Spinal cord defects
Various malformations of the spine—including spina bifida, meningocele, and myelomeningocele—result from defective embryonic neural tube closure during the first trimester of pregnancy. Generally, these defects occur in the lumbosacral area, but they’re occasionally found in the sacral, thoracic, and cervical areas.
Spina bifida occulta is the most common and least severe spinal cord defect. It’s characterized by incomplete closure of one or more vertebrae without protrusion of the spinal cord or meninges.
However, in more severe forms of spina bifida, incomplete closure of one or more vertebrae causes protrusion of the spinal contents in an external sac or cystic lesion. In spina bifida with meningocele, this sac contains meninges and cerebrospinal fluid (CSF). In spina bifida with myelomeningocele (meningomyelocele), this sac contains meninges, CSF, and a portion of the spinal cord or nerve roots distal to the conus medullaris.
Spina bifida is relatively common and affects about 5% of the population. In the United States, about 12,000 neonates each year are born with some form of spina bifida; spina bifida with myelomeningocele is less common than spina bifida occulta and spina bifida with meningocele. Incidence is highest in persons of Welsh or Irish ancestry.
The prognosis varies with the degree of accompanying neurologic deficit. It’s worst in patients with large open lesions, neurogenic bladders (which predispose to infection and renal failure), or total paralysis of the legs.
Clinical TipThe prognosis is better in patients with spina bifida occulta and meningocele than in those with myelomeningocele. Many patients with these conditions can lead normal lives.
Causes
Normally, about 20 days after conception, the embryo develops a neural groove in the dorsal ectoderm. This groove rapidly deepens, and the two edges fuse to form the neural tube.
By about day 23, this tube is completely closed except for an opening at each end. Theoretically, if the posterior portion of this neural tube fails to close by the 4th week of gestation, or if it closes but then splits open from a cause such as an abnormal increase in CSF later in the first trimester, a spinal defect results.
Viruses, radiation, and other environmental factors may be responsible for such defects. However, spinal cord defects occur more often in offspring of women who have previously had children with similar defects, so genetic factors may also be responsible.
Signs and symptoms
Indications vary depending on the type of defect.
Spina bifida occulta
Although overt signs may be absent, spina bifida occulta is often accompanied by a depression or dimple, tuft of hair, soft fatty deposits, port wine nevi, or a combination of these abnormalities on the skin over the spinal defect.
Spina bifida occulta doesn’t usually cause neurologic dysfunction but occasionally is associated with foot weakness or bowel and bladder disturbances.
Such disturbances are especially likely during rapid growth phases, when the spinal cord’s ascent within the vertebral column may be impaired by its abnormal adherence to other tissues.
Such disturbances are especially likely during rapid growth phases, when the spinal cord’s ascent within the vertebral column may be impaired by its abnormal adherence to other tissues.
Meningocele and myelomeningocele
A saclike structure protrudes over the spine in both meningocele and myelo-meningocele.
Like spina bifida occulta, meningocele rarely causes neurologic deficit. But myelomeningocele, depending on the level of the defect, causes permanent neurologic dysfunction, such as flaccid or spastic paralysis and bowel and bladder incontinence.
Associated disorders include trophic skin disturbances (ulcerations, cyan-osis), clubfoot, knee contractures, and hydrocephalus (in about 90% of patients). Mental retardation, Arnold-Chiari syndrome (in which part of the brain protrudes into the spinal canal), and curvature of the spine can also occur.
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