The more benign forms of spinal dysraphism include occult bony abnormalities unaccompanied by any displacement of spinal canal contents and with or without cutaneous stigmata. In these cases, there is failure of bony fusion between the two laminae of the involved vertebra (see Plate 1-7). Spina bifida occulta is of no clinical significance when it occurs alone without intra-spinal involvement.
Cutaneous stigmata of spina bifida occulta include dimples, dermal sinuses, subcutaneous lipomas, tufts of hair, or hemangiomas. Cutaneous lesions may occur in isolation, or herald an underlying tethered cord due to a low-lying conus or fat-infiltrated filum. Only a small subset of cutaneous lesions is associated with an intra-spinal anomaly, and MRI performed within a few months of age can often exclude the diagnosis without radiation or sedation. The tethered spinal cord syndrome occurs when a hypertrophied filum terminale is too inflexible and causes progressive traction and relative caudal displacement of the conus medullaris as the spine grows. This traction can produce progressive ischemia in the conus medullaris and lead to symptoms of sphincter dysfunction and gait abnormalities. One third of infants with a congenital tethered cord are likely to eventually develop neurologic dysfunction if the tethered cord is not treated. Prophylactic detethering by microsurgical sectioning of the filum terminale, ideally before 1 year of age, allows immediate ascent of the conus medullaris toward a more normal location within the spinal canal and minimizes the chance of development of neurologic deficits as the spine grows. For older children who have a late symptomatic presentation, treatment may minimize the further progression of neurologic deficits. If spina bifida occulta occurs in conjunction with a dermal sinus (an epithelium-lined tract linking the dural sac with the skin surface), there is a potential for communication between the skin and intraspinal contents and subsequent infection. Dermal sinuses located above the sacrococcygeal region should be removed surgically after MRI imaging to evaluate for other associated lesions.
SPINA BIFIDA APERTA
Dysraphic conditions in which there are overt manifestations of the underlying bony defect are referred to as “spina bifida aperta” (see Plate 1-8). Within this group, the progression of neurologic sequelae is defined, to a large extent, by the degree to which the contents of the spinal canal are displaced from their normal location. In the case of a meningocele, the most benign form of spina bifida aperta, a meningeal cyst free of neural elements is extruded. Often, a meningocele can be completely removed surgically and the defect closed.
Diastematomyelia is a congenital malformation in which the spinal cord is split into two sections, or hemicords. It is frequently associated with a midline cutaneous tuft of hair, and all infants with a worrisome tuft should be screened with an MRI. Frequently, a bony or cartilaginous septum separates the divided sections. Patients with a split cord malformation are at risk of developing associated scoliosis and progressive myelopathy as the spine grows. Surgical excision of the midline septum can halt the deteriorating condition and, in some cases, lead to restored function.
A far more devastating variant of spina bifida aperta is myelomeningocele, in which the spinal cord or nerve roots, or both, protrude through the posterior bony and cutaneous defects due to failed closure of the posterior neuropore. The severity of deficits from a myelomeningocele correlates with its location along the spinal canal, with increasing deficits occurring with more rostral lesions. The neurologic deficits are due to abnormal in utero development throughout the entire CNS. Postnatal closure of the myelomeningocele in the term infant is performed within a few days of birth to minimize the risk of meningitis, and is associated with low morbidity. Prenatal fetal closure may be an option for a very select group of patients and is associated with complications, including preterm delivery.
Prenatal folate supplementation has markedly decreased both the incidence of infants born with myelomeningocele and lesion severity. Infants with sacral and low lumbar lesions often achieve some degree of ambulation, and approximately 80% can achieve social bladder and bowel continence. Approximately half of infants with a lumbar or sacral myelomeningocele will develop hydrocephalus that requires surgical treatment (see Plate 1-7). Most of these infants will have an associated Chiari II malformation, with displacement of the cerebellar vermis into the cervical canal, but only a few percent will become symptomatic at any point. Children are at risk for developing the tethered cord syndrome as the myelomeningocele scar adheres to the repair site while the spine grows. All repaired myelomeningoceles will appear adherent to some degree on MRI, and the diagnosis of a tethered cord in this population is made clinically. Although multidisciplinary care is needed throughout the life span of children born with a myelomeningocele, many will become in ependent productive adults with a good quality of life.