the chest. The hindlimbs will usually fall in a "frogleg" position out of the way of the x-ray beam. It is very difficult to get the spinal column truly straight without technical assistance, but every attempt should be made to do so if diagnostic radiographs are to be obtained. The numbers and types of VO views should correspond to those taken in the lateral position. In cases of vertebral instability, VO positioning may cause further luxation and damage to the nervous system, so caution is advisable. SPECIAL PROCEDURES
Myelography
Indications. Myelography is ·the most frequently performed neuroradiographic contrast procedure in veterinary medicine. It enables one to localize and characterize spinal cord lesions more specifically than is possible with survey radiographs. A myelogram must be preceded by a thorough neurologic examination and careful evaluation of survey radiographs of the spine. Myelography is indicated in cases in which survey spinal radiographs have not clearly demonstrated a neuroanatomically correlating lesion(s). Myelography is contraindicated in patients displaying signs of inflammatory CNS disease and any time increased ICP is suspected. Techniques. Myelography is not difficult to perform if the practitioner strictly adheres to the principles of subarachnoid puncture (see the article "Cerebrospinal Fluid"). The only special pieces of equipment required are styleted spinal needles and an appropriate neuroradiologic contrast agent. Iohexol (240 mg%)* is an excellent contrast medium that provides adequate radiographic contrast with a low incidence of adverse reactions. The sites of injection (cisterna magna; L4-L5 or L5-L6 lumbar subarachnoid space) are the same as for cerebrospinal fluid (CSF) collection (see the article "Cerebrospinal Fluid"). Needle placement and injection at the cisterna magna are easier and usually safer to accomplish than with lumbar puncture. Cisternal injection is preferable fQr lesions localized in *Omnipaque 240. Winthrop Pharmaceuticals, New York, New York.
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the cervical, cervicothoracic, and upper thoracic areas; excellent results are also possible in the thoracolumbar and lumbosacral areas if the animal is held with its spine perpendicular to the floor for a few minutes following injection of the medium . However, when caudal medium flow is obstructed, cisternal injection is not as good as lumbar injection in delineating the total extent of thoracolumbar or lumbosacral lesions. Because of the "dead-end" anatomy of the lumbar cistern, medium injected into the lumbar area can be introduced under more pressure and forced around an obstructing lesion. This is not possible from the cisterna magna, because medium injected under pressure will flow rostrally into the brain, which should be avoided. Placing the spinal needle in the dorsal lumbar subarachnoid space is extremely difficult, and placing it in the ventral lumbar subarachnoid space (passing the needle through the spinal cord) can cause extensive damage if the needle travels off the midline and into the ventral horn cells of the sciatic nerve segments. In addition, if the needle is improperly positioned at the time of injection, contrast medium may be driven into the lumbosacral spinal cord segments, causing irreversible flaccid paralysis of the hindlimbs. Aseptic techniques must be followed throughout the procedures of CSF withdrawal and contrast medium injection. * After CSF has been withdrawn in a quantity not to exceed the amount of medium to be injected, the contrast medium is slowly injected into the subarachnoid space. A dosage of 0.3 ml per kg of the contrast medium is injected. This dose will opacify the subarachnoid space up to the mid- to proximal thoracic region if injected by the lumbar puncture technique and will adequately opacify the cervical subarachnoid space if injected at the cisterna magna. A dosage of 0.4 ml per kg may be injected in the cisternal region to opaCify the lumbar subarachnoid space. or vice versa. Radiographs should be made immediately following the completion of the injection of the contrast material unless time needs to be allowed for flow from the cisterna magna to the thoracolumbar and/or lumbosacral areas. Interpretation. Spinal cord lesions will be identified myelographically as extradural, intradural-extramedullary, or intramedullary in location (Fig. 10). Extruded intervertebral disc material is probably the most common extradural lesion, although neoplasms, hypertrophied spinal ligamentous structures (see the article "Disc-Associated Wobbler Syndrome"), and hematomas may also be seen as extradural lesions. Intradural-extramedullary lesions are almost always neoplastic. Intramedullary lesions may be neoplastic but may also be caused by hemorrhage or edema within the spinal cord. Other Special Procedures Many other special neuroradiographic procedures are available in addition to myelography. Many of these procedures require specialized expertise and equipment that is often not available in a clinical practice situation. If one of these procedures is felt to be necessary, referral to an institution or specialty practice may be require~. *CSF withdrawn prior to myelography should always be analyzed rather than assumed to be normal and discarded. Valuable diagnostic information may be lost and, if found to be necessary later, will require an additional episode of anesthesia for the animal.
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Among the specialized procedures that may assist in making a diagnosis in cases of nervous system dysfunction are epidurograms, angiograms, venous sinograms, ventriculograms, computerized axial tomography (CT scans), and magnetic resonance imaging (MRI). Angiograms and ventriculograms can be very dangerous to perform and have generally been replaced by nuclear medicine scans and CT scans. The practitioner is advised to consult some of the excellent texts available IS, 21, 22 or specialists in neurology or radiology when deciding how helpful or necessary these tests might be in individual cases. RADIOGRAPHIC INTERPRETATION The brain and spinal cord are not visible radiographically, but the bony structures surrounding them can be easily evaluated and may reflect disease involving the CNS. Interpretation of skull and spinal radiographs depends on a thorough knowledge of the normal appearance of these structures and on recognition of changes from normal in size, shape, margination, position, number, and opacity. IS. 19,21,22 Myelography allows evaluation of the spinal subarachnoid space and the size and shape of the spinal cord. Evaluation of the brain requires more sophisticated imaging techniques that may be available at some institutions. Figures 11 to 25 illustrate radiographs depicting normal skull and spinal radiographs (Figs. 11 and 16) and common diseases affecting the skull, brain, spine, and spinal cord (Figs. 12 to 15, and 17 to 25). SUMMARY Neuroradiographs are an invaluable part of the diagnostic plan in most cases involving nervous system dysfunction in companion animals. High MAS, low KvP techniques used on standard radiographic equipment available in most veterinary practices will provide good-quality neuroradiographs of the skull and spine. Proper positioning of the animal, which necessitates use of general anesthesia, is required to obtain neuroradiographs of good diagnostic quality. A working knowledge of the normal anatomy of the skull and spine is required to make correct interpretations of the neuroradiographs . ACKNOWLEDGMENT The authors wish to acknowledge the artwork of Annie S. Miley of the Biomedical Learning Resource Center, College of Veterinary Medicine, Texas A & M University,
REFERENCES 1. Baldessarini RJ: Drugs and the treatment of psychiatric di50rders , In Goodman AG, Goodman LS, Rail TW, et al (eds): The Pharmacological Basis of Therapeutics. Edition 7. New York, Macmillan Publishing Co, 1985, pp 387-445 2. Bonhaus DW, Sawyer DC, Hook JB: Displacement of protein-bound thiopental by sodium
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6. 7.
8. 9. 10. 11 . 12.
13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23.
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methiodal may contribute to anesthetic complications during canine myelography. Am J Vet Res 42:1612, 1981 Bunch SE, Castleman WL, Baldwin BH, et al: Effects of long-term primidone and phenytoin administration on canine hepatic function and morphology. Am J Vet Res 46:105, 1985 Davis EM , Glickman L, Rendano VT, et al: Seizures in dogs folloWing metrizamide myelography. JAm Anim Hosp Assoc 17:642, 1981 Dawson B, Michenfelder JD, Theye RA: Effects of ketamine on canine ce rebral blood flow and metabolism: Modification by prior administration of thiopental. Anesth Analg 50:443, 1971 Gilroy BA: Neuroanesthesiology. In Slatter DH (ed): Textbook of Small Animal Surgery. Philadelphia, WB Saunders Co, 1985, pp 2643-2649 Gray PR, Lowrie CT, Wetmore LA: Effect of intravenous administration of dextrose or lactated Ringer's solution on seizure development in dogs after cervical myelography with metrizamide. Am J Vet Res 48:1600, 1987 Gray PR, Indrieri RJ, Lippert AC: Influence of anesthetic regimen on the frequency of seizures after cervical myelography in the dog. J Am Vet Med Assoc 190:527, 1987 Hartsfield SM, Thurmon JC: Anesthesia in small animal neurological patients. J Am Anim Hosp Assoc 11:396, 1975 Hubbell JAE: Anesthesia for the emergency surgical patient. In Bright RM (ed): Surgical Emergencies. New York, Churchill Livingstone, 1986, pp 45-59 Lumb WV, Jones EW: Veterinary Anesthesia. Philadelphia, Lea & Febiger, 1984, p 168 Menheusen MJ, Parker AJ: Clinical observations of cardiovascular and respiratory changes during contrast myelography with water soluble media. J Am Anim Hosp Assoc 11:401 , 1975 Muir WW III, Hubbell J, Skarda R, et al: An Outline of Veterinary Anesthesia. Columbus, Ohio State University, 1986, p 25 Puglisi TA, Green RA, Hall CL: Comparison of metrizamide and iohexol for cisternal myelographic examination of dogs. Am J Vet Res 47:1863, 1986 Ryan GD: Radiographic Positioning of Small Animals. Philadelphia, Lea & Febiger, 1981 Saidman LJ, Eger EI II: Change in cerebrospinal fluid pressure during pneumoencephalography under nitrous oxide anesthesia. Anesthesiology 26:67, 1965 Shapiro HM: Anesthetic agents and techniques and cerebral blood flow . American Society of Anesthesiologists' Refresher Course Lectures, 1986, p 121 Shores A, Burns J: Technique and indications for metrizamide myelography in small animals. Compend Con tin Ed Pract Vet 9:361, 1987 Slatter DH: Textbook of Small Animal Surgery. Philadelphia, WB Saunders Co, 1985 Stowater JL, Kneller SK: Clinical evaluation of metrizamide as a myelographic agent in the dog. J Am Vet Med Assoc 175:191, 1979 Thrall DE : Textbook of Veterinary Diagnostic Radiology. Philadelphia, WB Saunders Co, 1986 Ticer JW: Radiographic Techniques in Veterinary Practice. Edition 2. Philadelphia, WB Saunders Co, 1984 Tomas PN , Walker MA, Paddleford RR, et al: Prevention of postmetrizamide seizures in dogs using 5% dextrose solution. J Am Vet Med Assoc 188:710, 1986
Department of Small Animal Medicine and Surgery College of Veterinary Medicine Texas A & M University College Station, Texas 77843
0195-5616/88 $0.00
+ .20
Neuroradiology Patricia]. Luttgen, DVM, MS, * Robert D. Pechman, DVM , MS,t and Sandee M. Hartsfield, DVM:j:
The majority of clinical cases presenting with neurologic dysfunction will require some form of neuroradiography during the diagnostic process. Specialized x-ray equipment is not necessary to produce high-quality radiographs of the skull or spine, but attention to detail during radiography is required to yield the best results. High MAS, low KvP radiographic techniques produce radiographs of high contrast and detail and should be used for neuroradiographic procedures. Careful collimation of the x-ray beam will help reduce scatter radiation and will increase radiographic contrast and detail. A grid, although not absolutely essential, should be used to further improve radiographic quality. Par-speed x-ray films and screens will proVide adequate detail for neuroradiographic procedures. ANESTHESIA AND RESTRAINT Neuroradiographic procedures usually require general anesthesia in animals .6 Proper positioning is essential for excellent radiographs. Anesthesia is needed to provide the muscle relaxation necessary for skull and spinal radiographs of diagnostic quality, subarachnoid puncture for myelography, and other special neuroradiographic procedures. Because of the central nervous system (CNS) effects of contrast media, myelograms are not performed without general anesthesia.
*Diplomate,
American College of Veterinary Internal Medicine (Neurology); Associate Professor, Neurology and Neurosurgery, Department of Small Animal Medicine and Surgery, Texas A & M University College of Veterinary Medicine, College Station, Texas tDiplomate, American College of Veterinary Radiology; Associate Professor of Veterinary Radiology, Veterinary Clinical Sciences, Louisiana State University School of Veterinary Medicine, Baton Rouge, Louisiana :t:Diplomate, American College of Veterinary Anesthesiologists; Professor and Chief of AnestheSiology, Department of Small Animal Medicine and Surg.. ry, Texas A & M University College of Veterinary Medicine, College Station, Texas Veterinary Clinics of North America: Small Animal Practice-Vol. 18, No.3, May 1988
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Preanesthetic Evaluation The approach to general anesthesia for neuroradiographics should include the patient's historical, physical, and laboratory data. This information will allow proper selection of anesthetic drugs and techniques based on the needs of each individual patient. A complete discussion of the selection of drugs and techniques for general anesthesia in patients with CNS disease is beyond the scope of this article, but a few recommendations are in order. First, historical data should be taken to elucidate any unusual responses to previous anesthetics or tranquilizers. Such responses may eliminate some drugs or suggest a more extensive evaluation of hepatic function. A history of seizures will contraindicate certain drugs (for example, ketamine, phenothiazine tranquilizers, enflurane, and methohexital) from the anesthetic regimen. 6 If an animal has received anticonvulsant therapy (phenobarbital, for example), tolerance to barbiturates may have developed. 6 Conversely, hepatic function may be impaired by chronic administration of anticonvulsants. 3 If hepatic disease is present, anesthetic drugs should be selected that do not require hepatic function for clinical recovery of the patient. Second, the physical examination should emphasize cardiopulmonary evaluation. Murmurs, arrhythmias, or abnormal respiratory sounds should signal a detailed evaluation (for example, electrocardiogram and thoracic radiographs) prior to selecting an anesthetic regimen. In addition, the impact of CNS disease on cardiovascular and respiratory functions should be considered in monitoring and managing neurologic patients. Third, a minimum database including a complete blood count, a biochemical profile with electrolytes, a urinalysis, and, if deemed necessary for the individual case, a blood gas analysis should be evaluated before selection of anesthetic drugs and supportive techniques. Particular attention should be paid to hepatic and renal functions. Also, plasma glucose levels should be evaluated prior to anesthesia because hypoglycemia can affect recovery. General Anesthesia With certain exceptions (for example, compressive lesions in the brain), routine anesthetic regimens can be used with confidence for dogs or cats showing only neurologic disease. Premedications can include the anticholinergic glycopyrrolate (0.01 mg per kg intravenously or subcutaneously), which crosses the blood-brain barrier to only a limited extent, II and tranquilizers such as diazepam (0.2 to 0.4 mg per kg intramuscularly), which is anticonvulsant in nature, or lenperone '(0.2 to 0.4 mg per kg intramuscularly or intravenously). Lenperone may not lower the threshold for seizures. 13 However, the butyrophenones have variable effects on seizure threshold, I and their use in periodically seizing animals remains somewhat equivocal. For anesthesia, injection of thiobarbiturates (for example, thiopental 8 to 14 mg per kg intravenously) for induction and administration of inhalation anesthetics (halothane, isoflurane, or methoxyflurane in oxygen) for maintenance of anesthesia are appropriate. Alternate regimens may be indicated in debilitated animals or animals with hepatic, renal, cardiac, or respiratory dysfunction . The practitioner is referred to anesthesiology texts for a complete discussion of other possible anesthetic regimens. 6. II
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Table 1. Effects of Various Anesthetics and Related Drugs on Cerebral Blood Flow* DRUG
CBF
DEGREE OF EFFECT
Thiopental Morphine Droperidol Midazolam Diazepam Ketamine Etomidate Lidocaine Halothane Enllurane Isollurane Methoxyllurane Nitrous oxide
Decrease Decrease Decrease Decrease Decrease Increase Decrease Decrease Increase Increase Increase Increase Increase
Large Small Small Moderate Small Large Large Moderate Large Moderate Small Moderate Small
*Based on data collected from both animal and human subjects. 5. 6.17
Positioning is important for neuroradiology and may require extreme cervical flexion in some animals. Such manipulation may impair air movement through the trachea or endotracheal tube. Also, some radiographic views require removal or adjustment of the endotracheal tube. When these manipulations are necessary, constant attention to the airway and to ventilation is important. In the neurologic patient, one should consider the effects of drugs and procedures on cerebral blood flow (CBF) and consequently on intracranial pressure (ICP), especially if compressive lesions are suspected (Table 1). Inhalation anesthetics and ketamine increase CBF and potentially ICP, whereas narcotics and thiobarbiturates decrease CBF. 5 . 6. 17 Increases in ICP or intraspinal pressure can decrease perfusion or cause cerebral or cerebellar herniation. 6 Carbon dioxide retention, hypoxia, positive pressure ventilation, and endotracheal intubation may affect CBF and ICP. 9 Thus, drugs and techniques to minimize these changes should be employed when ICP appears to be critical. In particular, controlled ventilation to a PaC0 2 of about 28 to 30 mmHg may reduce the magnitude of the increase in ICP associated with inhalation anesthetics. 6 Radiographic Contrast Studies Contrast studies in neuroradiology are commonly performed and require general anesthesia. Various media are used, and injection of radiopaque materials like methiodal, metrizamide, and iohexol may be associated with postmyelographic seizures as well as significant cardiopulmonary effects. The potential for cardiovascular effects such as arrhythmias and elevations in heart rate and blood pressure creates the need for good, continuous cardiopulmonary monitoring during contrast studies. 12 Likewise, the propensity for postmyelographic seizures requires careful patient management in the period immediately following anesthesia. The incidence of cardiovascular and respiratory (apnea) complications appears to be of more significance with methiodal than newer agents. The prevalence of postmyelographic seizures seems to be greater with methiodal,12 less with metrizamide,4. 20 and much less with iohexol. 14
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Several studies have been done to evaluate methods to control seizures associated with myelography. Two studies have shown that the use of 5 per cent dextrose during maintenance of anesthesia reduces the incidence of postanesthetic seizures in dogs given metrizamide. 7 , 23 The mechanisms postulated were diuresis to hasten contrast clearance23 and increased CNS glucose to overcome contrast inhibition of neuronal metabolism. 7 Another study reported reduced postmyelographic convulsions using pentobarbital (5 mg per kg) for premedication and methoxyflurane for maintenance of anesthesia. S A common anesthetic regimen for myelography includes an optional anticholinergic, no phenothiazine tranquilizers, induction with a thiobarbiturate, inhalation maintenance, and use of anticonvulsants (diazepam, pentobarbital and/or phenobarbital) in the postanesthetic period if they are required. IS Others routinely include preanesthetic dexamethasone (0.5 to l.0 mg per kg intravenously). In addition, it is appropriate to maintain anesthesia for at least 45 minutes after the time of contrast injection, to position the patient's head higher than the median plane of the body, to continue fluid administration (5 to 10 ml per kg per hour) throughout the anesthetic period, and to prevent hypothermia during anesthesia and recovery. Finally, other potential complications related to neuroradiography include the interactions of contrast media with anesthetics. Methiodal is known to displace thiopental from protein and can alter the amount of active barbiturate,2 possibly increasing depth or duration of anesthesia. Air contrast studies (ventriculograms) should not be done with nitrous oxide in the anesthetic regimen because of the potential for increasing the volume or pressure of the gas within the CNS, which leads to significant tissue damage. 16 Management in Critical Patients In traumatized or other critical patients, neuroradiographic screening for gross abnormalities may be needed before the patient's overall condition can be stabilized. Patients with increased ICP are poor candidates for anesthesia. lo In these animals, general anesthesia is risky and should be avoided, even if the quality of radiographs or proper positioning must be sacrificed. However, such radiographs are usually adequate to diagnose and prognose obvious fractures, subhixations, and luxations. Some patients that are considered to be greater risks for general anesthesia may be sedated or tranquilized to make them more cooperative. Oxymorphone (0.1 to 0.2 mg per kg intravenously), diazepam (0.1 to 0.2 mg per kg intravenously), and butorphanol (0.2 to 0.4 fig per kg intravenously) are possible choices. Diazepam has been recommended in patients with head trauma because of its minimal cardiopulmonary effects.1O The advantages of these drugs are that they are either of relatively short duration or potentially reversible. Opioids can be antagonized by naloxone, and antagonists for benzodiazepines are presently being evaluated. When opioids or other respiratory depressants are used, ventilation must be carefully monitored as long as the patient is under the influ"ence of the drug. In conclusion, management of patients to facilitate neuroradiography should be done using sound principles of anesthesia, including preanesthetic
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preparation prior to induction, careful selection of drugs, maintenance of a patent airway, regular monitoring, and institution of supportive measures as needed. Drug selection should be based on evaluation of the various body systems, with special attention paid to monitoring and supporting cardiopulmonary function. The recovery period must be considered a crucial time for monitoring the patient, especially following neuroradiographic contrast studies that can promote seizures. Drugs for controlling seizures should be readily available and used as soon as hyperactivity is recognized (for example, muscle fasciculations, extensor rigidity, or seizures).
PATIENT POSITIONING Proper patient positioning is critical to obtain radiographs of good diagnostic quality of the skull and spine. The skull and vertebral column are complex bony structures. Positioning that is less than perfect may result in radiographs with confusing shadows that severely impair accurate interpretation. General anesthesia will permit the best possible positioning of the patient for neuroradiologic procedures. Protective aprons and gloves should be worn by any person manually restraining the patient during the examination. However, mechanical restraint of the patient is safer and is easily achieved if the patient is adequately sedated or is under general anesthesia. Excellent references are available that detail the use of restraint devices for neuroradiologic examinations. 15, 22 Figures 1 through 11 illustrate proper and improper positioning for radiographs of the skull and spine. Skull Films
Routine Skull Series. A routine skull series consists of lateral and either ventrodorsal (VD) or dorsoventral (DV) views. This series is valuable for general screening of conditions such as hydrocephalus, cranial trauma, sinusitis with possible extension through the cribriform plate, opaque neoplastic masses, and conditions causing lysis of bone. It is extremely important that these views be symmetric for proper interpretation. Foam rubber can be used to elevate the nose and cervical spine so that the lateral skull profile is parallel to the table surface (Fig. 1). For the VD view, the animal's head should be positioned perfectly straight with the dorsum of the nose parallel to the table so that the x-ray beam passes equally through both halves (Fig. 2). To accomplish this, the chest and neck must also be held in a straight position or the head will be rotated (Fig. 7A). If a DV view is preferred (easier to position without assistants), the animal should be placed in sternal position with the forelimbs drawn caudally. The head needs to be perfectly straight for the x-ray beam to pass equally through both halves (Fig. 3). Tympanic Bullae. Evaluation of the tympanic bullae is indicated in animals displaying vestibular signs, Horner's syndrome, facial nerve paralysis, and/or otitis media-interna. The tympanic bullae can be more clearly viewed on specially positioned radiographs of the head. The "open-mouth" view (Fig. 4), with the x-ray beam directed at the back of the oropharynx, affords excellent simultaneous visualization of the "bullae with minimal overlying bone. This is the best view for a direct comparison of bullae Text continued on page 523
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Figure 1. Proper positioning of the head for lateral skull films. Note the foam rubber used to elevate the nose and cervical spine, so that the lateral skull profile is parallel to the surface of the table.
Figure 2. Proper positioning of the head for ventrodorsal skull films .
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Figure 3. Proper positioning of the head for dorsoventral skull films viewed from the top (:\) and side (B).
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c Figure 4. Proper positioning of the head for an open-mouth rostrocaudal view of the tympanic bullae seen from the side (A) and top (8). The x-ray beam should be directed at the open mouth toward the back of the oropharynx (C). Note that the endotracheal tube has been momentarily removed for the exposures.
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Figure 5. A view of the back of a dog lying in lateral recumbency (A) demonstrates the scoliotic position (B) the spinal column assumes if padding is not used to keep the entire spine parallel to the tabletop. If padding is not placed under the sternum and between the limbs, axial rotation occurs as the sternum and upper limbs roll down toward the tabletop (C).
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D
Figure 6. Proper positioning of the head, neck, and shoulders for lateral cervical radiographs (A). Foam should be placed under the nose, head, and neck (B and C), so that the cervical spine from the occiput to the thoracic vertebrae is straight and perfectly parallel to the tabletop. Tucking the nose under slightly while applying gentle cranial traction on the ears further assists in straightening the cervical spine (D).
,
.-----~---------'~ B Figure 7. Proper ventrodorsal positioning of the cervical area requires the sternum to be held directly over the thoracic spinal column with forelimbs drawn back while placing gentle traction on the head or ears to straighten the cervical spine (A and B).
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~,-------------------- - -----------------------------------
c
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Figure 8. Proper lateral positioning for thoracic, lumbar, sacral, and coccygeal vertebrae requires foam padding to prevent scoliotic sagging and axial rotation (A, B, and C) . The cervical spine should also be supported to maintain proper positioning of the upper thoracic vertebrae.
Figure 9. Proper ventrodorsal positionIng for thoracic, lumbar, and sacral views requires the sternum and midline of the abdomen to be held directly over the spinal column to avoid axial rotation. The forelimbs should be drawn cranially and the hindlimbs caudally.
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Figure 10. Schematic drawings of myelographic changes seen in extradural (A) , intradural extramedullary (B), and intramedullary (C) spinal cord lesions.
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Figure 11. Lateral (A), dorsoventral (B), and open-mouth rostrocaudal (C and D [closeup view of C]) views of the skull of a normal dog. The dorsoventral and rostrocaudal projections are especially helpful because the skull is a symmetric structure and the two sides may be compared to determine the presence of disease.
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Figure 12. Lateral radiograph of the skull of a dog with multiple fractures of the calvarium (black arrows). There is considerable soft tissue swelling surrounding the skull (white arrows). Additional views could be made to thoroughly evaluate the skull for other fractures and to determine the extent of the evident fracture.
Figure 13. Dorsoventral (A) and open-mouth rostrocaudal (B) radiographs of the skull of a dog with clinical signs of otitis media. The left petrous temporal bone is increased in opacity and the left osseous bulla is thickened and increased in opacity (black arrows) (compare with Fig. liB and lie). Mineral opacity in the left external ear canal (A) indicates chronic otitis extern a (white arrow).
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Figure 14. A lateral radiograph of a young dog with hydrocephalus. The skull is domeshaped and the bones of the calvarium are thinned. This puppy had an "open fontanelle" at the frontoparietal suture (white arrow). The inner surface of the calvarium lacks the prominent gyral markings found in normal dogs (compare with Fig. 1lA). Additionally, the first cervical vertebra (C 1) is hypoplastic, and atlantoaxial subluxation is present.
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Figure 15. Lateral radiographs of two dogs with neoplastic lesions of the skull. A, A 9year-old German Shepherd with an aggressive adenocarcinoma involving the left frontal sinus. Bone lysis and proliferation is present as well as superimposed soft tissue swelling (arrows). B, An 8-year-old Poodle with seizures. There is nearly complete destruction of the dorsal portion of the ethmoturbinates (small arrow) and the cribriform plate (large arrow), with increased soft tissue opacity in the same region. Nasal adenocarcinoma with extension to the brain through the cribriform plate was diagnosed at necropsy.
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Figure 16. Lateral radiographs of the cervical spine (A), thoracolumbar spine (8), and lumbar spine (C) of a normal dog. Complete radiographic evaluation should also include ventrodorsal views of each region. Intervertebral disc spaces are clear and normal in width. Superimposed rib shadows can make interpretation of the caudal thoracic spine difficult and great care should be exercised.
Figure 17. Degenerative joint disease of the dorsal articular facets of the spine (A) and spondylosis deformans (8) often may be seen, particularly in large breeds of dogs. The severity of either disease can differ widely among affected dogs. Spondylosis deformans is unlikely to produce clinical signs of disease, but degenerative joint disease of the articular facets can produce pain and reduced mobility. Spondylosis deformans must not be confused with discospondylitis.
Figure 18. A lateral radiograph of the caudal lumbar spine of a dog with discospondylitis. There is bone lysis of the opposing vertebral endplates and ventral bony bridging of L6-L7. Bone lysis of the vertebral endplates is the critical radiographic abnormality in this disease (compare with Fig. 178).
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Figure 19. Spinal fractures occur often in traumatized patients. A, Transverse fracture of the second cervical vertebra with dorsal displacement and overriding occurred when this dog fell from a table. B, Oblique compression fracture of the third lumbar vertebra occurred when this dog was hit by an automobile. All spinal fractures are not this apparent. C, In young dogs, a physis of the vertebral body may fracture and displace only slightly (arrow) . These fractures can be very difficult to identifY unless one searches for them carefully. In any patient with a spinal fracture, a complete neurologic examination should be performed to assess the integrity of the spinal cord.
Figure 20. Lateral radiograph of the cervicothoracic spine of a 7-year-old dog with osteosarcoma of the first thoracic vertebra (Tl). Bone lysis and proliferation are present and the vertebral body is compressed due to pathologic fracture. Neoplasia of the spine is not common. However, both primary and metastatic neoplasia may involve the spine.
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Figure 21. Lateral radiograph of the cervical spine of a quadriparetic Poodle. There is atlantoaxial subluxation (white arrows) and absence of the dens (solid black arrow). Note the complete absence of an intervertebral disc space (open black arrow) between the second and third cervical vertebrae. Congenital anomalies of the spine are not uncommon.
Figure 22 See legend on opposite page
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Figure 23. Lateral radiograph of a myelogram in a Dachshund with intervertebral disc herniation at L3-U . The myelogram illustrates the appearance of an extradural lesion (arrows). There is dorsal displacement of the ventral contrast column and compression of the dorsal contrast column. Mineralized disc material is seen on the floor of the neural canal.
Figure 24. Lateral view of a myelogram in a dog with left hindleg weakness. The myelogram depicts an intradural-extramedullary mass that is outlined by the contrast medium (arrows). The "golf-tee" sign at the cranial end of the mass is characteristic of intraduralextramedullary lesions.
Figure 22. Survey radiographic signs of intervertebral disc herniation include narrowing or wedging of the intervertebral space, reduced size and increased opacity of the intervertebral foramen, decreased width of the dorsal vertebral articulation, and mineralized (opaque) material in the neural canal. A, Mineralized intervertebral disc material is clearly seen in the neural canal (arrow) at L3-U and the disc space is narrowed. B, The L2-L3 disc space is narrowed and slightly wedged, the intervertebral foramen is decreased in size and increased in opacity (solid arrow), and the width of the dorsal articulation is reduced (open arrow). If the radiographic changes do not correspond with the findings derived from neurologic examination, a myelogram should be performed.
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Figure 25. Intramedullary spinal cord lesions display evidence of an expanding spinal cord mass on all radiographic views (arrows). Thoracic and lumbar ventrodorsal views may be difficult to interpret because of superimposed thoracic and abdominal structures such as the sternum and intestine. A, Lateral view of the myelogram of a dog with a T12-T13 intramedullary astrocytoma. B, Ventrodorsal view of a dog with an L2-L3 intramedullary ependymoma. (Two different dogs are shown for better photographic detail.)
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anatomy and density. Intentionally obliqued lateral views of the skull can also be performed to place each bulla directly into the x-ray beam without overlying bone. This view is helpful in evaluating the thickness of the bulla wall and in determining the presence of fluid in the middle ear, but does not allow simultaneous comparison of bullae. Nasal Sinuses. Occasionally, an inflammatory or neoplastic process in the nasal sinuses will cause neurologic signs by direct extension through the cribriform plate. In addition to the routine lateral and VD views, the frontal sinuses can be evaluated by directing the x-ray beam through them in an rostrocaudal direction while the patient is lying with the occipital bone on the table, which is similar to the position for "open-mouth" views. Direct evaluation of sinus density and symmetry can then be made. Spinal Films A routine spinal series consists of lateral and VD views . The extent of the spinal column to be radiographed will depend on the neuroanatomic localization of the suspected lesion or lesions. The most common cause of undiagnostic spinal films is poor positioning (Fig. 5). It is imperative that time be taken to position the animal properly so that the portion of the spine in the x-ray beam is perfectly straight and parallel to the tabletop. Cervical and Cervicothoracic Films. Lateral Views . For diagnostic lateral cervical radiographs, the cervical spine must be positioned straight and parallel to the tabletop (no rotation down the long axis of the spine; no scoliosis). When an animal is lying in lateral recumbency, the neck falls into a bowed (scoliotic) position off the elevated shoulders and head, and the upper cervical area rotates as the nose drops to the tabletop (see Fig. 5). To position the cervical and upper thoracic spinal column correctly, foam rubber will be needed to elevate the nose, head, neck, and sternum so that the cervical column is parallel to the tabletop from the occiput to the thoracic area (Figs. 6A to C). If technical help is available, one person can tuck the elevated nose under slightly while pulling gently cranially on the ears . Another person then holds the thoracic spinal column in position while drawing the forelimbs caudally, thus moving the shoulders out of the way of the caudal cervical area (Fig. 6D) . In most animals, two lateral cervical exposures should be made: one centered over the C2-C3 area (upper cervical) and one centered over the C5-C6 area (lower cervical). In very small animals, one view aimed at the C3-C4 area may suffice. Additional "coned-down" views may be desirable after examination of the initial films. In addition, a view centered over the C7-T2 area can be made without repositioning. Ventrodorsal Views . For VD views, the animal should be placed in dorsal recumbency. The cervical spine should be positioned perfectly straight without rotation. The head should be left resting on the occipital bone with the nose angled upward. Applying gentle traction on the ears will help straighten the spine. The forelimbs should be drawn caudally while the sternum is aligned directly over the thoracic vertebral column in a line perpendicular to the tabletop (Fig. 7). If technical assistance is not available, it will be ne"cessary to stabilize the head and chest with tape and a tray or sand bags. The head should be
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taped pulling the nose down as little as possible to avoid lifting the upper cervical vertebrae. The forelimbs should be drawn back and tape applied across the chest. Foam rubber will be needed under the neck to eliminate kyphosis created by pulling the nose down with the tape. Traction to straighten the spine is not possible in this method of positioning and often scoliosis will be evident on the films. It is possible to have the upper cervical spinal column parallel to the tabletop in the VD position. However, the caudal cervical vertebrae make a natural ventral curvature into the thoracic area, making it difficult to position the cervicothoracic vertebrae parallel to the tabletop. Some obliquity may be present in VD caudal cervical views, especially in large, deep-chested dogs. This can be corrected most easily by tilting the head of the radiographic unit to direct the x-ray beam perpendicularly through the spinal column of the cervicothoracic views. If this is not possible, repositioning the animal after taking the upper cervical view may be necessary. Views of the Dens. Evaluation of the dens of C2 is important in suspected cases of atlantoaxial subluxation or dens fracture/luxation. The dens is best visualized by the open-mouth view previously described for the tympanic bullae (Fig. 4). Thoracolumbar, Lumbosacral, and Sacrococcygeal Films. The same principles of positioning discussed for the cervical and upper thoracic spine apply when taking radiographs of the thoracic, lumbar, sacral, and coccygeal spinal segments; the spine should be straight and as parallel to the tabletop as possible. Lateral Views. In lateral recumbency without padding, the lumbar spine normally falls into a bowed position. The tail drops to the table at an angle (Fig. 5A and B). In the thorax, the sternal area is narrower than the epaxial area. Without padding under the sternum, the vertebral column is rotated on its long axis. The weight of the upper limbs dropping across the chest and hindquarters also causes vertebral rotation (Fig. 5C). Foam rubber padding should be placed under the lumbar spine and hips as necessary to align the thoracic, lumbar, and sacral vertebrae in a straight line parallel to the tabletop (Fig. 8A, B and C) . To prevent axial rotation, a wedge of foam should be placed under the lower chest wall to place the sternum in the same plane above the tabletop as the vertebral column. Foam should also separate the two forelimbs and two hindlimbs so that the body/limb relationship is more nearly like that of the standing animal with limbs perpendicular to the body. If the coccygeal vertebrae are of interest, foam may be placed under the tail to elevate them into line with the spinal column. With this padding in place, a series of lateral radiographs may be taken along the thoracic, lumbar, and sacral spine 'with no technical assistance. If desired, technicians can apply mild traction to further straighten the spinal column by pulling the forelimbs and head cranially and the hindlimbs and tail caudally. They should be careful to hold the limbs perpendicular to the body to avoid axial rotation. The number of lateral views taken will depend on the nature of the neurologic problem, the size of the animal, and how well the site can be localized on neurologic examination. In external tr~uma victims, "screening" radiographs of large numbers of vertebrae may be advisable to quickly evaluate the entire thoracic, lumbar, and sacral spinal column and to judge
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the safety of performing VO views. Once the extent of damage and location is known, "coned-down" views (three to five vertebrae per field) centered over the affected area(s) can be made for better detail. If thoracolumbar disc disease is suspected, coned-down views centered over T12-T13 and L2-L3 are usually adequate to confirm the diagnosis. Ventrodorsal Views . The animal should be positioned in dorsal recumbency for the VO views. When technical assistance is available, the forelimbs should be held cranially and the hindlimbs caudally with gentle traction to help straighten the spine. The sternum and midline of the abdomen should be aligned directly over the vertebral column to prevent axial rotation (Fig. 9). Without technical assistance, a tray or sand bags, in combination with tape across the chest and abdomen, will be necessary to hold the animal in dorsal recumbency. The forelimbs can be taped forward or allowed to fall as long as the paws are not lying across the chest. The hindlimbs will usually fall in a "frogleg" position out of the way of the x-ray beam. It is very difficult to get the spinal column truly straight without technical assistance, but every attempt should be made to do so if diagnostic radiographs are to be obtained. The numbers and types of VO views should correspond to those taken in the lateral position. In cases of vertebral instability, VO positioning may cause further luxation and damage to the nervous system, so caution is advisable. SPECIAL PROCEDURES
Myelography
Indications. Myelography is ·the most frequently performed neuroradiographic contrast procedure in veterinary medicine. It enables one to localize and characterize spinal cord lesions more specifically than is possible with survey radiographs. A myelogram must be preceded by a thorough neurologic examination and careful evaluation of survey radiographs of the spine. Myelography is indicated in cases in which survey spinal radiographs have not clearly demonstrated a neuroanatomically correlating lesion(s). Myelography is contraindicated in patients displaying signs of inflammatory CNS disease and any time increased ICP is suspected. Techniques. Myelography is not difficult to perform if the practitioner strictly adheres to the principles of subarachnoid puncture (see the article "Cerebrospinal Fluid"). The only special pieces of equipment required are styleted spinal needles and an appropriate neuroradiologic contrast agent. Iohexol (240 mg%)* is an excellent contrast medium that provides adequate radiographic contrast with a low incidence of adverse reactions. The sites of injection (cisterna magna; L4-L5 or L5-L6 lumbar subarachnoid space) are the same as for cerebrospinal fluid (CSF) collection (see the article "Cerebrospinal Fluid"). Needle placement and injection at the cisterna magna are easier and usually safer to accomplish than with lumbar puncture. Cisternal injection is preferable fQr lesions localized in *Omnipaque 240. Winthrop Pharmaceuticals, New York, New York.
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the cervical, cervicothoracic, and upper thoracic areas; excellent results are also possible in the thoracolumbar and lumbosacral areas if the animal is held with its spine perpendicular to the floor for a few minutes following injection of the medium . However, when caudal medium flow is obstructed, cisternal injection is not as good as lumbar injection in delineating the total extent of thoracolumbar or lumbosacral lesions. Because of the "dead-end" anatomy of the lumbar cistern, medium injected into the lumbar area can be introduced under more pressure and forced around an obstructing lesion. This is not possible from the cisterna magna, because medium injected under pressure will flow rostrally into the brain, which should be avoided. Placing the spinal needle in the dorsal lumbar subarachnoid space is extremely difficult, and placing it in the ventral lumbar subarachnoid space (passing the needle through the spinal cord) can cause extensive damage if the needle travels off the midline and into the ventral horn cells of the sciatic nerve segments. In addition, if the needle is improperly positioned at the time of injection, contrast medium may be driven into the lumbosacral spinal cord segments, causing irreversible flaccid paralysis of the hindlimbs. Aseptic techniques must be followed throughout the procedures of CSF withdrawal and contrast medium injection. * After CSF has been withdrawn in a quantity not to exceed the amount of medium to be injected, the contrast medium is slowly injected into the subarachnoid space. A dosage of 0.3 ml per kg of the contrast medium is injected. This dose will opacify the subarachnoid space up to the mid- to proximal thoracic region if injected by the lumbar puncture technique and will adequately opacify the cervical subarachnoid space if injected at the cisterna magna. A dosage of 0.4 ml per kg may be injected in the cisternal region to opaCify the lumbar subarachnoid space. or vice versa. Radiographs should be made immediately following the completion of the injection of the contrast material unless time needs to be allowed for flow from the cisterna magna to the thoracolumbar and/or lumbosacral areas. Interpretation. Spinal cord lesions will be identified myelographically as extradural, intradural-extramedullary, or intramedullary in location (Fig. 10). Extruded intervertebral disc material is probably the most common extradural lesion, although neoplasms, hypertrophied spinal ligamentous structures (see the article "Disc-Associated Wobbler Syndrome"), and hematomas may also be seen as extradural lesions. Intradural-extramedullary lesions are almost always neoplastic. Intramedullary lesions may be neoplastic but may also be caused by hemorrhage or edema within the spinal cord. Other Special Procedures Many other special neuroradiographic procedures are available in addition to myelography. Many of these procedures require specialized expertise and equipment that is often not available in a clinical practice situation. If one of these procedures is felt to be necessary, referral to an institution or specialty practice may be require~. *CSF withdrawn prior to myelography should always be analyzed rather than assumed to be normal and discarded. Valuable diagnostic information may be lost and, if found to be necessary later, will require an additional episode of anesthesia for the animal.
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Among the specialized procedures that may assist in making a diagnosis in cases of nervous system dysfunction are epidurograms, angiograms, venous sinograms, ventriculograms, computerized axial tomography (CT scans), and magnetic resonance imaging (MRI). Angiograms and ventriculograms can be very dangerous to perform and have generally been replaced by nuclear medicine scans and CT scans. The practitioner is advised to consult some of the excellent texts available IS, 21, 22 or specialists in neurology or radiology when deciding how helpful or necessary these tests might be in individual cases. RADIOGRAPHIC INTERPRETATION The brain and spinal cord are not visible radiographically, but the bony structures surrounding them can be easily evaluated and may reflect disease involving the CNS. Interpretation of skull and spinal radiographs depends on a thorough knowledge of the normal appearance of these structures and on recognition of changes from normal in size, shape, margination, position, number, and opacity. IS. 19,21,22 Myelography allows evaluation of the spinal subarachnoid space and the size and shape of the spinal cord. Evaluation of the brain requires more sophisticated imaging techniques that may be available at some institutions. Figures 11 to 25 illustrate radiographs depicting normal skull and spinal radiographs (Figs. 11 and 16) and common diseases affecting the skull, brain, spine, and spinal cord (Figs. 12 to 15, and 17 to 25). SUMMARY Neuroradiographs are an invaluable part of the diagnostic plan in most cases involving nervous system dysfunction in companion animals. High MAS, low KvP techniques used on standard radiographic equipment available in most veterinary practices will provide good-quality neuroradiographs of the skull and spine. Proper positioning of the animal, which necessitates use of general anesthesia, is required to obtain neuroradiographs of good diagnostic quality. A working knowledge of the normal anatomy of the skull and spine is required to make correct interpretations of the neuroradiographs . ACKNOWLEDGMENT The authors wish to acknowledge the artwork of Annie S. Miley of the Biomedical Learning Resource Center, College of Veterinary Medicine, Texas A & M University,
REFERENCES 1. Baldessarini RJ: Drugs and the treatment of psychiatric di50rders , In Goodman AG, Goodman LS, Rail TW, et al (eds): The Pharmacological Basis of Therapeutics. Edition 7. New York, Macmillan Publishing Co, 1985, pp 387-445 2. Bonhaus DW, Sawyer DC, Hook JB: Displacement of protein-bound thiopental by sodium
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4. 5.
6. 7.
8. 9. 10. 11 . 12.
13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23.
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methiodal may contribute to anesthetic complications during canine myelography. Am J Vet Res 42:1612, 1981 Bunch SE, Castleman WL, Baldwin BH, et al: Effects of long-term primidone and phenytoin administration on canine hepatic function and morphology. Am J Vet Res 46:105, 1985 Davis EM , Glickman L, Rendano VT, et al: Seizures in dogs folloWing metrizamide myelography. JAm Anim Hosp Assoc 17:642, 1981 Dawson B, Michenfelder JD, Theye RA: Effects of ketamine on canine ce rebral blood flow and metabolism: Modification by prior administration of thiopental. Anesth Analg 50:443, 1971 Gilroy BA: Neuroanesthesiology. In Slatter DH (ed): Textbook of Small Animal Surgery. Philadelphia, WB Saunders Co, 1985, pp 2643-2649 Gray PR, Lowrie CT, Wetmore LA: Effect of intravenous administration of dextrose or lactated Ringer's solution on seizure development in dogs after cervical myelography with metrizamide. Am J Vet Res 48:1600, 1987 Gray PR, Indrieri RJ, Lippert AC: Influence of anesthetic regimen on the frequency of seizures after cervical myelography in the dog. J Am Vet Med Assoc 190:527, 1987 Hartsfield SM, Thurmon JC: Anesthesia in small animal neurological patients. J Am Anim Hosp Assoc 11:396, 1975 Hubbell JAE: Anesthesia for the emergency surgical patient. In Bright RM (ed): Surgical Emergencies. New York, Churchill Livingstone, 1986, pp 45-59 Lumb WV, Jones EW: Veterinary Anesthesia. Philadelphia, Lea & Febiger, 1984, p 168 Menheusen MJ, Parker AJ: Clinical observations of cardiovascular and respiratory changes during contrast myelography with water soluble media. J Am Anim Hosp Assoc 11:401 , 1975 Muir WW III, Hubbell J, Skarda R, et al: An Outline of Veterinary Anesthesia. Columbus, Ohio State University, 1986, p 25 Puglisi TA, Green RA, Hall CL: Comparison of metrizamide and iohexol for cisternal myelographic examination of dogs. Am J Vet Res 47:1863, 1986 Ryan GD: Radiographic Positioning of Small Animals. Philadelphia, Lea & Febiger, 1981 Saidman LJ, Eger EI II: Change in cerebrospinal fluid pressure during pneumoencephalography under nitrous oxide anesthesia. Anesthesiology 26:67, 1965 Shapiro HM: Anesthetic agents and techniques and cerebral blood flow . American Society of Anesthesiologists' Refresher Course Lectures, 1986, p 121 Shores A, Burns J: Technique and indications for metrizamide myelography in small animals. Compend Con tin Ed Pract Vet 9:361, 1987 Slatter DH: Textbook of Small Animal Surgery. Philadelphia, WB Saunders Co, 1985 Stowater JL, Kneller SK: Clinical evaluation of metrizamide as a myelographic agent in the dog. J Am Vet Med Assoc 175:191, 1979 Thrall DE : Textbook of Veterinary Diagnostic Radiology. Philadelphia, WB Saunders Co, 1986 Ticer JW: Radiographic Techniques in Veterinary Practice. Edition 2. Philadelphia, WB Saunders Co, 1984 Tomas PN , Walker MA, Paddleford RR, et al: Prevention of postmetrizamide seizures in dogs using 5% dextrose solution. J Am Vet Med Assoc 188:710, 1986
Department of Small Animal Medicine and Surgery College of Veterinary Medicine Texas A & M University College Station, Texas 77843