Diagnostic Imaging in Children with Spinal Disorders

Symposium on Pediatric Radiology

Diagnostic Imaging in Children with Spinal Disorders Charles R. Fitz, M.D. *

Spinal disease may be investigated by a variety of radiological methods. These include plain films, tomography, myelography, CT, nuclear medicine, ultrasound, and magnetic resonance imaging (MRI), and, infrequently, arteriography. Many of these are pathways that consume considerable time and money, and should be embarked on in consultation with other specialists, especially orthopedic surgeons, neurosurgeons, and neuroradiologists. Whatever the symptoms, plain films are almost always the starting point. They will usually help decide which direction to pursue. Some generalities can be made about the usefulness of individual exams. Tomography. This has been supplanted by CT in most instances, and often is no longer available in smaller departments. It remains a useful technique for looking at bony disease, especially trauma and complex congenital abnormalities in which the sagittal or coronal tomographic image can be more revealing than the CT. Ultrasound. This has limited usefulness because the posterior arches make a complete bony ring by the end of the second year. When there is a defect in the ring from a congenital spina bifida or a surgical defect, ultrasound may be used to view the intraspinal contents. 15 Intraoperative ultrasound can be helpful to locate tumors and cysts in the cord. 22 The advantages of ultrasound are its portability, the ease of obtaining images at various planes, and the fact that it does not use x-rays. The disadvantages are a relatively poor definition compared with CT or MRI, and the inability to see through bone. Myelography. Although MRI will probably supplant it, myelography remains the primary means of examination of the cord and nerve roots with or without associated CT. The longitudinal view of the spinal canal quickly localizes abnormalities that may be examined in more detail with axial CT slices if needed. The newer water-soluble contrast agents are very safe and do not require removal, being absorbed with the cerebrospinal fluid. The

*Director,

Section of Neuroradiology, Department of Radiology, Children's Hospital National Medical Center, Professor of Pediatrics and of Radiology, George Washington University, School of Medicine, Washington, D.C.

Pediatric Clinics of North America-Vol. 32, No.6, December 1985

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original aqueous agent, metrizamide, is soon to be replaced by even safer agents, such as iohexol and iothalomate. 8, 16 The disadvantages of myelography are the need for sedation or anesthesia, the side effects of nausea and vomiting in up to 30 per cent of children, and a small incidence of postprocedural seizures-about 1 per cent. 20 As with any contrast agent, there is also the very small risk of a serious allergic reaction. The risk is much less than that of intravascular contrast material. Computed Tomography. The resolution of CT is not adequate to reliably see the cord or nerve roots anywhere but in the upper cervical region unless a contrast agent such as metrizamide is in the spinal canal. The ability to see bone is excellent, as is the capacity to visualize the paras pinal soft tissues. This makes CT very useful for a wide range of diseases from congenital anomalies to infection, vertebral tumors, and injuries. As bone and metrizamide have a high density compared with adjacent soft tissues, a lower radiation dose can be used to examine the spine in comparison with most other CT exams. Direct sagittal or coronal slices are easily done in infants, if needed, and sitting direct coronals may be obtained in older children. ' Magnetic Resonance Imaging. Experience with MRI in pediatric spinal disease is very limited. 18, 19 Its disadvantages are cost, with the average exam being in the $600 to $700 range, or two times the cost of CT. There is a long imaging time compared with CT, and the claustrophobia some people feel in the "tunnel" makes the exam impossible for a few. Cortical bone gives off no MR signal, but the marrow and all soft tissues do, making MRI potentially useful in nearly every disease except injuries. The ability to obtain detailed sagittal views of the spinal canal and its contents is probably its most exciting feature in regard to spinal disease. Nuclear Medicine. Nuclear medicine has the ability to examine the dynamics of disease, and is therefore primarily used when looking for inflammatory, metabolic, or metastatic disease and trauma. While sometimes less specific in diagnosis, it is often more sensitive in locating pathology. ANATOMY The spine may be thought of as two parallel connected parts: the vertebral bodies and posterior elements or neural canal (Fig. 1). The body connects to the pedicle at the neurocentral synchondrosis, which fuses throughout the spine by age seven. The pedicles attach to the transverse processes and the facet joints, which continue posteriorly into the laminae, which fuse to form the spinous process in the midline. Within the canal the cord is visible when surrounded by a contrast agent, or on MRI as an increased signal. It varies slightly in shape and diameter in each part of the spinal canal, and stays somewhat on the inside of each spinal curve. It is therefore posterior in the cervical and lumbar regions and slightly anterior in the mid thoracic. The cord is normally in the midline, except when scoliosis or pathology is present. The first and second cervical vertebrae are different from the rest of

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Figure 1. A, CT of the cervical spine without metrizamide in a 6-month-old. The neurocentral synchondrosis (solid arrow) is wide. The pedicles (open arrow) are continuous with the laminae (curved arrow). The spinous process is widely OPen posteriorly between the laminae. B, Lower thoracic CT in a 7-year-old. The laminae and neurocentral synchondroses have fused. The synchondrosis scar remains visible into the teen years. The spinal cord has an irregular outline owing to metastatic disease. C, Lumbar metrizamide CT in a 2-year-old. The neurocentral synchondroses are starting to fuse, and the laminae have already fused to form the spinous process. The nerve roots, below the conus at this level, have a symmetric pattern.

the cervical spine with odontoid embryologically belonging to Cl. The odontoid may be unfused to its base until age 10. It is variable in length, often not reaching the superior aspect of Cl in early childhood (Fig. 2). There also may be an accessory ossicle or os terminale atop the odontoid, which may be confused with a fracture. SPECIFIC DISEASE STATES CONGENITAL ANOMALIES

Although spina bifida and dysraphism simply mean a split, common usage of the terms refers to abnormality of the posterior elements. As the laminae fuse from lumbar to cervical from the first through the third years, physiologic nonfusion is mistaken for and masks true spina bifida (Fig. 3). A small number of normal adults have occult spina bifida of L5, SI, or Cl that is of no significance. This is a simple open line between the normal halves of the spinous processes, whereas the pathologic spina bifida is usually a more irregular or wider separation. A rule for congenital anomalies is that the younger the child, the more obvious the abnormality must be to make a clinical diagnosis. While the posterior arches of the vertebrae are unfused, minor, but significant, dysraphism will be missed by x-rays. Mild clinical symptoms such as

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Figure 2. Normal flexion view of the cervical spine in a one-yearold. C2 appears to be subluxed forward on C3. The tip of the odontoid (0) does not reach the upper border of Cl. The anterior arch of Cl is poorly ossified.

Figure 3. Normal AP chest in a lO-month-old. There are multiple levels of unfused laminae (arrow).

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enuresis and weakness that are apparent in later childhood are also not likely to be picked up during infancy. In infancy, one depends on external abnormalities ranging from the subtle midline dimples and hemangiomas to obvious protruding masses. The most severe abnormality, myelomeningocele, is usually not studied beyond plain film radiography because of the need to quickly free the placode of cord tissue from the overlying skin, and to close the meningocele to prevent cerebrospinal fluid leakage and infection. Myelomeningocele is invariably accompanied by the Chiari II hind-brain malformation and hydrocephalus, which sometimes does not appear until after repair of the myelomeningocele. The plain film diagnosis of myelomeningocele is often easy, because only this entity will cause a kyphosis in the lumbothoracic junction or lumbar region in the newborn (Fig. 4). These patients may later have a recurrent tethering of their placode to the dura, usually signaled by a worsening of lower limb function or sensation. 10 Metrizamide CT is needed to find the areas of attachment (Fig. 5). The central canal of the cord is usually a structure of 1 to 2 mm in diameter that is patent with the floor of the fourth ventricle at birth. This connection normally closes, but in myelomeningocele it often stays open and expands, causing symptoms, commonly upper limb motor and sensory loss. MRI or metrizamide CT easily diagnoses this complication, showing a

Figure 4. Myelomeningocele. A lateral spine film in a newborn with marked local kyphosis in the mid lumbar spine.

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Figure 5. Retethering of a myelomeningocele. CT with me, trizamide reveals the dark image of the cord (C) surrounded by the whiter,density metrizamide except where it is adherent (arrow) to the darker,density fat. Note the lami, nae are widely split and actually divergent.

cord that has expanded or collapsed to a ribbon6 (Fig. 6). MRI can directly show the fluid as a different signal within the cystic cord (Fig. 7). CT may show metrizamide in the cord or depend on the enlarged size or flattened shape. While this hydromyelic expansion of the cord is common in patients with myelomeningocele, it occasionally occurs in lipomyelomeningocele and in patients with the Chiari I malformation, which is a minor "herniation" of the cerebellar tonsils below the foramen magnum, on a developmental basis. Kyphosis does not occur with lipomyelomeningocele or simple meningocele. These two pathologic states may superficially resemble myelomeningocele, with a bulging, skin-covered mass in the back, usually lumbar, but carry a less severe prognosis of neurologic disablity and a less urgent need for treatment. Metrizamide myelography with CT remains the best

Figure 6. Hydromyelia. A, A collapsed cervical cord with a "ribbon" of metrizamide (arrow) in the central canal follOwing myelography. B, An expanded thoracic cord in another patient with no metrizamide entry into the central canal. Both patients had Chiari,II malformations.

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Figure 7. Hydromyelia. Sagittal MRI with spin echo technique at 0.5 Tesla with a cervical hydromyelia (arrows) giving off a lower signal intensity than the surrounding cord. Case courtesy of Dr. C. Citrin and J. Sherman, Magnetic Imaging of Washington.

method of diagnosis, allowing visualization of the varying complex mixtures of neurologic tissue, fat, and CSF (Fig. 8). Ultrasound may also be used in such cases. Though less sharply defined, the CSF, adipose' tissue, and neural elements can usually be discerned (Fig. 9). Simple meningocele with only CSF within and meninges covering the mass is much less common than lipomyelomeningocele by a factor of about 10. More common than these entities is the tethered conus or tethered filum syndrome. The embryologic origin and cause of symptoms of this is unclear.27 Sarawar23 believes the cause to be abnormalities in the growth of

Figure 8. Lipomyelomeningocele. Metrizamide CT in a 6month-old at the L3 level with the cord (black arrow) in a large dural space extending (arrowheads) into the large menigocele sac (M), which is partially fatty (white arrows).

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Figure 9. Lipomyelomeningocele. A sagittal ultrasound on an infant. The cord (solid arrows) is seen along the dorsal aspect of the spinal canal entering an hypoechoic meningocele sac (S). The central canal is visible within the cord as an echogenic interrupted line. A placode (P) of neural tissue enters a dorsal, slightly echoic lipomatous portion (open arrows). This in turn is surrounded by more echogenic fibrous elements.

Figure 10. Tethered conus. Myelogram in a 3-year-old with the conus (arrows) ending at the bottom of L3. The filum (open arrow) is larger than the surrounding nerve roots.

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the peripheral nerve roots that fix the tip of the cord at a lower level than normal. Symptoms are variable. Enuresis is common. Orthopedic abnormalities such as clubfoot or leg asymmetry are likewise common. Neurologic findings may be very subtle. External skin signs such as hemangiomas or hairy patches are often present. 13 In the author's experience, spina bifida in the lumbar or upper sacral region is always present on x-ray, though often masked before fusion occurs. A widened canal is also a common finding. Myelography or MRI is diagnostic, showing a pattern varying from a thick filum of 2 mm or greater diameter with a conus that ends caudal to the normal level (mid-L2 in the first two years of life, Ll-2 interspace by age 6) (Fig. 10) to a pattern like that of lipomyelomeningocele with a large lipoma. The difference is that the lipoma remains within the canal. In the more complicated case, CT is necessary following myelography to properly localize the parts of the abnormality. All of the described entities may be accompanied by diastematomyelia, which occurs in two types. 24 The simpler "split cord" is a midline sagittal separation of the cord, usually beginning above the abnormal conus or other pathology (Fig. 11). It may extend through the pathologic area or rejoin into a single cord above it. Occasionally it continues, into separate filL The less common and more dramatic type has a thin vertical spur that "stabs" the cord, splitting it and the surrounding meninges into separate longitudinal compartments. The spur is usually visible on plain films along with associated spina bifida, canal widening, and disk space narrowing12 (Fig. 12A), but the spur may be hard to see if it travels obliquely. CT myelography is the best diagnostic exam, showing both the cord and bone to advantage (Fig. 12B). Symptoms are usually thought to be caused by traction at the lower end of the spur, where the two halves join again into a single cord and often are tight against the bony spur. Other anomalies are much less common than those mentioned here. Sacral dysgenesis (caudal regression) frequently occurs in children whose parents, especially mothers, are diabetic. They may present early

Figure 11. Diastematomyelia-split cord. Metrizamide CT in the mid-lumbar level in an 11-yearold with a tethered conus. The cord is divided with a thin fibrous or neural band still connecting the two halves. Nerve roots are visible off the lateral borders of the two hemicords.

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Figure 12. Diastematomyelia with a bony spur. A, The plain film reveals a locally widened canal, narrowed disk spaces, and the bony spikes (arrows) at T5 and T7 in an 11month old. B, CT with metrizamide clearly shows the thick bone dividing the two cord halves. The spinous process is also large and anomalous.

with Battened buttocks and sensory or motor abnormalities. Plain films show the hypoplasia or absence of the sacrum easily, except in some infants in whom gas and feces obscure the sacral region. It is necessary to do myelography, or perhaps MRI, to determine if the cord is compressed by the stenotic canal, and if the cord is tethered low, which seems a paradoxic situation with a short dural sac. 4 The anomaly can occasionally be seen in meningomyelocele.

Figure 13. Anterior meningocele. A, A newborn infant with a sacrum that curves to the left (the scimitar) and is missing several of the pedicles on the right. B, Metrizamide CT reveals a meningocele pocket (arrow) bulging out into the pelvis through a partially absent lower sacral vertebral body.

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Radiographically similar to this is the scimitar sacrum, which has a portion of one side, or an entire hemisacrum, missing (Fig. 13) and has an associated meningocele that protrudes anteriorly into the pelvis. While the plain films will indicate the diagnosis, metrizamide myelography or MRI is necessary to define the full extent of the abnormality for proper surgical treatment. This diagnosis should be considered in children with pelvic masses. Some authors advocate cervical puncture in children with the various anomalies that cause a low-lying conus. The author's experience has been that a lumbar puncture is safe, as there is sufficient room to insert a spinal needle into the subarachnoid space anywhere except directly at a placode or bony spur. An off midline puncture will also help avoid hitting the cord, though passing a needle through the cord is rarely of any consequence. A cervical puncture is more likely to require a general anesthetic, and the same risk of cord puncture is present in an even more sensitive region. Dermoids and epidermoid cysts may occur anywhere in the spinal canal. Dermoids are much more common in the lower thoracolumbar region. These cysts often connect to an external tract seen on the midline back as a dimple. Not all tracts lead to a cyst, and not all cysts have ~ external opening. Those that do not infrequently present with bacterial meningitis. If the cyst breaks and secretions are released into the subarachnoid space, a chemical meningitis may occur. Most, but not all, of these cysts have spina bifida at the level of the cyst. The cyst may enter the cord, or remain extramedullary (Fig. 14). Neurenteric cysts (split notochord syndrome) are rare foregut duplication anomalies that usually have a cleft or hole anteriorly in a thoracic vertebral body through which the cyst traverses from the mediastinum to

Figure 14. Dermoid cyst. Metrizamide CT of the lower thoracic spine shows a tract (arrow) entering the subarachnoid space below the point at which it enters the cord, which is already larger than normal at this level.

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the spinal canal. They can occur throughout the spine and extend posteriorly.2 Symptoms may be meningitis, compression, or none, with the bony abnormalities being the only clue. CT myelography or MRI defines the intraspinal extent, which varies from extradural to intermedullary. Although neurofibromatosis is a hereditary disease, about a third or more are said to arise spontaneously. Though often a devastating illness, it fortunately does not frequently present with spinal problems until the teen years or adulthood. Scoliosis, intraspinal neurofibromas, and paraspinal masses are the most commbn abnormalities. Up to 50 per cent of neurofibromatosis patients have scoliosis, but not all of these have other abnormalities. The scoliosis is often fairly localized with a sharp curve over a short space (Fig. 15). A so-called "dural ectasia" causes scalloping of the posterior aspect of the vertebral bodies, widening of the foramina with bulging of the arachnoid through the foramina to the point of causing lateral "meningoceles." Local widening of the canal or foraminal widening should be investigated with CT myelography or MRI to be certain no intracanalicular neurofibromas are present. When present they are often multiple, and best seen by myelography. Because they can occur at multiple levels and grow slowly, only those causing symptoms are usually treated. Para-

Figure 15. Scoliosis in neurofibromatosis. There is a sharp curve of nearly 90° in the mid thoracic spine of this 14-year-old with neurofibromatosis and no intraspinal abnormality.

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spinal masses may arise, and are likely to be ganglioneuromas or even neuroblastomas rather than neurofibromas. Scoliosis is a common spinal disease in childhood, and is usually studied by plain films without the need for other studies. Exceptions are those cases with congenital scoliosis secondary to spinal anomalies usually seen in young boys, cases in which neurologic symptoms occur, and those cases that have a spina bifida and are going to be operatively treated. These three groups have a higher likelihood of having underlying neural anomalies, especially the tethered conus, which can become more symptomatic when traction is put on the cord by straightening the spine. Achondroplasia is the most common type of dwarfism, and these patients frequently develop symptoms secondary to cord or nerve root compression. These unfortunates have a narrow bony canal because of early neurocentral synchondrosis closure, thick pedicles, and prominent disks that compress their normal-sized cord, causing quadriplegia or paraplegia. Plain films visualize the narrow canals, but CT myelography best shows the relationship of the squeezed cord and canal. Symptoms are often at the lumbar level because the bony canal does not have its usual widening at that point. These patients also have a small foramen magnum, which is

Figure 16. MPS-VI. A lateral cervical myelogram showing the flattened vertebral bodies and progressive narrowing of the cervical canal as it reaches the cranio-cervical junction.

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thought to restrict CSF flow, adding to their propensity to hydrocephalus along with compressing the cord brain stem junction. The mucopolysaccharidoses (MPS disease) are variable in the amount of spinal and intraspinal abnormalities. The best known is the flattened bullet vertebral body of Rurler's (MPS-I). Deposits in the bone, dura, and ligaments may cause bony and ligamentous thickening, especially in MPSI, IRIS, IV, and VI6 (Fig. 16). The cervical canal in particular is constricted. Children with Down's syndrome often have a hypoplasia of the odontoid and lax ligaments, allowing the anterior arch of CI to slide over the odontoid with forced extension, and narrowing of the canal with flexion. 21 Lateral plain films in flexion and extension will demonstrate this abnormality, and should be done in patients who are planning active sports.

TUMORS

Bony Tumors Primary bony tumors of the spine are rare with the exception of aneurysmal bone cyst and osteoid osteoma or osteoblastoma. Osteoid osteoma is the most familiar tumor to clinicians because of its common pattern of pain that is worse at night and relieved by aspirin. It is usually located in the pedicle when involving the spine. Plain films, CT, and radionuclide scan all may be definitive in making a diagnosis. The first two exams can show the dense nidus within a sclerotic region. The third exam typically shows a very "hot" abnormality at the tumor site l l (Fig. 17). Osteoblastoma, or giant osteoid osteoma, although related to the osteoid osteoma, is a larger and more cystic expansile mass that can compress the cord. CT or MRI is probably the best exam for this tumor, which can closely resemble an aneurysmal bone cyst. Aneurysmal bone cyst commonly involves young male teens. Twenty per cent are in the spine, usually in posterior elements. The expanding and multilocular character of the tumor is best seen with CT, which also shows paraspinal soft tissue masses when present (Fig. 18). Giant cell tumor is less common than the other tumors, but also resembles the aneurysmal bone cyst on plain film and CT and more commonly will involve more than one vertebral level. Eosinophilic granuloma is usually considered with tumors, and is a relatively frequent vertebral mass. It can be differentiated from other tumors by the collapse of the vertebral body that it causes in comparison to the expansion seen in most other spinal tumors. Metastatic disease is best searched for by a radionuclide scan. Intraspinal Tumors Astrocytoma of the cord is the most common tumor in childhood, 9 usually presenting with pain, neurologic signs, and expansion of the spinal canal on plain x-ray. Astrocytomas may be local or may extend the length' of the cord. CT, and more recently MRI, shows they are often cystic,14 and newer operative techniques allow total or near total resection of these tumors. 7

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Figure 17. Osteoid osteoma. The radionuclide bone scan indicates a focal area of increased uptake (arrow) in the L3 pedicle in a 17-year-old girl with low back pain and a normal spine x-ray.

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Figure 18. Aneurysmal bone cyst. A, The oblique cervical spine film reveals the loss of the C3 facet. B, CT indicates the full extent of the mass, which expands the pedicle, lamina, and crosses the midline to the other lamina.

Teratomas, which usually occur in infancy or early childhood, may resemble astrocytoma on x-ray studies. Ependymomas are less common, and often occur in the filum rather than the cord. 5 They may be small tumors best seen on myelography or MRI. One must be certain that this tumor has not spread into the brain or is not a "drop" metastasis from an asymptomatic brain ependymoma. Metastatic lesions of the CSF space are nearly always medulloblastoma or ependymoma, and they should be routinely looked for by myelography when these intracranial tumors are diagnosed. Up to 100 per cent of medulloblastomas have been reported to have intraspinal metastases at the time of diagnosis. 25 The metastatic tumors have a typical appearance. They coat and distort the nerve roots of the cauda equina (Fig. 19) and cause expansion and irregularity of the cord itself (Fig. 13). Leukemia, though commonly spreading to the central nervous system, very uncommonly presents in this fashion. It occasionally is seen as an extradural tumor causing symptoms of compression. Paraspinal masses in childhood are usually neuroblastomas or their more benign counterparts, ganglioneuroblastomas or ganglioneuromas. Neuroblastoma in particular may invade the spinal canal. This may be clinically evident with signs of cord compression or may be clinically silent. Widening of a foramen or scoliosis convex toward the mass may be plain

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Figllre 19. Metastatic medulloblastoma. Metrizamide myelogram shows that the nerve roots are twisted and inferiorly are irregularly thickened by tumor coating them. Tumor fills the canal and blocks the flow of metrizamide at L'3 (arrow).

film signs of invasion. An abdominal CT, which is always done to evaluate such tumors, may also reveal tumor invasion of the foramen. If there is uncertainty as to extradural involvement, myelography or MRI should be done, as the intraspinal portion of the tumor may bleed and thereby expand its mass effect when the main extraspinal tumor is resected away from it. 1 If radiation is done before surgery, this becomes less important because the tumor will shrink from this treatment. INFECTION

Often presenting in a child with back pain and fever, osteomyelitis and diskitis are best diagnosed by radionuclide scan, which is discussed in

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the article by Majd. Childhood diskitis sometimes may be asymptomatic, revealing itself later as an incidentally discovered calcification in the disk space. In acute osteomyelitis, plain films and CT have a role in defining the exact extent of disease, and in following progressive changes, such as collapse of the disk space or vertebral body. CT also defines any paraspinal soft tissue involvement, as does MRI. Extradural, or rarely subdural abscesses are usually heralded by paralysis or less dramatic motor or sensory loss, and require immediate myelography to localize the extent of the process. With water-soluble dye, it is safe to force it past an obstructing mass, whether it is of infectious origin or tumor. This defines the upper borders for surgical intervention. Abscesses are usually localized to one or two segments. A longer section of involvement should make one suspicious of TB.

TRAUMA

In trauma, the cervical spine is a particular area of concern and some confusion. With flexion, the cervical vertebrae appear to slide forward slightly in childhood, so that there is a step pattern between each of the vertebrae, more pronounced at C2-C3 (Fig. 2). The posterior border of the anterior arch of Cl may also be separated from the anterior border of the odontoid by as much as 4 mm with flexion. Even for the experienced observer, it can be difficult to tell when the border between physiolog movement and pathologic displacement has occurred. The presence of soft tissue swelling is a helpful secondary sign, but is not always present. Flexion also allows vertical widening between the posterior arches of C 1 and C2, simulating a pathologic separation. Birth trauma is a special and dramatic pediatric problem. Usually occurring after a difficult forceps or breech delivery, the symptoms may vary from a single upper extremity weakness to quadriplegia, which is surprisingly at times mistaken for a "floppy baby" for several days. Because of the great elasticity of the ligaments of the newborn, plain x-rays may be completely normal with total disruption of the cord. Surrounding hematoma may obscure the severity of the damage at myelography, showing only irregularity and apparent cord narrowing when the cord is actually severed. In other respects, spinal trauma in children is not unlike that in adults. Plain films are excellent screening devices, and will pick up major spinal injuries. Both AP and lateral views are needed to be certain no fracture or dislocation exists. CT can be used on focal abnormalities to evaluate compression of the spinal cord, and subtle facet injuries and chip fractures. Abnormalities in the same plane as a CT slice, such as a horizontal fracture, may be missed on CT, though sagittal and coronal reconstruction of thin slices (3 to 5 mm) is sometimes helpful in improving resolution when required. Tomography may be simpler to perform and may give better results (Fig. 20). Minor anomalies and normal variants are one of the biggest problems for radiologists and clinicians. They are often found after trauma or unrelated symptoms that require an x-ray. It may be difficult to decide if an

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Figure 20. Trauma. An AP multidirectional tomogram clearly shows an unfused odontoid (0) that has been traumatically displaced laterally. The alignment on CT was not obviously abnormal. (Reprinted from the Clinical Neurosciences; Volume 4. Neuroradiology, Chapter 14. ChurchillLivingstone, New York, 1984 with permission of the publisher.)

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abnormality is significant. The upper cervical spine is especially prone to minor anomalies such as absent pedicles or laminae, which simulate fractures. CT will usually reveal the congenital nature of such cases if there is doubt on plain films. Spondylolisthesis This abnormality is usually thought of as secondary to multiple minor traumas in a spine that probably has a congenital weakness of the pedicles. Rarely seen before age two, and usually from age 5 to 10 years, it may cause low back pain in children. The spondylolysis, or fracture of the pars interarticularis, and/or the forward slippage (spondylolisthesis) is usually at the L4 to L5 level. It can nearly always be diagnosed on routine radiographs (Fig. 21). Myelography is needed if neurologic symptoms are present. This exam may reveal trapping of nerve roots that are draped over the posterior edge of the L5 vertebral body, which narrows the subarachnoid space. Disk Disease Disk disease is rare in childhood, and nearly always has a sudden onset after exertion, especially in athletic teenagers. It is therefore traumatic rather than degenerative. The radiologic findings are those of adult disk disease.

Figure 21. Spondylolysis and spondylolisthesis. A lateral spine film with clearly visible separation of the laminar parts (arrows), and forward slippage of U on LS.

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Figure 22. Rotary subluxation. C 1, mostly seen on A, is rotated to the left in relation to C2, seen on B. This should bring the right lateral mass of C1 (arrow) closer to the odontoid (arrowhead). Fixation holds the odontoid toward the left lateral mass. Note normal open posterior laminae of C1 in A.

Rotatory fixation or subluxation is a subject of some confusion. Although most often post-traumatic, it can be secondary to inflammation with ligamentous softening, or to simple muscle spasm. The child presents with torticollis, and on x-ray one finds the odontoid is fixed to one side when the head is turned in both directions. 26 It can be diagnosed by careful plain film examination, but CT is often easier (Fig. 22). The importance of the diagnosis is that it indicates that there is a physical abnormality causing the symptoms. It usually responds to treatment with a collar and muscle relaxants when needed but occasionally will require surgical fusion of the spine if it persists for several months.

REFERENCES 1. Armstrong, E. A., Harwood-Nash, D. C. F., Fitz, C. R, et al.: CT of neuroblastomas and ganglioneuromas in children. A.J.N.R, 3:401-406, 1982. 2. Bentley, J. F. R., and Smith, J. R: Developmental posterior enteric remnants and spinal malformations. Arch. Dis. Child., 35:76-86, 1960. 3. Bonafe, A., Manelfe, C., Espagno, J., et al.: Evaluation of syringomyelia with metrizamide computed tomographic myelography. J. Comput. Assist. Tomogr., 4:797-802, 1980. 4. Brooks, B. S., El Gammal, T., Hartlage, P., et al.: Myelography of sacral agenesis. A.J.N.R, 2:319-323, 1981. 5. Chan, H. S. L., Becker, L. E., Hoffman, H. J., et al.: Myxopapillary ependymoma of the filum terminale and cauda equina in childhood: report of seven cases and review of the literature. Neurosurgery, 14:204-310, 1984. 6. Edwards, M. K., Harwood-Nash, D. C., Fitz, C. R, et al.: CT metrizamide myelography of the cervical spine in Morquio syndrome. A.J.N.R, 3:666-669,1982. 7. Epstein, F., and Epstein, N.: Surgical management of holocord intramedullary spinal cord astrocytomas in children. J. Neurosurg., 54:829-832, 1981. 8. Gabrielsen, T. 0., Gebarski, S. S., Knake, J. E., et al.: Iohexol versus metrizamide for lumbar myelography: Double-blind trial. A.J.N.R, 5:181-183, 1984. 9. Harwood-Nash, D. C., Fitz, C. R: Chapter 20. Mass lesions of the spinal cord. In Neuroradiology in Infants and Children. St. Louis, C. V. Mosby Co, 1976, pp. 11671227. 10. Heinz, E. R., Rosenbaum, A. E., Scarff, T. B., et al.: Tethered spinal cord following meningomyelocele repair. Radiology, 131:153-160, 1979.

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11. Helms, C. A., Hattner, R. S., Vogler, III, J. B.: Osteoid osteoma: Radionuclide diagnosis. Radiology, 151:779-784, 1984. 12. Hilal, S. K., Marton, D., and Pollack, E.: Diastematomyelia in children. Radiology, 112:609-621, 1974. 13. Hoffman, H. J., Hendrick, E. B., Humphreys, R. P.: The tethered spinal cord: Its protean manifestations, diagnosis and surgical correction. Child's Brain, 2:145-155, 1976. 14. Kan, S., Fox, A. J., Vinuela, F., et al.: Delayed CT metrizamide enhancement of syringomyelia secondary to tumor. A.J.N.R., 4:73-78, 1983. 15. Kangarioo, H., Gold, R. H., Diament, M. J., et al.: High-resolution spinal sonography in infants. A.J.N.R., 5:191-195, 1984. 16. Kieffer, S. A., Binet, E. F., Davis, D.O., et al.: Lumbar myelography with iohexol and metrizamide: A comparative multicenter prospective study. Radiology, 151 :665-670, 1984. 17. List, C. F.: Intraspinal epidermoids, dermoids, and dermal sinuses. Surg. Gynecol. Obstet., 73:525-538, 1941. 18. Modic, M. T., Weinstein, M. A., Pavlicek, W, et al.: Magnetic resonance imaging of the cervical spine: Technical and clinical observations. A.J.N.R., 5:15-22, 1984. 19. Norman, D., Mills, C. M., Brant-Zawadzki, M., et al.: Magnetic resonance imaging of the spinal cord and canal: Potentials and limitations. A.J.N.R., 5:9-14, 1984. 20. Pettersson, H., Fitz, C. R., Harwood-Nash, D. C. F., et al.: Adverse reactions to myelography with metrizamide in infants, children, and adolescents. 1. General and CNS effects. Radiology, 148:880, 1983. 21. Pueschel, S. M., Scola, F. H., Perry, C. D., et al.: Atlanto-axial instability in children with Down syndrome. Pediatr. Radiol., 10:129-132, 1981. 22. Quencer, R. M., Montalvo, B. M., Green, B. A., et a\.; Intraoperative spinal sonography of soft-tissue masses of the spinal cord and spinal canal. A.J.N.R., 5:507-515,1984. 23. Sarwar, M., Virapongse, C., Bhimani, S.: Primary tethered cord syndrome: A new hypothesis of its origin. A.J.N.R., 5:235-242,1984. 24. Scotti, G., Musgrave, M. A., Harwood-Nash, D. C., etal.: Diastematomyelia in children: Metrizamide and CT metrizamide myelography. A.J.N.R., 1:403-410, 1980. 25. Stanley, P., Senac, Jr., M. 0., Segall, H. D., et al.: Intraspinal seeding from intracranial tumors in children. A.J.R., 144:157-161, 1985. 26. Wortzman, G., and Dewar, F. P.: Rotary fixation of the atlantoaxial joint: Rotational atlantoaxial subluxation. Radiology, 90:479-487, 1968. 27. Yamada, S., Zinke, D. E., and Sanders, D.: Pathophysiology of "tethered cord syndrome." J. Neurosurg., 54:494-503, 1981. Section of Neuroradiology Children's Hospital National Medical Center III Michigan Avenue, N.W. Washington, D.C. 20010