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Myelography and Epidurography
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DIAGNOSTIC IMAGING
MYELOGRAPHY AND EPIDUROGRAPHY Royce E. Roberts, DVM, MS, and Barbara A. Selcer, DVM
Diagnostic evaluation of the spinal canal often requires the introduction of contrast media into the subarachnoid or epidural space. These procedures are used in the evaluation of the spinal cord and cauda equina. Satisfactory results can be obtained in a private practice setting. This article provides indications and instructions for performing these diagnostic procedures and guidelines for interpreting the results. MYELOGRAPHY
Myelography is a radiographic technique of the spinal cord made after the injection of contrast medium into the spinal subarachnoid space. Myelograms are being performed on dogs and cats with increasing frequency by both academic and private veterinary practitioners. This is a result of the increased sophistication of small animal veterinary practice and the development of improved myelographic contrast media. Myelography is essential to the diagnosis of many spinal cord disorders in dogs and cats. Even though the currently used nonionic myelographic contrast media are much safer than earlier generations of contrast media, myelography is not an innocuous procedure and should be performed only by those who have acquired adequate training and knowledge. Indications
The clinical utility of myelography in small animals is well documented. Myelography should be considered for those patients exhibitFrom the Department of Anatomy and Radiology, University of Georgia College of Veterinary Medicine, Athens, Georgia
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ing spinal pain or neurologic deficits after noncontrast radiographs or other indicated laboratory tests have failed to identify the offending. problem. A careful neurologic examination must be performed to localize the affected cord s-egment(s). A myelogram is indicated when (1) there is absence of a spinal lesion on noncontrast radiographs; (2) the lesion seen on noncontrast radiographs is not compatible with clinical signs and/or neurologic examination; (3) noncontrast radiographs indicate multiple lesions consistent with clinical signs and/or neurologic examination; (4) it is necessary to confirm a lesion suspected on noncontrast radiographs; (5) it is desirable to determine the precise location of a spinal lesion to help choose an exact surgical approach; e.g., lateralized disc herniation; (6) it is desirable to determine the extent of spinal cord swelling or compression to aid surgical ·decisions; and (7) the diagnosis of a neurologic disorder is established by exclusion of a spinal cord compressive lesion; e.g., degenerative myelopathy. Contraindications
Myelography should be avoided in patients with a cerebrospinal fluid (CSF) analysis that indicates inflammation/infection. Myelography may unnecessarily potentiate the clinical signs or spread the infection within the subarachnoid space. Contrast Agents
Even though no myelographic contrast medium has been approved for animal use in the United States, veterinarians have been performing myelography for several decades. Characteristics of the ideal myelographic contrast agent should include nontoxic, miscible with cerebrospinal fluid, autoclavable, water-soluble, radiopaque in isotonic concentration, absorbable, and affordable. 21 Numerous contrast agents have been used for animal myelography. Iophendylate (Pantopaque, LaFayette Pharmaceutical, LaFayette, IN) and methiodal sodium (Skiodan, Winthrop-Breon, New York, NY) are older contrast agents that are inappropriate for use in contemporary small animal myelography. Both agents have been associated with an unacceptable incidence of arachnoiditis, and methiodal sodium myelography results in a high incidence of seizures, aggravation of neurologic signs, and poor myelographic contrast. Myelographic contrast agents currently recommended for small animal use are the newer nonionic media, which include iopamidol (Isovue, E.R. Squibb, Princeton, NJ) and iohexol (Omnipaque, Winthrop-Breon, New York, NY). Metrizamide (Amipaque, Winthrop-Breon, New York, NY) was the first nonionic contrast agent developed and resulted in significant reduction in neurotoxicity and postmyelographic
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complications compa-red to earlier contrast agents; however, a significant number of dogs experienced postmyelographic complications. 1· 2 Because the newer generation of nonionic myelographic agents are associated with a further reduction in neurotoxicity and significant improvement in myelographic quality, metrizamide is no longer recommended for small animal myelography. lohexol and iopamidol were approved for human use in 1986. Both contrast agents reportedly cause fewer postmyelographic complications than metrizamide in dogs and cats. 5• 13• 19· 20 However, one report17 that compared neurotoxic effects of iopamidol and metrizamide in dogs indicated no significant difference in adverse effects between these contrast agents. One repore3 suggests fewer complications with iohexol than with iopamidol. Mild transient changes in CSF have been reported with iopamidol and iohexol. 19 lopamidol and iohexol are rapidly removed from the subarachnoid space by diffusion and exhibit less chemotoxicity than metrizamide. 21 lotrolan (lotral, Schering, Berlin, Germany) is a third-generation nonionic myelographic agent and may be less neurotoxic than iohexol and iopamidol. lotrolan is more viscous than other nonionic agents, which results in difficulty when injecting it through the smaller gauge needles employed in small animal myelography.21 lotrolan is not recommended at this time for small animal myelography. lohexol and iopamidol are the contrast agents of choice for myelography in dogs and cats. 19~ 21 lopamidol is available in several concentrations ranging from 200 mg 1/mL to 370 mg 1/mL. The 200 mg II mL concentration is only slightly hyperosmolal and is .suitable for myelography in dogs and cats. lohexol also is available in various concentrations of 180 mg 1/mL to 350 mg 1/mL. The 180 mg 1/mL is near osmolal and is recommended by some for routine myelographic procedures in small animals. The higher concentrations of iohexol and iopamidol are hyperosmolal but can be safely used for small animal myelography. An excellent and detailed review of myelographic contrast agents in veterinary medicine has been conducted21 and is recommended reading for those wishing more detailed information regarding this topic. Technique
Patients selected for myelography are anesthetized using normal preanesthetic and anesthetic protocols. However, phenothiazine derivative tranquilizers (acetylpromazine) must be avoided. Diazepam has been recommended as a preanesthetic anticonvulsive agent; however, its short biologic half-life (30 to 45 minutes) limits its effectiveness as a premyelographic antiseizure drug. Diazepam is an effective postmyelographic anticonvulsive drug. 16 It has been reported that dogs receiving metrizamide for cervical myelography had a significant reduction in seizures when premedicated with pentobarbital (5 mg/kg intramuscu-
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larly) and maintained on methoxyflurane (Metofane, Pittman-Moore, Washingtons Crossing, NJ). This apparent anticonvulsive effect of preanesthetic pentobarbital was not seen when anesthesia was main~ tained with halothane (Fluothane, Ayerst, New York, NY)/ Phenothiazine derivative tranquilizers (e.g., acetylpromazine maleate) are contraindicated in myelographic patients. Their use increases the risk of seizures presumably by lowering the seizural threshold. Maintaining hydration is essential in minimizing the neurotoxic effects of myelographic contrast agents. An intravenous catheter is placed, and normal hydration is maintained with isotonic fluids. Premyelographic dehydration has been shown to slow resorption of contrast media, resulting in unnecessary neurotoxicity. After anesthesia is achieved, the patient is placed in lateral recumbency (some myelographers prefer sternal recumbency) on the radiographic table. The spinal tap site is clipped and aseptically prepared. The prepared site should be liberal enough to allow aseptic palpation of anatomic landmarks (i.e., the wings of the atlas and external occipital protuberance for cisternal injection or the wings of the ilia and the spinous processes of the L6 vertebrae for lumbar injection). The site selected for injection of contrast medium is dependent on myelographer preference, suspected location of lesion, and availability of fluoroscopy/ image intensification. Because fluoroscopy/image intensification is unavailable to most private veterinary practitioners, myelographic protocols may be different than those used in specialty or institutional practices where this technology is available. In the author's opinion, when fluoroscopic visualization is available, all myelographic procedures (cervical, thoracolumbar, or cervicothoracolumbar myelograms) can be performed satisfactorily from a lumbar subarachnoid tap. When fluoroscopy is unavailable, the following tap sites are recommended: Smaller dogs(< 50 lbs) and cats-lumbar tap at L5-L6 for cervical, thoracolumbar, and cervicothoracolumbar myelograms. Larger dogs (> 50 lbs)-lumbar tap at L5-L6 (L4-L5 in German Shepherds and possibly other large breed dogs) for thoracolumbar myelograms, and cerebellomedullary cistern (cisternal) tap for cervical and cervicothoracolumbar myelograms. Commercially available spinal needles with stylet are used in all cases and should not be reused. For cisternal puncture, a 1.5 inch, 22-ga needle is recommended for cats and most dogs. In some larger dogs, a 2.5 inch, 22-ga needle may be required. For lumbar subarachnoid puncture, a 1.5 inch, 22-ga needle is sufficient for most small dogs(< 20 lbs) and cats. Some obese small dogs require a 2.5 inch needle. In large dogs, a 2.5 inch, 22-ga needle is recommended. A 3.5 inch, 22- or 20-ga spinal needle may be required in obese large dogs or giant breed dogs. The precise procedure used for lumbar puncture (i.e., angle of entry to the lumbar subarachnoid space) influences the length of spinal needle required. All spinal taps are performed under aseptic conditions. Placing the spinal needle into the cerebellomedullary cistern is performed with the patient in lateral or sternal recumbency and the
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neck fully flexed ventrally. The needle is inserted on the midline with the bevel directed caudal, near the center of a triangle formed by the external occipital protuberance and the wings of the atlas. The dorsal midline can be identified by palpation of the junction of the paired dorsal cervical muscles. The needle is slowly advanced ventrally until the ligament flavum and dorsal dura are penetrated, which is indicated by a sudden reduced resistance to the needle and a characteristic "pop." The stylet is removed to check for flow of CSF. If additional manipulation of the needle is necessary to facilitate CSF flow, the stylet should be replaced and lateral or craniocaudal movement of the needle should not be allowed. Lumbar subarachnoid taps can be performed with the patient in lateral or ventral recumbency; the former is preferred by the author. The patient should be placed in an exact as possible lateral position. After the puncture site is aseptically prepared, the dorsocaudal edge of the spinous process of L6 is identified, which is normally the first spinous process palpated cranial to the wings of the ilium. The spinal needle is introduced through the skin slightly off the midline at this site. The needle is advanced cranioventrally and toward the mid line at a 30° to 60° angle just lateral to the spinous process (Fig. 1). It may be necessary to reposition the needle several times before it enters the interarcuate space. Occasionally, it is necessary to flex the spine to facilitate needle entry into the vertebral canal. This is most often necessary in older, large dogs with dorsal midline vertebral osteophytes. As the needle enters the spinal canal and penetrates the dorsal dura mater, a muscle twitch of the tail or rear limbs may occur. Repeated lumbar subarachnoid space puncture should be minimized, because cord compression can result from needle puncture alone, 18 and multiple dural punctures will increase the chances of epidural leakage. The needle is then slowly advanced to the ventral floor of the vertebral canal. The needle bevel should be directed cranially and the needle stylet is removed to check for CSF. In Dachshunds and other small dogs and in cats, at this location, the needle tip is often in the ventral subarachnoid space, and CSF will flow or can be identified in the needle hub. In larger dogs, it is often necessary to retract the needle slightly before CSF can be identified. Compressing the jugular veins will facilitate CSF flow. The contrast medium should be injected through a flexible extension tube that connects the needle and syringe. Extension tubes vary in length (4-21 inches) and volume (0.5-3 mL). Small-volume extension tubes should be used for small dog and cat myelograms. By using an extension tube, the possibility of needle movement during the injection is minimized. Excessive needle movement can result in direct injury to the spinal cord or inappropriate injection of contrast medium into the epidural space (Fig. 2) or the spinal cord (intramedullary). The recommended dose of contrast medium (e.g., iohexol, 300 mg 1/mL) can be calculated from the following: cervical myelogram (lumbar tap)-0.45 mL!kg cervical myelogram (cervical tap)-0.30 mL!kg
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Figure 1. Location of needle for lumbar injection myelography. A, Lateral view showing approximate entry angle and location of spinal needle. Note position of needle relative to spinous process of L6. B, Dorsocaudal view of needle position and location of interarcuate space.
Figure 2. Lateral radiograph of lumbar spine showing contrast medium outside the subarachnoid space (epidural). Accumulations of contrast medium are seen over the plane of the intervertebral foramina and ir. the dorsal epidural spaces (arrows). Epidural contrast medium can significantly reduce myelographic quality.
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thoracolumbar myelogram (lumbar tap)-0.30 mL!kg thoracolumbar myelogram (cervical tap)-0.30 mL!kg cervicothoracolumbar·,m yelogram (cervical or lumbar)-0.45 mL!kg The calculated dose of contrast medium is drawn into a syringe, which is then connected to the extension tube. The extension tube is filled completely with contrast medium; care should be taken to avoid introducing air bubbles into the tube. If the needle hub is free of fluid, it should be filled with contrast medium from the tube prior to connecting the needle and extension tube. This will prevent air bubbles from entering the spinal subarachnoid space (Fig. 3). After the extension tube is carefully connected to the spinal needle, injection of the contrast medium can proceed. Injection should be slow (2-3 mL!min), and little resistance should be felt. If fluoroscopy is available, subarachnoid filling can be confirmed by direct visualization. Without fluoroscopy, a test injection of 0.5 to 1 mL of contrast material should be performed, and a lateral radiograph of the injection site is made. The visualization of longitudinal-oblique linear striations of the cauda equina, filling defects of nerve rootlets, and/or thin discrete subarachnoid spaces indicate good subarachnoid space filling (Fig. 4). If satisfactory subarachnoid filling is evident, the remaining volume of contrast medium can be injected. In the larger volume (2 mL) extension tubes, that portion of the contrast medium remaining in the tube can be injected after the syringe is emptied. After completion of the injection, lateral radiographs are made of the spinal segments of interest. The needle may remain in place while the lateral radiographs are made, but great care must be taken not to move the patient or needle. The lateral radiographs should indicate satisfactory filling of the subarachnoid space. By positioning the patient appropriately, the contrast material can be gravity directed by elevating or lowering the head, depending on the desired flow. Occasionally, there is nonuniform filling or poor mixing of contrast medium with CSF. After removal of the spinal needle, it may be helpful to place the
Figure 3. Lateral radiograph of the lumbar spine showing multiple air bubbles within the
subarachnoid space. Air bubbles can mimic intradural lesions, but they typically are in different locations on subsequent radiographs.
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Figure 4. Lateral radiographs of the caudal lumbar spinal cord and cauda equina following injection of contrast medium. A, Normal subarachnoid filling is indicated by the oblique striations of the cauda equina (solid arrows) and the fine serpentine filling defects created by the leptomeningeal vessels (open arrows). 8, The normal caudal tapering of the spinal cord and cauda equina can be seen. Note the normal lack of discrete columns of contrast material that is seen with the spinal cord segments.
animal in dorsal or in the opposite lateral recumbency to evenly distribute or mix the CSF and contrast medium. After 2 to 3 minutes in this position, repeat the lateral radiographs as necessary. Ventrodorsal radiographs of the spine can be made after satisfactory lateral views are obtained. In patients suspected of lateralized spinal lesions (e.g., herniated disc, nerve root tumor), oblique myelographic views may be useful. When lesions are not located precisely ventral or lateral to the spinal cord, conventional lateral and ventrodorsal views may not completely demonstrate, characterize, or localize the lesion. Interpretation
The normal myelogram is characterized by discrete, thin columns of contrast medium that are nearly parallel except at the cauda equina where the subarachnoid space tapers to form the dural end sac (Figs. 5, 6, and 7). In most cases, the subarachnoid space approximates the margins of the vertebral canal. The spinal cord may not appear uniform in diameter owing to the normal widening of the spinal cord in the caudal cervical-cranial thoracic spinal cord segment and caudal lumbar spinal cord segment, as a result of the brachial and lumbosacral spinal intumescences, respectively (Figs. SB, 6B). In some small animals, the ventral thoracolumbar subarachnoid space may be thinner than its
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Figure 5. Normal canine cervical myelogram. A, Lateral view depicting thin, discrete subarachnoid spaces from C3 to the cervicothoracic junction. The increased width of the atlantoaxial subarachnoid space is normal. 8, Ventrodorsal radiograph shows normal widening of the caudal spinal cord indicating the area of the brachial plexus intumescence (arrows).
corresponding dorsal subarachnoid space. In dogs, the dorsal subarachnoid space at the atlantoaxial level is consistently wide (Fig. SA). Myelographic variations due to breed have been recognized. The location of the lumbar intumescence is located more cranially in German Shepherds than in DachshundsY Focal thinning of the ventral subarachnoid space over the intervertebral disc spaces can be normal in some small breed dogs, and larger breed dogs often demonstrate smaller spinal cords relative to the size of the vertebral canal. The location and appearance of the cauda equina and dural end sac are influenced by breed and position of the spine. 9• 12 Typically, the dural end sac of larger breed dogs terminates more cranial than in small or toy breed dogs. Also, the position of the cauda equina/dural end sac within the vertebral canal is inconsistent. It has been suggested that this inconsistency may be corrected by flexion-extension of the spine. 9 Feline myelograms are similar in appearance to canine myelograms; however, the cervical spinal cord may appear wider in comparison (Fig. 7). Focal thinning of the ventral subarachnoid space at the intervertebral disc spaces and the location of the dural end sac have been reportedly similar to that observed in chondrodystrophoid breed dogs. 20 The abnormal myelogram demonstrates alterations of the subarachnoid space and spinal cord. The myelogram can determine lesion
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Figure 6. Normal canine thoracolumbar myelogram. A, Lateral view showing subarachnoid space filling . Note the location and position of the cauda equina and dural end-sac. B, Ventrodorsal radiograph showing the normal widening of the lumbar spinal cord (L3-4), which indicates the location of the lumbosacral spinal intumescence (arrows).
location relative to the dura mater (extradural, intradural) and to the spinal cord (intramedullary). Four myelographic patterns can be identified: normal, extradural, intradural-extramedullary, and intramedullary (Fig. 8) . By characterizing spinal lesions in this fashion, diagnostic
Figure 7. Normal feline cervical myelogram. The spinal cord appears relatively wider than in canine cervical myelograms.
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VD VIEW
A
B
c
D Figure 8. Illustrations of myelographic patterns. A, Normal myelographic pattern, B, Extradural myelographic pattern. Example, extruded intervertebral disk. C, Intradural myelographic pattern. Example, intradural mass lesion. 0 , Intramedullary myelographic pattern.
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Table 1. MYELOGRAPHIC PATTERNS: COMMON DIAGNOSTIC POSSIBILITIES Extradural
Intradural/Extramedullary
Herniated intervertebral disc Vertebral neoplasia (primary or secondary)
Nerve tumors (e.g., neurofibromas)
Epidural soft tissue neoplasia (e.g. , feline lymphosarcoma) Hemorrhage/hematoma Vertebral fracture/luxation Spinal anomaly Hypertrophied spinal ligaments
Nerve sheath tumor (e.g., meningiomas) Hemorrhage/hematoma
Intramedullary
Spinal cord tumor (e.g., astrocytoma) Granulomatous meningoencephalitis (GME) (reticulosis) Hemorrhage
Spinal cord edema Ischemic myelopathy
possibilities, therapeutic choices, and prognoses can be established. Table 1 lists some common causes of these myelographic patterns. Extradural lesions involve tissues outside the dura and displace the subarachnoid space and spinal cord that is contiguous with the lesion (Figs. 9A and lOB). Depending on the severity of the lesion and degree of spinal cord compression, the affected subarachnoid spaces can appear thin or completely void of contrast medium. The degree and direction of deviation iri the subarachnoid space and spinal cord is
Figure 9. Cervical myelogram demonstrating extradural lesion. A, Lateral view showing dorsal deviation of the spinal cord and calcified intervertebral disk in situ at C4-C5, 8 , Ventrodorsal radiograph showing spinal cord enlargement at C4- C5 resulting from large ventral extradural lesion. Surgical diagnosis of extruded intervertebral disk.
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Figure 10. Thoracolumbar myelogram indicating lateralized intervertebral disk herniation. A, Lateral view shows attenuation of contrast medium at T13-L 1. Deviation and thinning of the ventral subarachnoid space at L1-L2 is indicative of a Type II disk herniation. B, Ventrodorsal view confirms a large extradural lesion located lateral and to the left of the spinal cord at T13-L 1. Surgical exploration confirmed disk extrusion at T13-L1.
best seen on that radiographic view that is tangential to the lesion (Figs. 9A and lOB). The opposite view (90°) will frequently demonstrate widening of the spinal cord segment with lateral displacement and/or thinning of the subarachnoid spaces (Fig. 9B). In some cases of acute extradural compressive lesions (disc herniation), spinal cord swelling is so severe as to obliterate the subarachnoid space. This can occur over several vertebral segments and results in nonvisualization of the extradural lesions (Fig. 11). A common cause of extradural lesions in dogs
Figure 11. Lateral thoracolumbar myelogram showing attenuation of the subarachnoid space from T12 to L4. Surgical exploration revealed disk extrusion at L 1- L2 and spinal cord swelling.
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is intervertebral disc herniation. Other diagnostic possibilities for extradural lesions in dogs and cats are listed in Table 1. Lesions characterized as intradural-extramedullary are located within the subarachnoid space but outside the spinal cord (i.e., between the dura mater and the spinal cord). This myelographic pattern is characterized by a "filling defect" within the subarachnoid space and the "golf tee" sign. The filling defect appears as a radiolucency surrounded by radiopaque contrast medium. The golf tee sign is formed by the diverging column of contrast medium as it attempts to go around the intradural space-occupying lesion (Fig. 12). The spinal cord is typically displaced away from the intradural lesion, and the spinal cord appears widened on the 90° opposite radiographic view, much in the same manner as seen with extradural lesions. Spinal meningiomas are common causes of intradural lesions. Intramedullary lesions are located within the spinal cord parenchyma. This myelographic lesion is characterized by widening of the spinal cord and deviation of the subarachnoid spaces away from the central canal of the spinal cord on both radiographic views (Fig. 13). Thinning of the subarachnoid spaces in all planes is observed typically. The latter change is helpful in differentiating intramedullary lesions from the normal widening of the spinal cord seen at the brachial and lumbosacral intumescences. Thinning of the subarachnoid spaces is not
Figure 12. Thoracolumbar myelogram demonstrating an intradural-extramedullary lesion. A, Lateral view shows a large filling defect within the contrast medium. Note the "golf tee" sign formed by the contrast medium as it attempts to circumvent the caudal edge of the mass lesion (arrow). 8, Ventrodorsal view showing widening of the spinal cord and filling defect.
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Figure 13. Lumbar myelogram of intramedullary lesion. A, Lateral radiograph showing focal widening of the spinal cord and thinning of the subarachnoid space (arrows). 8, Ventrodorsal view also exhibits spinal cord swelling and thinning of contrast medium (arrows) . Widening of the spinal cord on both lateral and ventrodorsal views is characteristic of intramedullary lesions.
normally observed with the spinal intumescences. Spinal cord parenchymal tumors and spinal cord edema are common causes of intramedullary lesions. Myelomalacia is an uncommon intramedullary lesion that is characterized by staining of the spinal cord parenchyma with contrast medium (Fig. 14). Many causes of spinal cord dysfunction can be identified by myelography; however, myelographic findings must be judged in
Figure 14. Lateral lumbar myelogram exhibiting contrast medium within the spinal cord parenchyma (arrows). This sign suggests spinal cord liquification or myelomalacia.
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Table 2. COMMON NEUROLOGIC DISORDERS THAT MAY SHOW A NORMAL MYELOGRAM Degenerative myelopathy Ischemic myelopathy Meningitis
Myelitis Spinal cord trauma GME (reticulosis)
context with all other clinical data before arriving at a diagnosis. It is important to remember that some spinal cord disease can result in profound neurologic signs without myelographic abnormalities (Table 2). In fact, the presumptive diagnosis of some common neurologic diseases in dogs and cats depends on a normal myelogram, establishing the diagnosis by exclusion (e.g., degenerative myelopathy). The role of myelography is to localize and characterize spinal lesions. The neurologic examination determines the clinical prognosis. Postmyelographic Patient Care
Routine postmyelographic care is directed at minimizing the neurotoxic effect of myelographic contrast agents. Because of the inherent nature and accepted risk associated with myelography, some degree of neurotoxicity may be unavoidable. Careful and prudent care of the patient before and during myelography can significantly reduce the potential for postmyelographic complications. Routine . postmyelographic procedures should include elevating the patient's head, maintaining normal hydration during recovery, and continuously monitoring for evidence of muscle fasciculations and partial or generalized motor seizures. Those patients receiving larger doses of contrast medium or cerebellomedullary cistern injection are at increased risk for seizures. 1 Muscle fasciculations and seizure activity are treated immediately with anticonvulsive drugs. Diazepam is widely used at a dose of 0.2 to 0.4 mg/kg intravenously, and it can be given every 10 to 30 minutes, not to exceed a total dose of 20 mg in small dogs and cats or 40 mg in large dogs. 16 If seizures are refractory to diazepam, intravenous phenobarbital at a dose of 2 to 4 mg/kg can be administered. The postmyelographic removal of contrast material to aid in preventing postmyelographic complications has been advocated. 22 Since the introduction of the newer, nonionic myelographic contrast agents (iopamidol and iohexol), the incidence of myelographic complications has been reduced significantly. However, these agents are not without neurotoxicity even when used properly. All myelographers must become proficient in performing spinal taps prior to performing myelograms on client animals. EPIDUROGRAPHY
Diseases that affect the canine lumbosacral vertebral canal are varied and produce a wide range of clinical signs, including rear limb
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lameness, pain, and fecal or urinary incontinence. 4• 10 Typically, clinical signs are the result of nerve root compression secondary to disc herniation, fibrous connective tissue proliferation, or spinal canal stenosis of congenital or developmental origin. 4 In some instances, lumbosacral subluxation will result in lumbosacral spinal canal compressive disease with dynamic or static compressive lesions as possible sequelae. Less frequently, infectious diseases such as discospondylitis or neoplastic disease may cause lumbosacral canal compression. Diagnostic imaging plays a major role in the diagnosis of lumbosacral disease. Survey radiographs of the lumbosacral junction are usually inconclusive. Survey radiographic signs that have been recognized in dogs with lumbosacral disease include spondylosis deformans, L7 Sl end plate sclerosis, and narrowing of the lumbosacral disc space. 14 However, these findings also can be seen in asymptomatic dogs whereas some dogs with lumbosacral compressive disease may have normal survey radiographs. Several special radiographic contrast studies have been used to evaluate canine lumbosacral spinal cord disease. These include vertebral sinus venography, flexion/extension myelography, and epidurography.3· 6• 8• 9• 11 • 14• 15 Vertebral sinus venography generally is thought to be technically more difficult to perform than myelography or epidurography, and results are often unreliable owing to technical inconsistencies in filling the vertebral venous sinuses. 6• 8 Myelography combined with flexion and extension has been reported to be successful in the diagnosis of compressive lumbosacral disease. 9 However, other J;eports 8• 15 suggest that myelography is of limited value at the lumbosacral junction, because in many dogs the dural sac ends cranial to the lumbosacral disc space (Fig. 15). Alternatively, epidurography can be a relatively simple radiographic procedure to perform and will anatomically delineate the lumbosacral vertebral canal. In addition, abnormalities identified on epidurograms have a high correlation with lesions identified at surgeryY For these reasons, epidurography may be the modality of choice for imaging the canine lumbosacral spinal canal.
Technique General anesthesia is required of patients scheduled for epidurography. Initial survey radiographs of the lumbosacral junction should be made and evaluated for diseases that may obviate epidurography. Lateral and ventrodorsal views should be made. Following survey film evaluation, if epidurography is indicated, the lumbosacral region should be aseptically prepared. The ileal crests caudally to the level of the fifth or sixth caudal vertebrae should be included in the preparation. Using sterile technique, an epidural puncture is made between the first and second caudal vertebrae or at the sacral-caudal vertebral junction. Spinal needle size is dependent on the size of the patient.
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Figure 15. A, Lateral myelographic view of the caudal lumbar spine and lumbosacral junction. The myelogram shows inadequate filling over the lumbosacral region. (Compare to Figure 1B.) 8, Lateral epidurographic view of the same dog shown in Figure 1A. The epidurogram illustrates dorsal and ventral compression of the cauda equina over the lumbosacral junction.
Usually, 1.5-inch long 20-to 22-ga spinal needles are adequate. Injections made at the lumbosacral junction or in more caudal vertebral spaces can result in unsatisfactory epidurograms. The bevel of the needle should be directed cranially and advanced to the ventral aspect of the vertebral canal. Fluoroscopic visualization during the procedure or a radiograph made prior to injection are helpful in assessing proper needle placement. Once the needle is placed, the stylet is removed. Unlike myelography, cerebrospinal fluid will not fill the needle if it is properly placed in the epidural space. Flexible extension tubing filled with contrast medium will facilitate injection. The dose of contrast medium varies with patient size and number of radiographs desired. In general, 0.1 to 0.2 mL/kg of contrast medium are injected into the epidural space. The contrast agents of choice are those recommended for myelography, especially the nonionic contrast media. If the needle is placed within the epidural space, contrast medium should fill the ventral and, to some extent, the dorsal epidural spaces.
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Figure 16. Dorsoventral epidurographic view showing compression of the cauda equina from the right side (arrow).
Lateral radiographs with the pelvis positioned in neutral, flexed, and hyperextended positions should be made, preferably near the end of each injection. These views usually supply the greatest diagnostic information. Dorsoventral views may be useful and are made if the patient can be repositioned into sternal recumbency. Dorsoventral radiographs are helpful when compressive lesions are located lateral to the midline (Fig. 16). Interpretation
Epidurographic findings correlate well with clinical compressive lumbosacral disease. Abnormalities identified on epidurograms fall into three major categories: (1) epidurograms with complete obstruction of cranial flow of contrast media over the lumbosacral junction (Fig. 17); (2) epidurograms with dorsal deviation of the ventral epidural space (Figs. 15B and 18); and (3) epidurograms with epidural space deviation/ attenuation recognized on dorsoventral views (Fig. 16). 14• 15 Dorsally compressive lumbosacral lesions can be recognized on epidurograms (Fig. 15B) but are identified less frequently owing to inadequate or inconsistent filling of the d orsal epidural space versus the ventral epidural space.
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Figure 17. Lateral epidurographic view that shows obstruction of cranial flow over the ventral epidural space at the lumbosacral junction.
Figure 18. Lateral epidurographic view that shows greater than 50% dorsal deviation of the ventral epidural space at the lumbosacral junction.
Figure 19. Lateral epidurographic view that shows less than 50% dorsal deviation of the ventral epidural space over the lumbosacral junction.
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Figure 20. Normal lateral epidurographic view at the lumbosacral junction.
In dogs in which the epidurogram shows complete obstruction of cranial flow or greater than 50% dorsal elevation of the ventral epidural space (Figs. 15B, 17, and 18), there is a high correlation with surgically confirmed compressive lumbosacral vertebral canal disease. 14 When the ventral epidural space is only slightly or equivocally elevated, i.e., less than 50% of the width of the vertebral canal (Fig. 19), confirmation of disease is less certain; however, a tentative diagnosis of compressive lumbosacral disease can still be made if epidurographic findings are consistent with clinical and neurologic data. Flexion and extension views of the epidurogram may accentuate some lesions or may identify lesions with dynamic characteristics. Other findings that can be found on epidurographic studies include soft-tissue contrast medium extravasation and filling of the vertebral venous sinuses. These findings are inconsistent, occurring in normal and abnormal dogs, and may be related to technique, i.e., concurrent filling of the epidural space and vertebral venous sinus. In contrast to the abnormalities identified on an epidurographic study, normal dogs generally demonstrate filling of the epidural space over the lumbosacral junction without significant deviation of the epidural spaces or obstruction to cranial flow of contrast medium (Fig. 20). SUMMARY
To summarize the points discussed in this article, Small animal myelography can be performed competently and safely without specialized equipment. The newer myelographic contrast media (iohexol, iopamidol) have less neurotoxic potential than those media previously used.
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Myelography is necessary for the diagnosis of many neurologic disorders in small animals. Metrizamide is no longer recommended for myelography in dogs and cats. Iohexol (Isovue) in concentration of 180 mg 1/mL to 300 mg 1/mL is recommended for small animal myelography. Some patients with serious spinal cord disease can exhibit normal myelograms. Recognizing abnormal myelographic patterns is the hallmark of myelographic interpretation. Anticonvulsive drugs (diazepam, pentobarbital, and phenobarbital) should be available for therapy of postmyelographic seizures. Epidurography is an effective diagnostic technique in the evaluation of canine lumbosacral compressive disease. Abnormalities identified with epidurography correlate well with surgically confirmed lesions.
References 1. Adams WM, Stowater JL: Complications of metrizamide myelography in the dog: A
summary of 107 clinical case histories. Vet Radio! 22:27-34, 1981 2. Bartels JE, Bramid KG: Experimental arachnoid fibrosis produced by metrizamide in the dog. Vet Radio! 21:78-81, 1980 3. Blevins W: Transosseous vertebral venography: A diagnostic aid in lumbosacral disease. Vet Radio! 21:50-54, 1980 4. Chambers J, Selcer B, Oliver J: Results of treatment of degenerative lumbosacral stenosis in dogs by exploration and excision. Vet Comp Orthop Trauma 3:130-133, 1988 5. Cox FH, Jakovljevic S: The use of iopamidol for myelography in dogs: A study of twenty-seven cases. J Small Anim Pract 27:159-165, 1986 6. Feeney D, Wise M: Epidurography in the normal dog: Technic and radiographic findings. Vet Radio! 22:35-39, 1981 7. Gray PR, Idrieri RJ, Lippert AC: Influence of anesthetic regimen on the frequency of seizures after cervical myelography in the dog. J Am Vet Med Assoc, 190:527-529, 1987 8. Hathcock J, Pechman R, Dillon A, et a!: Comparison of three radiographic contrast procedures in the evaluation of the canine lumbosacral spinal canal. Vet Radiol29:415, 1988 9. Lang, J: Flexion-extension myelography of the canine cauda equina. Vet Radio! 29:242-257, 1988 10. LenehanT: Canine cauda equina syndrome. Compend Contin Educ Pract Vet 5:941951, 1983 11. McNeel S, Morgan J: Intraosseous vertebral venography: A technic for examination of the canine lumbosacral junction. JAm Vet Radio! Soc 19:168-175, 1978 12. Morgan JP, Atiola M, Bailey CS: Vertebral canal and spinal cord mensuration: A comparative study of its effects on lumbosacral myelography in the dachshund and german shepherd dog. JAm Vet Med Assoc 191:951-957, 1988 13. Puglisi TA, Green RW, et a!: Comparison of metrizamide and iohexol for cisternal myelographic examination of dogs. Am J Vet Res 47:1863-1869, 1986 14. Selcer B, Chambers J, Schwensen K, et a!: Epidurography as a diagnostic aid in canine lumbosacral compressive disease: 47 cases (1981-1986). Vet Comp Orthop Trauma 2:97-103, 1988 15. Selcer B: Radiographic imaging in canine lumbosacral disease. Vet Med Rep 1:282290, 1989
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16. Shores A, Burns J: Technique and indications for metrizamide myelography in small animals. Comp Cont Ed Pract Vet 9:361-364, 1987 17. Spencer C, Crisman CL, eta!: Neurotoxicologic effects of the nonionic contrast agent iopamidol on the leptomeninges of the dog. Am J Vet Res, 43:11, 1982 18. Tilmant L, Ackerman N, Spencer C: Mechanical aspects of subarachnoid space puncture in the dog. Vet Radio! 50:227-232, 1984 19. Wheeler SJ, Davis JV: Iohexol myelography in the dog and cat: A series of one hundred cases, and a comparison with metrizamide and iopamidol. J Small Anim Pract 26:247-256, 1985 20. Wheeler SJ, Jones C, Wright JA: Myelography in the cat. J Small Anim Pract 26:143152, 1985 2L Widmer WR, Blevins WE: Veterinary myelography: A review of contrast media, adverse effects, and technique. JAm Anim Hosp Assoc 27:163-177, 1991 22. Widmer WR, Blevins WE, et a!: Effects of post-myelographic removal of metrizamide in dogs. Vet Radio! 31(1):2-10, 1990. 23. Wood KW: Iohexol and i'opamidol: New nonionic contrast media for myelography in dogs. Comp Cont Ed Pract Vet 10:32-36, 1988
Address reprint requests to Royce E. Roberts, DVM, MS Department of Anatomy and Radiology University of Georgia College of Veterinary Medicine Athens, GA 30602
Key Words
Myelography radiography of the spinal cord after injection of a contrast medium. subarachnoid space the space between the arachnoid and pia mater (spinal cord). nonionic an atom or group of atoms not having a positive or negative electrical charge. cauda equina the collection of spinal roots that continue from the spinal cord and occupy the vertebral canal caudad to the spinal cord. spinal intumescences normal swelling of the spinal cord where the nerves of the brachial plexus and lumbosacral plexus originate. epidurography radiographic visualization of the epidural space following the regional installation of a radiopaque contrast medium.
DIAGNOSTIC IMAGING
MYELOGRAPHY AND EPIDUROGRAPHY Royce E. Roberts, DVM, MS, and Barbara A. Selcer, DVM
Diagnostic evaluation of the spinal canal often requires the introduction of contrast media into the subarachnoid or epidural space. These procedures are used in the evaluation of the spinal cord and cauda equina. Satisfactory results can be obtained in a private practice setting. This article provides indications and instructions for performing these diagnostic procedures and guidelines for interpreting the results. MYELOGRAPHY
Myelography is a radiographic technique of the spinal cord made after the injection of contrast medium into the spinal subarachnoid space. Myelograms are being performed on dogs and cats with increasing frequency by both academic and private veterinary practitioners. This is a result of the increased sophistication of small animal veterinary practice and the development of improved myelographic contrast media. Myelography is essential to the diagnosis of many spinal cord disorders in dogs and cats. Even though the currently used nonionic myelographic contrast media are much safer than earlier generations of contrast media, myelography is not an innocuous procedure and should be performed only by those who have acquired adequate training and knowledge. Indications
The clinical utility of myelography in small animals is well documented. Myelography should be considered for those patients exhibitFrom the Department of Anatomy and Radiology, University of Georgia College of Veterinary Medicine, Athens, Georgia
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ing spinal pain or neurologic deficits after noncontrast radiographs or other indicated laboratory tests have failed to identify the offending. problem. A careful neurologic examination must be performed to localize the affected cord s-egment(s). A myelogram is indicated when (1) there is absence of a spinal lesion on noncontrast radiographs; (2) the lesion seen on noncontrast radiographs is not compatible with clinical signs and/or neurologic examination; (3) noncontrast radiographs indicate multiple lesions consistent with clinical signs and/or neurologic examination; (4) it is necessary to confirm a lesion suspected on noncontrast radiographs; (5) it is desirable to determine the precise location of a spinal lesion to help choose an exact surgical approach; e.g., lateralized disc herniation; (6) it is desirable to determine the extent of spinal cord swelling or compression to aid surgical ·decisions; and (7) the diagnosis of a neurologic disorder is established by exclusion of a spinal cord compressive lesion; e.g., degenerative myelopathy. Contraindications
Myelography should be avoided in patients with a cerebrospinal fluid (CSF) analysis that indicates inflammation/infection. Myelography may unnecessarily potentiate the clinical signs or spread the infection within the subarachnoid space. Contrast Agents
Even though no myelographic contrast medium has been approved for animal use in the United States, veterinarians have been performing myelography for several decades. Characteristics of the ideal myelographic contrast agent should include nontoxic, miscible with cerebrospinal fluid, autoclavable, water-soluble, radiopaque in isotonic concentration, absorbable, and affordable. 21 Numerous contrast agents have been used for animal myelography. Iophendylate (Pantopaque, LaFayette Pharmaceutical, LaFayette, IN) and methiodal sodium (Skiodan, Winthrop-Breon, New York, NY) are older contrast agents that are inappropriate for use in contemporary small animal myelography. Both agents have been associated with an unacceptable incidence of arachnoiditis, and methiodal sodium myelography results in a high incidence of seizures, aggravation of neurologic signs, and poor myelographic contrast. Myelographic contrast agents currently recommended for small animal use are the newer nonionic media, which include iopamidol (Isovue, E.R. Squibb, Princeton, NJ) and iohexol (Omnipaque, Winthrop-Breon, New York, NY). Metrizamide (Amipaque, Winthrop-Breon, New York, NY) was the first nonionic contrast agent developed and resulted in significant reduction in neurotoxicity and postmyelographic
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complications compa-red to earlier contrast agents; however, a significant number of dogs experienced postmyelographic complications. 1· 2 Because the newer generation of nonionic myelographic agents are associated with a further reduction in neurotoxicity and significant improvement in myelographic quality, metrizamide is no longer recommended for small animal myelography. lohexol and iopamidol were approved for human use in 1986. Both contrast agents reportedly cause fewer postmyelographic complications than metrizamide in dogs and cats. 5• 13• 19· 20 However, one report17 that compared neurotoxic effects of iopamidol and metrizamide in dogs indicated no significant difference in adverse effects between these contrast agents. One repore3 suggests fewer complications with iohexol than with iopamidol. Mild transient changes in CSF have been reported with iopamidol and iohexol. 19 lopamidol and iohexol are rapidly removed from the subarachnoid space by diffusion and exhibit less chemotoxicity than metrizamide. 21 lotrolan (lotral, Schering, Berlin, Germany) is a third-generation nonionic myelographic agent and may be less neurotoxic than iohexol and iopamidol. lotrolan is more viscous than other nonionic agents, which results in difficulty when injecting it through the smaller gauge needles employed in small animal myelography.21 lotrolan is not recommended at this time for small animal myelography. lohexol and iopamidol are the contrast agents of choice for myelography in dogs and cats. 19~ 21 lopamidol is available in several concentrations ranging from 200 mg 1/mL to 370 mg 1/mL. The 200 mg II mL concentration is only slightly hyperosmolal and is .suitable for myelography in dogs and cats. lohexol also is available in various concentrations of 180 mg 1/mL to 350 mg 1/mL. The 180 mg 1/mL is near osmolal and is recommended by some for routine myelographic procedures in small animals. The higher concentrations of iohexol and iopamidol are hyperosmolal but can be safely used for small animal myelography. An excellent and detailed review of myelographic contrast agents in veterinary medicine has been conducted21 and is recommended reading for those wishing more detailed information regarding this topic. Technique
Patients selected for myelography are anesthetized using normal preanesthetic and anesthetic protocols. However, phenothiazine derivative tranquilizers (acetylpromazine) must be avoided. Diazepam has been recommended as a preanesthetic anticonvulsive agent; however, its short biologic half-life (30 to 45 minutes) limits its effectiveness as a premyelographic antiseizure drug. Diazepam is an effective postmyelographic anticonvulsive drug. 16 It has been reported that dogs receiving metrizamide for cervical myelography had a significant reduction in seizures when premedicated with pentobarbital (5 mg/kg intramuscu-
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larly) and maintained on methoxyflurane (Metofane, Pittman-Moore, Washingtons Crossing, NJ). This apparent anticonvulsive effect of preanesthetic pentobarbital was not seen when anesthesia was main~ tained with halothane (Fluothane, Ayerst, New York, NY)/ Phenothiazine derivative tranquilizers (e.g., acetylpromazine maleate) are contraindicated in myelographic patients. Their use increases the risk of seizures presumably by lowering the seizural threshold. Maintaining hydration is essential in minimizing the neurotoxic effects of myelographic contrast agents. An intravenous catheter is placed, and normal hydration is maintained with isotonic fluids. Premyelographic dehydration has been shown to slow resorption of contrast media, resulting in unnecessary neurotoxicity. After anesthesia is achieved, the patient is placed in lateral recumbency (some myelographers prefer sternal recumbency) on the radiographic table. The spinal tap site is clipped and aseptically prepared. The prepared site should be liberal enough to allow aseptic palpation of anatomic landmarks (i.e., the wings of the atlas and external occipital protuberance for cisternal injection or the wings of the ilia and the spinous processes of the L6 vertebrae for lumbar injection). The site selected for injection of contrast medium is dependent on myelographer preference, suspected location of lesion, and availability of fluoroscopy/ image intensification. Because fluoroscopy/image intensification is unavailable to most private veterinary practitioners, myelographic protocols may be different than those used in specialty or institutional practices where this technology is available. In the author's opinion, when fluoroscopic visualization is available, all myelographic procedures (cervical, thoracolumbar, or cervicothoracolumbar myelograms) can be performed satisfactorily from a lumbar subarachnoid tap. When fluoroscopy is unavailable, the following tap sites are recommended: Smaller dogs(< 50 lbs) and cats-lumbar tap at L5-L6 for cervical, thoracolumbar, and cervicothoracolumbar myelograms. Larger dogs (> 50 lbs)-lumbar tap at L5-L6 (L4-L5 in German Shepherds and possibly other large breed dogs) for thoracolumbar myelograms, and cerebellomedullary cistern (cisternal) tap for cervical and cervicothoracolumbar myelograms. Commercially available spinal needles with stylet are used in all cases and should not be reused. For cisternal puncture, a 1.5 inch, 22-ga needle is recommended for cats and most dogs. In some larger dogs, a 2.5 inch, 22-ga needle may be required. For lumbar subarachnoid puncture, a 1.5 inch, 22-ga needle is sufficient for most small dogs(< 20 lbs) and cats. Some obese small dogs require a 2.5 inch needle. In large dogs, a 2.5 inch, 22-ga needle is recommended. A 3.5 inch, 22- or 20-ga spinal needle may be required in obese large dogs or giant breed dogs. The precise procedure used for lumbar puncture (i.e., angle of entry to the lumbar subarachnoid space) influences the length of spinal needle required. All spinal taps are performed under aseptic conditions. Placing the spinal needle into the cerebellomedullary cistern is performed with the patient in lateral or sternal recumbency and the
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neck fully flexed ventrally. The needle is inserted on the midline with the bevel directed caudal, near the center of a triangle formed by the external occipital protuberance and the wings of the atlas. The dorsal midline can be identified by palpation of the junction of the paired dorsal cervical muscles. The needle is slowly advanced ventrally until the ligament flavum and dorsal dura are penetrated, which is indicated by a sudden reduced resistance to the needle and a characteristic "pop." The stylet is removed to check for flow of CSF. If additional manipulation of the needle is necessary to facilitate CSF flow, the stylet should be replaced and lateral or craniocaudal movement of the needle should not be allowed. Lumbar subarachnoid taps can be performed with the patient in lateral or ventral recumbency; the former is preferred by the author. The patient should be placed in an exact as possible lateral position. After the puncture site is aseptically prepared, the dorsocaudal edge of the spinous process of L6 is identified, which is normally the first spinous process palpated cranial to the wings of the ilium. The spinal needle is introduced through the skin slightly off the midline at this site. The needle is advanced cranioventrally and toward the mid line at a 30° to 60° angle just lateral to the spinous process (Fig. 1). It may be necessary to reposition the needle several times before it enters the interarcuate space. Occasionally, it is necessary to flex the spine to facilitate needle entry into the vertebral canal. This is most often necessary in older, large dogs with dorsal midline vertebral osteophytes. As the needle enters the spinal canal and penetrates the dorsal dura mater, a muscle twitch of the tail or rear limbs may occur. Repeated lumbar subarachnoid space puncture should be minimized, because cord compression can result from needle puncture alone, 18 and multiple dural punctures will increase the chances of epidural leakage. The needle is then slowly advanced to the ventral floor of the vertebral canal. The needle bevel should be directed cranially and the needle stylet is removed to check for CSF. In Dachshunds and other small dogs and in cats, at this location, the needle tip is often in the ventral subarachnoid space, and CSF will flow or can be identified in the needle hub. In larger dogs, it is often necessary to retract the needle slightly before CSF can be identified. Compressing the jugular veins will facilitate CSF flow. The contrast medium should be injected through a flexible extension tube that connects the needle and syringe. Extension tubes vary in length (4-21 inches) and volume (0.5-3 mL). Small-volume extension tubes should be used for small dog and cat myelograms. By using an extension tube, the possibility of needle movement during the injection is minimized. Excessive needle movement can result in direct injury to the spinal cord or inappropriate injection of contrast medium into the epidural space (Fig. 2) or the spinal cord (intramedullary). The recommended dose of contrast medium (e.g., iohexol, 300 mg 1/mL) can be calculated from the following: cervical myelogram (lumbar tap)-0.45 mL!kg cervical myelogram (cervical tap)-0.30 mL!kg
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Figure 1. Location of needle for lumbar injection myelography. A, Lateral view showing approximate entry angle and location of spinal needle. Note position of needle relative to spinous process of L6. B, Dorsocaudal view of needle position and location of interarcuate space.
Figure 2. Lateral radiograph of lumbar spine showing contrast medium outside the subarachnoid space (epidural). Accumulations of contrast medium are seen over the plane of the intervertebral foramina and ir. the dorsal epidural spaces (arrows). Epidural contrast medium can significantly reduce myelographic quality.
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thoracolumbar myelogram (lumbar tap)-0.30 mL!kg thoracolumbar myelogram (cervical tap)-0.30 mL!kg cervicothoracolumbar·,m yelogram (cervical or lumbar)-0.45 mL!kg The calculated dose of contrast medium is drawn into a syringe, which is then connected to the extension tube. The extension tube is filled completely with contrast medium; care should be taken to avoid introducing air bubbles into the tube. If the needle hub is free of fluid, it should be filled with contrast medium from the tube prior to connecting the needle and extension tube. This will prevent air bubbles from entering the spinal subarachnoid space (Fig. 3). After the extension tube is carefully connected to the spinal needle, injection of the contrast medium can proceed. Injection should be slow (2-3 mL!min), and little resistance should be felt. If fluoroscopy is available, subarachnoid filling can be confirmed by direct visualization. Without fluoroscopy, a test injection of 0.5 to 1 mL of contrast material should be performed, and a lateral radiograph of the injection site is made. The visualization of longitudinal-oblique linear striations of the cauda equina, filling defects of nerve rootlets, and/or thin discrete subarachnoid spaces indicate good subarachnoid space filling (Fig. 4). If satisfactory subarachnoid filling is evident, the remaining volume of contrast medium can be injected. In the larger volume (2 mL) extension tubes, that portion of the contrast medium remaining in the tube can be injected after the syringe is emptied. After completion of the injection, lateral radiographs are made of the spinal segments of interest. The needle may remain in place while the lateral radiographs are made, but great care must be taken not to move the patient or needle. The lateral radiographs should indicate satisfactory filling of the subarachnoid space. By positioning the patient appropriately, the contrast material can be gravity directed by elevating or lowering the head, depending on the desired flow. Occasionally, there is nonuniform filling or poor mixing of contrast medium with CSF. After removal of the spinal needle, it may be helpful to place the
Figure 3. Lateral radiograph of the lumbar spine showing multiple air bubbles within the
subarachnoid space. Air bubbles can mimic intradural lesions, but they typically are in different locations on subsequent radiographs.
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Figure 4. Lateral radiographs of the caudal lumbar spinal cord and cauda equina following injection of contrast medium. A, Normal subarachnoid filling is indicated by the oblique striations of the cauda equina (solid arrows) and the fine serpentine filling defects created by the leptomeningeal vessels (open arrows). 8, The normal caudal tapering of the spinal cord and cauda equina can be seen. Note the normal lack of discrete columns of contrast material that is seen with the spinal cord segments.
animal in dorsal or in the opposite lateral recumbency to evenly distribute or mix the CSF and contrast medium. After 2 to 3 minutes in this position, repeat the lateral radiographs as necessary. Ventrodorsal radiographs of the spine can be made after satisfactory lateral views are obtained. In patients suspected of lateralized spinal lesions (e.g., herniated disc, nerve root tumor), oblique myelographic views may be useful. When lesions are not located precisely ventral or lateral to the spinal cord, conventional lateral and ventrodorsal views may not completely demonstrate, characterize, or localize the lesion. Interpretation
The normal myelogram is characterized by discrete, thin columns of contrast medium that are nearly parallel except at the cauda equina where the subarachnoid space tapers to form the dural end sac (Figs. 5, 6, and 7). In most cases, the subarachnoid space approximates the margins of the vertebral canal. The spinal cord may not appear uniform in diameter owing to the normal widening of the spinal cord in the caudal cervical-cranial thoracic spinal cord segment and caudal lumbar spinal cord segment, as a result of the brachial and lumbosacral spinal intumescences, respectively (Figs. SB, 6B). In some small animals, the ventral thoracolumbar subarachnoid space may be thinner than its
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Figure 5. Normal canine cervical myelogram. A, Lateral view depicting thin, discrete subarachnoid spaces from C3 to the cervicothoracic junction. The increased width of the atlantoaxial subarachnoid space is normal. 8, Ventrodorsal radiograph shows normal widening of the caudal spinal cord indicating the area of the brachial plexus intumescence (arrows).
corresponding dorsal subarachnoid space. In dogs, the dorsal subarachnoid space at the atlantoaxial level is consistently wide (Fig. SA). Myelographic variations due to breed have been recognized. The location of the lumbar intumescence is located more cranially in German Shepherds than in DachshundsY Focal thinning of the ventral subarachnoid space over the intervertebral disc spaces can be normal in some small breed dogs, and larger breed dogs often demonstrate smaller spinal cords relative to the size of the vertebral canal. The location and appearance of the cauda equina and dural end sac are influenced by breed and position of the spine. 9• 12 Typically, the dural end sac of larger breed dogs terminates more cranial than in small or toy breed dogs. Also, the position of the cauda equina/dural end sac within the vertebral canal is inconsistent. It has been suggested that this inconsistency may be corrected by flexion-extension of the spine. 9 Feline myelograms are similar in appearance to canine myelograms; however, the cervical spinal cord may appear wider in comparison (Fig. 7). Focal thinning of the ventral subarachnoid space at the intervertebral disc spaces and the location of the dural end sac have been reportedly similar to that observed in chondrodystrophoid breed dogs. 20 The abnormal myelogram demonstrates alterations of the subarachnoid space and spinal cord. The myelogram can determine lesion
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Figure 6. Normal canine thoracolumbar myelogram. A, Lateral view showing subarachnoid space filling . Note the location and position of the cauda equina and dural end-sac. B, Ventrodorsal radiograph showing the normal widening of the lumbar spinal cord (L3-4), which indicates the location of the lumbosacral spinal intumescence (arrows).
location relative to the dura mater (extradural, intradural) and to the spinal cord (intramedullary). Four myelographic patterns can be identified: normal, extradural, intradural-extramedullary, and intramedullary (Fig. 8) . By characterizing spinal lesions in this fashion, diagnostic
Figure 7. Normal feline cervical myelogram. The spinal cord appears relatively wider than in canine cervical myelograms.
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VD VIEW
A
B
c
D Figure 8. Illustrations of myelographic patterns. A, Normal myelographic pattern, B, Extradural myelographic pattern. Example, extruded intervertebral disk. C, Intradural myelographic pattern. Example, intradural mass lesion. 0 , Intramedullary myelographic pattern.
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Table 1. MYELOGRAPHIC PATTERNS: COMMON DIAGNOSTIC POSSIBILITIES Extradural
Intradural/Extramedullary
Herniated intervertebral disc Vertebral neoplasia (primary or secondary)
Nerve tumors (e.g., neurofibromas)
Epidural soft tissue neoplasia (e.g. , feline lymphosarcoma) Hemorrhage/hematoma Vertebral fracture/luxation Spinal anomaly Hypertrophied spinal ligaments
Nerve sheath tumor (e.g., meningiomas) Hemorrhage/hematoma
Intramedullary
Spinal cord tumor (e.g., astrocytoma) Granulomatous meningoencephalitis (GME) (reticulosis) Hemorrhage
Spinal cord edema Ischemic myelopathy
possibilities, therapeutic choices, and prognoses can be established. Table 1 lists some common causes of these myelographic patterns. Extradural lesions involve tissues outside the dura and displace the subarachnoid space and spinal cord that is contiguous with the lesion (Figs. 9A and lOB). Depending on the severity of the lesion and degree of spinal cord compression, the affected subarachnoid spaces can appear thin or completely void of contrast medium. The degree and direction of deviation iri the subarachnoid space and spinal cord is
Figure 9. Cervical myelogram demonstrating extradural lesion. A, Lateral view showing dorsal deviation of the spinal cord and calcified intervertebral disk in situ at C4-C5, 8 , Ventrodorsal radiograph showing spinal cord enlargement at C4- C5 resulting from large ventral extradural lesion. Surgical diagnosis of extruded intervertebral disk.
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Figure 10. Thoracolumbar myelogram indicating lateralized intervertebral disk herniation. A, Lateral view shows attenuation of contrast medium at T13-L 1. Deviation and thinning of the ventral subarachnoid space at L1-L2 is indicative of a Type II disk herniation. B, Ventrodorsal view confirms a large extradural lesion located lateral and to the left of the spinal cord at T13-L 1. Surgical exploration confirmed disk extrusion at T13-L1.
best seen on that radiographic view that is tangential to the lesion (Figs. 9A and lOB). The opposite view (90°) will frequently demonstrate widening of the spinal cord segment with lateral displacement and/or thinning of the subarachnoid spaces (Fig. 9B). In some cases of acute extradural compressive lesions (disc herniation), spinal cord swelling is so severe as to obliterate the subarachnoid space. This can occur over several vertebral segments and results in nonvisualization of the extradural lesions (Fig. 11). A common cause of extradural lesions in dogs
Figure 11. Lateral thoracolumbar myelogram showing attenuation of the subarachnoid space from T12 to L4. Surgical exploration revealed disk extrusion at L 1- L2 and spinal cord swelling.
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is intervertebral disc herniation. Other diagnostic possibilities for extradural lesions in dogs and cats are listed in Table 1. Lesions characterized as intradural-extramedullary are located within the subarachnoid space but outside the spinal cord (i.e., between the dura mater and the spinal cord). This myelographic pattern is characterized by a "filling defect" within the subarachnoid space and the "golf tee" sign. The filling defect appears as a radiolucency surrounded by radiopaque contrast medium. The golf tee sign is formed by the diverging column of contrast medium as it attempts to go around the intradural space-occupying lesion (Fig. 12). The spinal cord is typically displaced away from the intradural lesion, and the spinal cord appears widened on the 90° opposite radiographic view, much in the same manner as seen with extradural lesions. Spinal meningiomas are common causes of intradural lesions. Intramedullary lesions are located within the spinal cord parenchyma. This myelographic lesion is characterized by widening of the spinal cord and deviation of the subarachnoid spaces away from the central canal of the spinal cord on both radiographic views (Fig. 13). Thinning of the subarachnoid spaces in all planes is observed typically. The latter change is helpful in differentiating intramedullary lesions from the normal widening of the spinal cord seen at the brachial and lumbosacral intumescences. Thinning of the subarachnoid spaces is not
Figure 12. Thoracolumbar myelogram demonstrating an intradural-extramedullary lesion. A, Lateral view shows a large filling defect within the contrast medium. Note the "golf tee" sign formed by the contrast medium as it attempts to circumvent the caudal edge of the mass lesion (arrow). 8, Ventrodorsal view showing widening of the spinal cord and filling defect.
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Figure 13. Lumbar myelogram of intramedullary lesion. A, Lateral radiograph showing focal widening of the spinal cord and thinning of the subarachnoid space (arrows). 8, Ventrodorsal view also exhibits spinal cord swelling and thinning of contrast medium (arrows) . Widening of the spinal cord on both lateral and ventrodorsal views is characteristic of intramedullary lesions.
normally observed with the spinal intumescences. Spinal cord parenchymal tumors and spinal cord edema are common causes of intramedullary lesions. Myelomalacia is an uncommon intramedullary lesion that is characterized by staining of the spinal cord parenchyma with contrast medium (Fig. 14). Many causes of spinal cord dysfunction can be identified by myelography; however, myelographic findings must be judged in
Figure 14. Lateral lumbar myelogram exhibiting contrast medium within the spinal cord parenchyma (arrows). This sign suggests spinal cord liquification or myelomalacia.
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Table 2. COMMON NEUROLOGIC DISORDERS THAT MAY SHOW A NORMAL MYELOGRAM Degenerative myelopathy Ischemic myelopathy Meningitis
Myelitis Spinal cord trauma GME (reticulosis)
context with all other clinical data before arriving at a diagnosis. It is important to remember that some spinal cord disease can result in profound neurologic signs without myelographic abnormalities (Table 2). In fact, the presumptive diagnosis of some common neurologic diseases in dogs and cats depends on a normal myelogram, establishing the diagnosis by exclusion (e.g., degenerative myelopathy). The role of myelography is to localize and characterize spinal lesions. The neurologic examination determines the clinical prognosis. Postmyelographic Patient Care
Routine postmyelographic care is directed at minimizing the neurotoxic effect of myelographic contrast agents. Because of the inherent nature and accepted risk associated with myelography, some degree of neurotoxicity may be unavoidable. Careful and prudent care of the patient before and during myelography can significantly reduce the potential for postmyelographic complications. Routine . postmyelographic procedures should include elevating the patient's head, maintaining normal hydration during recovery, and continuously monitoring for evidence of muscle fasciculations and partial or generalized motor seizures. Those patients receiving larger doses of contrast medium or cerebellomedullary cistern injection are at increased risk for seizures. 1 Muscle fasciculations and seizure activity are treated immediately with anticonvulsive drugs. Diazepam is widely used at a dose of 0.2 to 0.4 mg/kg intravenously, and it can be given every 10 to 30 minutes, not to exceed a total dose of 20 mg in small dogs and cats or 40 mg in large dogs. 16 If seizures are refractory to diazepam, intravenous phenobarbital at a dose of 2 to 4 mg/kg can be administered. The postmyelographic removal of contrast material to aid in preventing postmyelographic complications has been advocated. 22 Since the introduction of the newer, nonionic myelographic contrast agents (iopamidol and iohexol), the incidence of myelographic complications has been reduced significantly. However, these agents are not without neurotoxicity even when used properly. All myelographers must become proficient in performing spinal taps prior to performing myelograms on client animals. EPIDUROGRAPHY
Diseases that affect the canine lumbosacral vertebral canal are varied and produce a wide range of clinical signs, including rear limb
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lameness, pain, and fecal or urinary incontinence. 4• 10 Typically, clinical signs are the result of nerve root compression secondary to disc herniation, fibrous connective tissue proliferation, or spinal canal stenosis of congenital or developmental origin. 4 In some instances, lumbosacral subluxation will result in lumbosacral spinal canal compressive disease with dynamic or static compressive lesions as possible sequelae. Less frequently, infectious diseases such as discospondylitis or neoplastic disease may cause lumbosacral canal compression. Diagnostic imaging plays a major role in the diagnosis of lumbosacral disease. Survey radiographs of the lumbosacral junction are usually inconclusive. Survey radiographic signs that have been recognized in dogs with lumbosacral disease include spondylosis deformans, L7 Sl end plate sclerosis, and narrowing of the lumbosacral disc space. 14 However, these findings also can be seen in asymptomatic dogs whereas some dogs with lumbosacral compressive disease may have normal survey radiographs. Several special radiographic contrast studies have been used to evaluate canine lumbosacral spinal cord disease. These include vertebral sinus venography, flexion/extension myelography, and epidurography.3· 6• 8• 9• 11 • 14• 15 Vertebral sinus venography generally is thought to be technically more difficult to perform than myelography or epidurography, and results are often unreliable owing to technical inconsistencies in filling the vertebral venous sinuses. 6• 8 Myelography combined with flexion and extension has been reported to be successful in the diagnosis of compressive lumbosacral disease. 9 However, other J;eports 8• 15 suggest that myelography is of limited value at the lumbosacral junction, because in many dogs the dural sac ends cranial to the lumbosacral disc space (Fig. 15). Alternatively, epidurography can be a relatively simple radiographic procedure to perform and will anatomically delineate the lumbosacral vertebral canal. In addition, abnormalities identified on epidurograms have a high correlation with lesions identified at surgeryY For these reasons, epidurography may be the modality of choice for imaging the canine lumbosacral spinal canal.
Technique General anesthesia is required of patients scheduled for epidurography. Initial survey radiographs of the lumbosacral junction should be made and evaluated for diseases that may obviate epidurography. Lateral and ventrodorsal views should be made. Following survey film evaluation, if epidurography is indicated, the lumbosacral region should be aseptically prepared. The ileal crests caudally to the level of the fifth or sixth caudal vertebrae should be included in the preparation. Using sterile technique, an epidural puncture is made between the first and second caudal vertebrae or at the sacral-caudal vertebral junction. Spinal needle size is dependent on the size of the patient.
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Figure 15. A, Lateral myelographic view of the caudal lumbar spine and lumbosacral junction. The myelogram shows inadequate filling over the lumbosacral region. (Compare to Figure 1B.) 8, Lateral epidurographic view of the same dog shown in Figure 1A. The epidurogram illustrates dorsal and ventral compression of the cauda equina over the lumbosacral junction.
Usually, 1.5-inch long 20-to 22-ga spinal needles are adequate. Injections made at the lumbosacral junction or in more caudal vertebral spaces can result in unsatisfactory epidurograms. The bevel of the needle should be directed cranially and advanced to the ventral aspect of the vertebral canal. Fluoroscopic visualization during the procedure or a radiograph made prior to injection are helpful in assessing proper needle placement. Once the needle is placed, the stylet is removed. Unlike myelography, cerebrospinal fluid will not fill the needle if it is properly placed in the epidural space. Flexible extension tubing filled with contrast medium will facilitate injection. The dose of contrast medium varies with patient size and number of radiographs desired. In general, 0.1 to 0.2 mL/kg of contrast medium are injected into the epidural space. The contrast agents of choice are those recommended for myelography, especially the nonionic contrast media. If the needle is placed within the epidural space, contrast medium should fill the ventral and, to some extent, the dorsal epidural spaces.
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Figure 16. Dorsoventral epidurographic view showing compression of the cauda equina from the right side (arrow).
Lateral radiographs with the pelvis positioned in neutral, flexed, and hyperextended positions should be made, preferably near the end of each injection. These views usually supply the greatest diagnostic information. Dorsoventral views may be useful and are made if the patient can be repositioned into sternal recumbency. Dorsoventral radiographs are helpful when compressive lesions are located lateral to the midline (Fig. 16). Interpretation
Epidurographic findings correlate well with clinical compressive lumbosacral disease. Abnormalities identified on epidurograms fall into three major categories: (1) epidurograms with complete obstruction of cranial flow of contrast media over the lumbosacral junction (Fig. 17); (2) epidurograms with dorsal deviation of the ventral epidural space (Figs. 15B and 18); and (3) epidurograms with epidural space deviation/ attenuation recognized on dorsoventral views (Fig. 16). 14• 15 Dorsally compressive lumbosacral lesions can be recognized on epidurograms (Fig. 15B) but are identified less frequently owing to inadequate or inconsistent filling of the d orsal epidural space versus the ventral epidural space.
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Figure 17. Lateral epidurographic view that shows obstruction of cranial flow over the ventral epidural space at the lumbosacral junction.
Figure 18. Lateral epidurographic view that shows greater than 50% dorsal deviation of the ventral epidural space at the lumbosacral junction.
Figure 19. Lateral epidurographic view that shows less than 50% dorsal deviation of the ventral epidural space over the lumbosacral junction.
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Figure 20. Normal lateral epidurographic view at the lumbosacral junction.
In dogs in which the epidurogram shows complete obstruction of cranial flow or greater than 50% dorsal elevation of the ventral epidural space (Figs. 15B, 17, and 18), there is a high correlation with surgically confirmed compressive lumbosacral vertebral canal disease. 14 When the ventral epidural space is only slightly or equivocally elevated, i.e., less than 50% of the width of the vertebral canal (Fig. 19), confirmation of disease is less certain; however, a tentative diagnosis of compressive lumbosacral disease can still be made if epidurographic findings are consistent with clinical and neurologic data. Flexion and extension views of the epidurogram may accentuate some lesions or may identify lesions with dynamic characteristics. Other findings that can be found on epidurographic studies include soft-tissue contrast medium extravasation and filling of the vertebral venous sinuses. These findings are inconsistent, occurring in normal and abnormal dogs, and may be related to technique, i.e., concurrent filling of the epidural space and vertebral venous sinus. In contrast to the abnormalities identified on an epidurographic study, normal dogs generally demonstrate filling of the epidural space over the lumbosacral junction without significant deviation of the epidural spaces or obstruction to cranial flow of contrast medium (Fig. 20). SUMMARY
To summarize the points discussed in this article, Small animal myelography can be performed competently and safely without specialized equipment. The newer myelographic contrast media (iohexol, iopamidol) have less neurotoxic potential than those media previously used.
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Myelography is necessary for the diagnosis of many neurologic disorders in small animals. Metrizamide is no longer recommended for myelography in dogs and cats. Iohexol (Isovue) in concentration of 180 mg 1/mL to 300 mg 1/mL is recommended for small animal myelography. Some patients with serious spinal cord disease can exhibit normal myelograms. Recognizing abnormal myelographic patterns is the hallmark of myelographic interpretation. Anticonvulsive drugs (diazepam, pentobarbital, and phenobarbital) should be available for therapy of postmyelographic seizures. Epidurography is an effective diagnostic technique in the evaluation of canine lumbosacral compressive disease. Abnormalities identified with epidurography correlate well with surgically confirmed lesions.
References 1. Adams WM, Stowater JL: Complications of metrizamide myelography in the dog: A
summary of 107 clinical case histories. Vet Radio! 22:27-34, 1981 2. Bartels JE, Bramid KG: Experimental arachnoid fibrosis produced by metrizamide in the dog. Vet Radio! 21:78-81, 1980 3. Blevins W: Transosseous vertebral venography: A diagnostic aid in lumbosacral disease. Vet Radio! 21:50-54, 1980 4. Chambers J, Selcer B, Oliver J: Results of treatment of degenerative lumbosacral stenosis in dogs by exploration and excision. Vet Comp Orthop Trauma 3:130-133, 1988 5. Cox FH, Jakovljevic S: The use of iopamidol for myelography in dogs: A study of twenty-seven cases. J Small Anim Pract 27:159-165, 1986 6. Feeney D, Wise M: Epidurography in the normal dog: Technic and radiographic findings. Vet Radio! 22:35-39, 1981 7. Gray PR, Idrieri RJ, Lippert AC: Influence of anesthetic regimen on the frequency of seizures after cervical myelography in the dog. J Am Vet Med Assoc, 190:527-529, 1987 8. Hathcock J, Pechman R, Dillon A, et a!: Comparison of three radiographic contrast procedures in the evaluation of the canine lumbosacral spinal canal. Vet Radiol29:415, 1988 9. Lang, J: Flexion-extension myelography of the canine cauda equina. Vet Radio! 29:242-257, 1988 10. LenehanT: Canine cauda equina syndrome. Compend Contin Educ Pract Vet 5:941951, 1983 11. McNeel S, Morgan J: Intraosseous vertebral venography: A technic for examination of the canine lumbosacral junction. JAm Vet Radio! Soc 19:168-175, 1978 12. Morgan JP, Atiola M, Bailey CS: Vertebral canal and spinal cord mensuration: A comparative study of its effects on lumbosacral myelography in the dachshund and german shepherd dog. JAm Vet Med Assoc 191:951-957, 1988 13. Puglisi TA, Green RW, et a!: Comparison of metrizamide and iohexol for cisternal myelographic examination of dogs. Am J Vet Res 47:1863-1869, 1986 14. Selcer B, Chambers J, Schwensen K, et a!: Epidurography as a diagnostic aid in canine lumbosacral compressive disease: 47 cases (1981-1986). Vet Comp Orthop Trauma 2:97-103, 1988 15. Selcer B: Radiographic imaging in canine lumbosacral disease. Vet Med Rep 1:282290, 1989
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16. Shores A, Burns J: Technique and indications for metrizamide myelography in small animals. Comp Cont Ed Pract Vet 9:361-364, 1987 17. Spencer C, Crisman CL, eta!: Neurotoxicologic effects of the nonionic contrast agent iopamidol on the leptomeninges of the dog. Am J Vet Res, 43:11, 1982 18. Tilmant L, Ackerman N, Spencer C: Mechanical aspects of subarachnoid space puncture in the dog. Vet Radio! 50:227-232, 1984 19. Wheeler SJ, Davis JV: Iohexol myelography in the dog and cat: A series of one hundred cases, and a comparison with metrizamide and iopamidol. J Small Anim Pract 26:247-256, 1985 20. Wheeler SJ, Jones C, Wright JA: Myelography in the cat. J Small Anim Pract 26:143152, 1985 2L Widmer WR, Blevins WE: Veterinary myelography: A review of contrast media, adverse effects, and technique. JAm Anim Hosp Assoc 27:163-177, 1991 22. Widmer WR, Blevins WE, et a!: Effects of post-myelographic removal of metrizamide in dogs. Vet Radio! 31(1):2-10, 1990. 23. Wood KW: Iohexol and i'opamidol: New nonionic contrast media for myelography in dogs. Comp Cont Ed Pract Vet 10:32-36, 1988
Address reprint requests to Royce E. Roberts, DVM, MS Department of Anatomy and Radiology University of Georgia College of Veterinary Medicine Athens, GA 30602
Key Words
Myelography radiography of the spinal cord after injection of a contrast medium. subarachnoid space the space between the arachnoid and pia mater (spinal cord). nonionic an atom or group of atoms not having a positive or negative electrical charge. cauda equina the collection of spinal roots that continue from the spinal cord and occupy the vertebral canal caudad to the spinal cord. spinal intumescences normal swelling of the spinal cord where the nerves of the brachial plexus and lumbosacral plexus originate. epidurography radiographic visualization of the epidural space following the regional installation of a radiopaque contrast medium.