As the older names imply, spina bifida and cranium bifidum share a failure of bone fusion in the posterior midline of the skull (cranium bifidum) or the vertebral column (spina bifida). The result is a bony cleft through which the meninges and varying quantities of brain or spinal cord tissue protrude. In cranium bifidum, the neural herniation is termed encephalocele and can consist of brain parenchyma and meninges or only of meninges. These form the wall of a saclike cyst filled with cerebrospinal fluid (CSF). Posterior encephaloceles may contain only supratentorial structures, only posterior fossa structures, or both. In spina bifida, the herniation is called meningocele or meningomyelocele, depending on whether the meninges herniate alone or together with spinal cord parenchyma and nerve roots. The traditional names spina bifida and cranium bifidum are now less frequently used than in the older literature because of the recognition that the bony cleft may not be the primary defect in all cases, but instead that the pathogenesis may involve neural induction of mesodermal tissues in the dorsal midline, including leptomeninges, dura mater, and bone.
Spina bifida occulta is a minor fusion failure of the posterior vertebral arches unaccompanied by herniation of meninges or neural tissue. Spina bifida cystica collectively designates meningocele, meningomyelocele, and other cystic lesions (FIGURE). Similarly, in the head, cranium bifidum comprises meningocele, a herniation of meninges containing only CSF, and the more commonly occurring encephalocele, in which the sac contains neural and glial tissue. Rachischisis refers to a severe condition with an extensive defect of the craniovertebral bone with exposure of the brain, spinal cord, and meninges. Myeloschisis is another defect in the tissues over the lower spinal cord. Neural tissue is exposed at the surface as a flat, red lesion with a velvety appearance over the sacral region, without protruding as a myelomeningocele sac.
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