Normal closure of the neural tube occurs during the fourth week of development. Many anomalies can occur here as the different tissues (meninges, vertebral arches, muscles, and skin) close around the spinal cord. Spina bifida refers to a failure of the vertebral arches to close and ranges from clinically insignificant to extremely dangerous. The anomaly is broken up into two groups:

• spina bifida occulta
  
• spina bifida cystica (due to a cystlike sac they all have in common) which includes
  -spina bifida with meningocele
  -spina bifida with meningomyelocele
  -spina bifida with myeloschisis
  

Spina bifida occulta - usually no clinical symptoms; about 10% of people; the embryonic halves of the vertebrae do not fuse but are covered by skin; may be visible as a small dimple covered by a patch of hair

Spina bifida cystica - 1:1000 births; spinal cord and/or meninges protrude through vertebral arches; associated with a cystlike sac; may involve neurological abnormalities, lack of sensation of corresponding dermatome, partial/complete paralysis of skeletal muscle; suspected at high levels of alpha-fetoprotein (AFP) in the amniotic fluid (via amniocentesis), confirmed by ultrasound
-Spina bifida with meningocele - sac contains meninges and cerebrospinal fluid; the spinal cord
and root are in normal position but may be abnormal; rare

-Spina bifida with meningomyelocele (myelo - spinal cord) - the spinal cord and/or nerve roots are included in the sac, covered by skin or very fine membrane; more common and more severe than meningoceles; may be associated with meroanencephaly or anencephaly, partial absence of the brain (seen below)

Spina bifida with myeloschisis - most severe type of spina bifida; the caudal neuropore fails to close at the end of the fourth week, resulting in an open, affected area

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