Other rare entities are neurocutaneous melanosis, neuroectodermal melanolysosomal disease with mental retardation, progeria, Cockayne syndrome, and ataxia-telangiectasia (see further on; also Gomez, 1987).
Dysraphism, or Rachischisis: Meningocele, Encephaloceles, and Spina Bifida
Included under this heading is the large number of disorders of fusion of dorsal midline structures of the primitive neural tube, a process that takes place during the first 3 weeks of postconceptual life. Exogenous factors are presumed to be operative in most cases but there are genetic forms. The most extreme form is anencephaly, as described earlier; it is characterized by the absence of the entire cranium at birth, and the undeveloped brain lies in the base of the skull, a small vascular mass without recognizable nervous structures.
An eventration of brain tissue and its coverings through an unfused midline defect in the skull is called an encephalocele. Frontal encephaloceles may deform the forehead or remain occult. Associated defects of the frontal cortex, anterior corpus callosum, and optic-hypothalamic structures, as well as cerebrospinal fluid (CSF) leakage into frontal or ethmoid sinuses, pose a risk of meningitis. Some of these children are relatively cognitively normal. Far more severe are the posterior encephaloceles, some of which are enormous and are attended by grave neurologic deficits such as blindness, ataxia, and cognitive impairment. However, lesser degrees of the defect are well known and may be small or hidden, such as a meningoencephalocele connected with the rest of the brain through a small opening in the skull. Small nasal encephaloceles may cause no neurologic signs, but if they are mistaken for nasal polyps and excised, CSF rhinorrhea may result.
A failure of development of the midline portion of the cerebellum referred to earlier, forms the basis of the Dandy-Walker syndrome (Fig. 37-2). A cyst-like structure, representing the greatly dilated fourth ventricle, expands in the midline, causing the occipital bone to bulge posteriorly and displace the tentorium and torcula upward. In addition, the cerebellar vermis is aplastic, the corpus callosum may be deficient or absent, and there is dilatation of the aqueduct as well as the third and lateral ventricles.
Even more frequent are abnormalities of closure of the vertebral arches. These take the form of a spina bifida occulta, meningocele, and meningomyelocele of the lumbosacral or other regions. In spina bifida occulta, the cord remains inside the canal and there is no external sac, although a subcutaneous lipoma or a dimple or wisp of hair on the overlying skin may mark the site of the lesion. In meningocele, there is a protrusion of only the dura and arachnoid through the defect in the vertebral laminae, forming a cystic swelling usually in the lumbosacral region; the cord remains in the canal, however. In meningomyelocele, which is 10 times as frequent as meningocele, the cord (more often the cauda equina) is extruded also and is closely applied to the fundus of the cystic swelling.
The incidence of spinal dysraphism (myeloschisis), like that of anencephaly, varies widely from one locale to another, and the disorder is more likely to occur in a second child if one child has already been affected (the incidence then rises from 1 per 1,000 to 40 to 50 per 1,000).
Etiology Exogenous factors (e.g., potato blight in