ASSOCIATED CENTRAL NERVOUS SYSTEM MALFORMATIONS
Extensive neuropathologic studies have demonstrated that NTDs are associated with a high incidence of gross and microscopic malformations of the forebrain and hindbrain (59). Additional anomalies in the spinal cord may complicate the original local dysraphic defect (52,53,57,60,61). Table 15.1 lists associated anomalies and malformations by location.
The spinal cord defect usually results in a lower motor neuron process. Spasticity is present in most individuals with spina bifida across their lifetime (62). The presence
TABLE 15.1 ASSOCIATED CENTRAL NERVOUS SYSTEM MALFORMATIONS
or gradual development of spasticity above the level of the spinal cord lesion may be related to tethering of the spinal cord, Chiari type II malformation exerting pressure on the cervical spinal cord, decompensating hydrocephalus, ventriculitis, syringohydromyelia, or coexistent encephalopathy sustained at birth (63-66).
Tethered cord refers to an abnormal attachment of the spinal cord at its distal end (57). Under normal circumstances, the conus medullaris ascends from its distal position to the LI to L2 vertebral level during the first year of life (67). Focal abnormalities—including thickened and shortened filum terminale, supernumerary fibrous bands, persistent membrane reunions, dural sinus, diastematomyelia, entrapment by lumbosacral tumors, and adhesions in the scar tissue of the repaired myelomeningocele—interfere with this process (57). Almost all children born with spina bifida have a low-lying cord on MRI, and approximately one-third develop neurologic, urologic, or orthopedic complications or symptoms (68) (Figure 15.2).
Tethering of the spinal cord is the second most common cause of neurologic decline in children with myelomeningocele (68). The most common clinical signs or symptoms of a tethered cord include spasticity in the lower extremities, decline in lower extremity strength,
FIGURE 15.2 T2-weighted magnetic resonance image of tethered cord. There is tethering of the spinal cord with conus seen down to the L5 vertebral level, heterogeneous signal intensity characteristics, and areas of fibrofatty tissue.
Source: Molnar GE. Spina bifida: clinical correlations of associated central nervous system malformations. Phys Med Rehabil State Art Rev. 1991;5:289, with permission.
and worsening scoliosis. Other signs and symptoms that strongly suggest tethering of the spinal cord include back pain, changes in urologic function, changes in gait, and development of lower extremity contractures. In patients who are suspected of having a symptomatic tethered cord, the function of their shunt must be evaluated prior to proceeding forward with surgical management (69), as shunt malfunction can present similarly.
The reported functional outcome of surgical management of a tethered cord is variable. One study reported improvements in gait in almost 80% of patients following untethering, whereas another study reported improvement in as few as 7% (70). (Note: All cords tether to some extent following repair.) Less than 20% of children with a tethered cord experience back pain. However, this is the symptom most likely to improve with surgery (63,71). A recent article comparing prophylactic and symptomatic tethered cord surgery in those with lumbosacral lipomas found no change in long-term outcome. This suggests that delaying surgical intervention may decrease the number of tethered cord surgeries (72).
Diastematomyelia is a postneurulation defect that results in a sagittal cleavage of the spinal cord, most commonly affecting the lumbar and thoracolumbar levels of the spinal cord. It is more common in females (73,74). Diastematomyelia may have both neurologic and orthopedic presentations. Orthopedic symptoms include scoliosis, Sprengel's deformity (especially when associated with Klippel-Feil sequence), hip subluxation, and lower extremity limb-length discrepancies (74,75).
Neurologic symptoms include gait abnormalities, asymmetric motor and sensory deficits of the lower extremities, and neurogenic bladder and bowel (76). Symptoms of diastematomyelia may present in childhood or, less commonly, in adulthood (77).
It is not uncommon for individuals to develop syringomyelia—a tubular cavitation in the spinal cord parenchyma extending more than two spinal segments (78). Syringomyelia is present in up to 40% of individuals with myelomeningocele (79). The syrinx may be located anywhere along the spinal cord, medulla, or pons, but is most common in the cervical spinal region (52,54,80). MRI is used to detect syringomyelia (81) (Figure 15.3).
Often, a syrinx is of little clinical significance; however, if a patient develops decreasing function above the level of his or her lesion, syringomyelia must be considered in the differential diagnosis. Although shunt malfunction and spinal cord tethering are more common complications, symptomatic hydromyelia may explain a slower-than-expected progression through gross motor and fine motor developmental milestones or a decrease in strength or change in function. Early progression of scoliosis above the initial neurologic level may be the earliest sign of a syrinx. A shunt malfunction may contribute to a symptomatic syrinx, and shunt function should be evaluated. Placement of a syringopleural shunt may be necessary to decompress the syrinx.
FIGURE 15.3 T2-weighted magnetic resonance image showing sagittal (A) and axial (B) views. There is a large syrinx present, beginning at the mid portion of C6 and extending to L4.