Myelomeningocele is a neural tube defect that disrupts proper neuralization and closure of the spinal column. This defect occurs at 4–6 weeks gestation and results in evagination of the dural sac with neuronal tissue. Myelomeningoceles are the most common neural tube defect, with 3.4 out of every 10 000 live births being affected. Although the etiology of myelomeningocele is not completely understood, low levels of folic acid during pregnancy are thought to play a role. Prenatal screening, early termination, and folic acid supplementation have decreased the incidence of myelomeningoceles. Advances in fetal screening tests including maternal alpha-fetoprotein levels, amniocentesis, and fetal ultrasound have helped identify cases of myelomeningocele allowing for earlier interventions.
The most common form of myelomeningocele occurs in the lumbar spinal region. Lesions with greater cephalic spread and those larger in size carry more severe risks and consequences. Myelomeningocele predisposes patients to infection and sensorimotor dysfunction. Sensory and motor nerve injury has been shown in animal studies to originate from neuronal tissue trauma within the uterus. Myelomeningoceles are associated with a high morbidity and the degree neural damage may present with paraparesis, incontinence, frequent urinary tract infections (UTIs), and seizures. A distinction should be made for meningoceles, as they do not include neuronal tissue within the dural sac, and subsequently, have a lower risk of sensorimotor dysfunction.
Patients typically present at less than 2 days of life following a scheduled cesarean section delivery, when the defect has been prenatally diagnosed. The most immediate concern is for infection, which relies on timely closure. Patching of the dural sac within the first 3 days of life can significantly decrease the risk of meningitis or arachnoiditis. Additional goals of surgical intervention are to preserve neural function and prevent increases in intracranial pressure. Even with prompt intervention, many patients continue to suffer sensorimotor sequela.
Myelomeningocele occurs in high association with Arnold Chiari II malformations and hydrocephalus. The caudal herniation of the brainstem and fourth ventricle seen with Arnold Chiari II malformation obstructs cerebrospinal fluid flow, resulting in hydrocephalus and elevated intracranial pressures. Arnold Chiari II by itself poses a risk of respiratory compromise secondary to the pressure exerted on the respiratory centers of the brainstem.
A persistent tract distracting the spinal column caudally is commonly associated with myelomeningoceles. The tethered cord may cause nerve dysfunction from structural distraction or diminished perfusion of the spinal column.
Cardiac Defects and Other Conditions
Preoperative echocardiographic evaluation of the patient with myelomeningocele is recommended to assess for potential cardiac defects. Atrial septal defects are the most commonly associated cardiac lesion. Practitioners should also be mindful of higher frequency of prematurity and associated conditions in neonates with myelomeningocele. In addition, intestinal, renal, and tracheoesophageal malformations can occur in conjunction with myelomeningocele.