Congenital Spinal Malformations

When imaging children with suspected congenital spinal malformations, one must be aware that multiple anomalies such as myelomeningocele, split-cord malformation, syringohydromyelia, and others may co-exist. Anomalies of the caudal spine must be considered in patients with urogenital or anorectal malformations. Sagittal and coronal imaging of the entire spine with MRI is recommended to identify the location of the conus medullaris (normal level: T10 to L2), associated lipomas or syrinxes, anomalous segments of spinal cord, and anomalies of dorsal closure or segmentation. Transaxial T1- and T2-WI should be obtained from the conus through the bottom of the sacrum to assess for a fatty (T1 hyperintense on MRI) and/or thickened filum terminale. If a split-cord malformation (diastem-atomyelia) is detected, additional transaxial T2*-WI images should be performed through the levels of the split cord to best demonstrate a bony or fibrous spur. In patients with complex bone anomalies, MRI should be augmented with radiographs and CT scans.

Spinal dysraphism (see Chapter 8) results from abnormal neurulation, and includes a group of malformations characterized by incomplete dorsal midline closure of the dermal, muscular, osseous and/or neural tissues. In spina bifida aperta, part or all of the contents of the spinal canal protrude posteriorly through the dorsal dysraphic spinal defect. The neural arch is widened, and pedicles and laminae of the abnormal vertebral bodies are flared outward and oriented posterolaterally instead of posteromedially. These findings are typically easy to see on both obstetrical ultrasound and MRI. Examples of open spinal dysraphism include myeloceles and myelomeningoceles. In these two entities, the spinal cord is tethered, and neural tissue (neural placode) is directly exposed without any covering skin. In myeloceles, the placode is flush with the skin surface, whereas the neural tissue is elevated by protrusion of underlying CSF-filled meninges in myelomeningoceles. Chiari II malformations are almost always seen in association with myelomeningoceles.

In occult spinal dysraphism, neural tissue is not directly exposed but is instead covered with intact skin. Examples include meningocele, dermal sinus tract, spinal lipoma, fatty filum terminale and split-cord malformations. In simple meningoceles, meninges containing CSF (without neural tissue) protrude through the dysraphic area. Dermal sinus tracts result from incomplete separation ("disjunction") of neural and cutaneous ectoderm during neurulation. They consist of epithelium-lined tracts extending from the skin surface to the subcutaneous soft tissues or extending to the dura, subarachnoid space or spinal cord. They are most commonly located in the lumbosacral and occipital regions, and approximately half are associated with dermoid or epidermoid cysts. They may lead to recurrent infection or to focal symptoms secondary to compression of the cord or cauda equina. Their delineation by MRI is improved by addition of contrast-enhanced Tl-weighted sequences with fat saturation.

Spinal lipomas are the most common occult spinal dysraphism, and are likely also due to improper disjunction during neurulation. They can be further categorized as intradural lipoma, lipomyelocele or lipomyelomeningocele, and fibrolipomas of the filum terminale. Intradural lipomas may not have an associated defect in the bone of the spinal canal, and usually appear as well-defined fatty dorsal intradural masses (hypodense on CT and Tl hyperintense on MRI) that are in direct contact with neural tissue. Occasionally these lipomas are large enough to compress the spinal cord. The most frequent locations are the thoracic and cervical spine. Lipomyeloceles and lipomyelomeningoceles refer to myeloceles and myelomeningoceles in which the placode (dorsal surface of the unclosed neural tube) is adherent to a lipoma, which is covered by intact skin. They are not associated with Chiari II malformations, although approximately 5% are associated with Chiari I. Fibrolipomas of the filum terminale appear as linear foci of hypodensity and T1 hyperintensity along an abnormally thickened filum terminale (diameter >1 mm at the L5-S1 level). These can be very subtle but still cause symptoms due to associated tethering of the spinal cord. The normal filum is usually not seen on MRI, because it extends inferiorly from the conus medullaris to the bottom of the subarachnoid space and exits the dura to attach to the first coccygeal vertebral body.

Figure 13. Split-cord malformation in the upper thoracic spine. (transaxial T1-WI)

On MRI, the abnormal filum is short and thick, and usually accompanies an abnormally low conus medullaris, located caudal to the L2 level. Affected patients can present at any age with back pain, bladder dysfunction, and sensory and motor changes in the lower extremities. Neurological abnormalities are presumably caused by hydromyelia and/ or injury to the spinal cord due to chronic tension, and may manifest as low Tl and high T2 SI within the central cord on MRI. Children can also have orthopedic deformities such as scoliosis and clubfeet.

Split-cord malformations are considered a subtype under the group of split notochord syndromes, which also include dorsal enteric fistulae, sinuses, diverticulae and cysts. All are probably due to abnormal splitting of the notochord during embryogenesis. The latter four entities will not be discussed here, but are thought to result from abnormal persistent connection of the neural ectoderm and enteric endoderm. Split-cord malformations, also known as diastematomyelia, refer to partial or complete sagittal splitting of the spinal cord into two symmetric or asymmetric hemicords (Fig. 13). The intervening septum may be bony (in about half of all cases) or fibrous. The splitting usually occurs along the lower thoracic and lumbar spine. Children frequently have cutaneous lesions over the spine (e.g., dimples, nevi, hairy patches, hemangiomas), scoliosis and clubfeet. Split-cord malformations are almost always associated with significant segmentation anomalies, especially fusion of the lamina of adjacent vertebral bodies. Transaxial Tl - and T2-WI must be obtained to evaluate for a fatty and/or thickened filum and any associated hydromy-elia. Transaxial T2*-WI are useful for investigating the septum, which appears hypointense if ossified or large.

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