Atlas of pathophysiology, 2 Edition
Part II - Disorders
Spina bifida and other neural tube defects (NTDs) are serious birth defects that involve the spine and spinal cord. In spina bifida, the neural tube fails to close at approximately 28 days after conception.
Exact cause and most of the specific environmental triggers unknown
· Maternal folic acid deficiency
· Fetal exposure to a teratogen such as valproic acid
· Multiple malformation syndrome (for example, chromosomal abnormalities such as trisomy 18 or 13 syndrome)
· Isolated NTDs (not due to a specific teratogen or associated with other malformations) believed to be caused by a combination of genetic and environmental factors
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, the spinal contents protrude in an external sac or cystic lesion (spina bifida cystica). Spina bifida cystica has two forms:myelomeningocele (meningomyelocele) and meningocele. In myelomeningocele, the external sac contains meninges, cerebrospinal fluid (CSF), and a portion of the spinal cord or nerve roots distal to the conus medullaris. When the spinal nerve roots end at the sac, motor and sensory function below the sac is abolished. Arnold-Chiari syndrome is a form of meningomyelocele in which part of the brain protrudes into the spinal canal. Meningocele, in which the sac contains only meninges and CSF, is less severe and may not produce symptoms.
Signs and symptoms
Spina bifida occulta
· 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
· Occasionally associated with foot weakness or bowel and bladder disturbances
· Saclike structure protrudes over the spine
· Seldom causes neurologic deficit
· Saclike protrusion containing nerve tissue
· Depending on the level of the defect, causes permanent neurologic dysfunction
· Trophic skin disturbances (ulcerations, cyanosis), clubfoot, knee contractures, curvature of the spine, hydrocephalus (in about 90% of patients) and, possibly, mental retardation
Diagnostic test results
· Amniocentesis detects elevated alpha fetoprotein (AFP) levels in amniotic fluid, which indicates the presence of an open NTD.
· Fetal karyotype detects chromosomal abnormalities.
· Maternal serum AFP screening is used in combination with other serum markers, such as human chorionic gonadotropin (HCG), free beta-HCG, or unconjugated estriol (for patients with a lower risk of NTDs and those who will be under age 34½ at the time of delivery) to estimate a fetus's risk of NTD as well as possible increased risk of perinatal complications, such as premature rupture of the membranes, abruptio placentae, or fetal death.
· Ultrasound is performed when increased risk of open NTD exists, based on family history or abnormal serum screening results (not conclusive for open NTDs or ventral wall defects).
If the NTD isn't diagnosed before birth, other tests are used to make the diagnosis, including:
· palpation and spinal X-ray for spina bifida occulta
· myelography to differentiate spina bifida occulta from other spinal abnormalities, especially spinal cord tumors
· transillumination of the protruding sac to distinguish between myelomeningocele (typically doesn't transilluminate) and meningocele (typically transilluminates)
· pinprick examination of the legs and trunk to show the level of sensory and motor involvement in myelomeningocele
· skull X-rays, cephalic measurements, and computed tomography scan to demonstrate associated hydrocephalus.
· Spina bifida occulta: usually no treatment
· Meningocele: surgical closure of protruding sac
· Myelomeningocele: repair of the sac (doesn't reverse neurologic defects); shunt to relieve hydrocephalus if needed; supportive measures to promote independence and prevent further complications
TYPES OF SPINA BIFIDA