Defective closure of the caudal neural tube at the end of week 4 of gestation results in anomalies of the lumbar and sacral vertebrae or spinal cord called spina bifida. These anomalies range in severity from clinically insignificant defects of the L5 or S1 vertebral arches to major malformations, leaving the spinal cord uncovered by skin or bone on the infant’s back. The latter severe defect, called a myelomeningocele, results in flaccid paralysis and loss of sensation in the legs and incontinence of bowel and bladder, with the extent and degree of neurologic deficit dependent on the location of the myelomeningocele. In addition, affected children usually have an associated Chiari type II malformation (downward displacement of the cerebellar tonsils and medulla), resulting in hydrocephalus and weakness of face and swallowing. In a meningocele, the spinal canal and cystic meninges are exposed on the back, but the underlying spinal cord is anatomically and functionally intact. In spina bifida occulta, the skin of the back is apparently intact, but defects of the underlying bone or spinal canal are present. Meningoceles and spina bifida occulta may be associated with a lipoma, dermoid cyst, or tethering of the cord to a thick filum terminale. A dimple or tuft of hair may be present over the affected area. Patients may also have an associated dermoid sinus, an epithelial tract extending from the skin surface to the meninges; this increases the risk of meningitis. Patients with spina bifida occulta or meningocele may have weakness and numbness in the feet that can result in recurrent ulcerations, or difficulties controlling bowel or bladder function that may result in recurrent urinary tract infections, reflux nephropathy, and renal insufficiency. In diastematomyelia, a bone spicule or fibrous band divides the spinal cord into two longitudinal sections. An associated lipoma that infiltrates the cord and tethers it to the vertebrae may be present. Symptoms include weakness and numbness of the feet and urinary incontinence.
Myelomeningocele in the fetus is suggested by an elevated alpha-fetoprotein in the mother’s blood and confirmed by ultrasound and high concentrations of alpha-fetoprotein and acetylcholinesterase in the amniotic fluid. After birth, screening ultrasound may be used with magnetic resonance imaging to confirm less dramatic underlying spinal abnormalities.
Neonates with myelomeningocele must undergo operative closure of the open spinal defects (fetal or postnatal surgery) and often require treatment of hydrocephalus by placement of a ventriculoperitoneal shunt. Toddlers and children with lower spinal cord dysfunction require physical therapy, bracing of the lower extremities, and intermittent bladder catheterization. In the absence of associated brain anomalies, most survivors have normal intelligence, but learning problems and epilepsy are more common than in the general population.
Spina bifida can be prevented in many cases by folate administration to the pregnant mother. Because the defect occurs so early in gestation, all women of childbearing age are advised to take oral folic acid daily.
Defective closure of the rostral neural tube produces anencephaly or encephaloceles. Neonates with anencephaly have a rudimentary brainstem or midbrain, but no cortex or cranium. This is rapidly fatal after delivery. Patients with encephalocele usually have a skull defect and exposure of meninges alone or meninges and brain. The recurrence risk in subsequent pregnancies for either cranial or spinal neural tube defects is 3% to 4%. Within a family, an anencephalic birth may be followed by the birth of a child affected with a lumbosacral myelomeningocele. The inheritance of neural tube defects is polygenic.