Defective Neural Tube Formation

An encephalocele is a protrusion of a portion of the brain or meninges through a skull defect. Although an encephalocele usually occurs in the occipital region in patients from Europe and North America, it can develop frontally or in the nasal passages, especially in children in Southeast Asia (see Plate 1-5). The herniated brain tissue is connected through a narrow isthmus. With occipital encephaloceles, there may be associated abnormalities of the cerebellum and midbrain. The Meckel-Gruber syndrome includes a posterior encephalocele, microcephaly, microphthalmus, cleft lip and palate, polydactyly, and polycystic kidneys. This syndrome is inherited in an autosomal recessive manner, whereas for parents of a child with simple encephalocele, the risk of recurrence is 5%. Myelomeningocele results from failure of caudal closure of the neural tube, with an 80% incidence in the lumbar region. Because closure of the central canal is essential to subsequent development of the rostral CNS, myelomeningocele also causes numerous associated brain anomalies. Prenatally, fetal ultrasonography is used to diagnose a myelomeningocele. Postnatally, magnetic resonance imaging (MRI) is particularly valuable in delineating the extent of the structural abnormalities. All affected infants require neurosurgical intervention. The prognosis depends on the degree of CNS involvement, which may be difficult to assess in the neonate.

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