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23. Malformations of the nervous system

Learning objectives

  • What are neural tube defects?
  • When do the neural folds close?
  • Which are the three most common neural tube defects?
  • How can most neural tube defects be prevented?
  • What is spina bifida?
  • What is spina bifida occulta?
  • What is meningocoele?
  • What is myelomeningocoele?
  • What is anencephaly?
  • What is encephalocoele?
  • What is holoprosencephaly, and what is the underlying cause?
  • What is microcephaly, and how does it relate to brain development?

Malformations of the nervous system

Neural tube defects

Neural tube defects (NTDs) are a collection of developmental malformations which occur due to abnormal closure of the neural folds, which occurs in weeks 3 and 4. The most common neural tube defects are myelomeningocoele, anencephaly, and encephalocoele.

Etiology and prevention

Maternal supplementation of folic acid (vitamin B9) prevents most neural tube defects. For this reason, all women trying to conceive should take 400 µg folic acid supplementation 1 month before conception and during the whole pregnancy.

Medications which reduce maternal folic acid supply, including several antiepileptics and antibiotics, increase the risk for NTDs. Child-wanting women taking any of these medications should take a much larger daily dose of folic acid.

Spina bifida

Those neural tube defects which affect the spinal regions are called spina bifida. There are three types of spina bifida. They are spina bifida occulta, meningocoele, and myelomeningocoele. These defects affect the lumbosacral regions most commonly.

Spina bifida occulta is the mildest form. In this developmental malformation one or more vertebrae fail to close correctly, but the spinal cord, spinal meninges, and overlying skin remain intact. This defect is usually asymptomatic and is therefore detected incidentally. The overlying region is often covered with a tuft of hair, for some reason.

Meningocoele is the intermediate form. In this malformation one or more vertebrae fail to close correctly, and the spinal meninges protrude through the defect. The defect is not covered by skin. The spinal cord remains in its normal position.

Myelomeningocoele is the most severe form, and unfortunately the most common neural tube defect. It’s similar to meningocoele, except the spinal cord also protrudes through the defect. Almost all newborns with myelomeningocoele experience symptoms related to spinal cord dysfunction, most commonly urinary and faecal incontinence, and paralysis of the legs.

Almost all newborns with myelomeningocoele also have another developmental anomaly called the Chiari II malformation. It is characterised by herniation of part of the cerebellum into the foramen magnum. This obstructs outflow of CSF, causing accumulation of CSF in the brain, a condition called hydrocephalus.

Anencephaly

Anencephaly is a type of neural tube defect where the cranial neural folds don’t close, causing the developing brain to turn into a necrotic mass. The defect is incompatible with life.

Encephalocoele

Encephalocoele is a neural tube defect where brain and the leptomeninges protrude through an opening in the skull. The protrusion is covered by skin.

Holoprosencephaly

Holoprosencephaly is a developmental malformation of the brain characterised by failure of the prosencephalon to divide into two hemispheres. This failure may be complete, forming a telencephalic vesicle instead of two hemispheres, a defect which is incompatible with life. In less severe cases of holoprosencephaly, there is some division of the prosencephalon, but there is incomplete development of midline structures like the olfactory bulbs and corpus callosum.

Severe cases of holoprosencephaly are associated with midline defects of the face as well. This can range from the presence of a single upper incisor tooth to the presence of a single midline eye.

Microcephaly

Microcephaly refers to an abnormally small cranial vault. Contrary to what would seem logical, microcephaly is a result of an underdeveloped brain, and not the other way around. Head growth is normally driven by brain growth.

This defect is often a result of a genetic defect, or due to in utero exposure to a teratogen. Most people with this defect experience intellectual disability.

Summary

  • What are neural tube defects?
    • They’re malformations which occur due to abnormal closure of the neural folds
  • When do the neural folds close?
    • Weeks 3 and 4
  • Which are the three most common neural tube defects?
    • Myelomeningocoele, anencephaly, encephalocoele
  • How can most neural tube defects be prevented?
    • By maternal folic acid supplementation, 400 µg daily beginning 1 month before conception and lasting the whole pregnancy
  • What is spina bifida?
    • It’s a group of neural tube defects which affect the spinal region
  • What is spina bifida occulta?
    • The mildest form of spina bifida, where one or more vertebrae fail to close but no other abnormality is present
  • What is meningocoele?
    • A form of spina bifida where spinal meninges protrude through a defect in the vertebrae
  • What is myelomeningocoele?
    • The most severe form of spina bifida, where spinal meninges and spinal cord protrude through a defect in the vertebrae
  • What is anencephaly?
    • A neural tube defect where the cranial neural folds don’t close, causing the brain to turn into a necrotic mass
  • What is encephalocoele?
    • A neural tube defect where brain and leptomeninges protrude through an opening in the skull
  • What is holoprosencephaly, and what is the underlying cause?
    • A malformation caused by failure of the prosencephalon to divide into the two hemispheres
    • The severity depends on the degree of failure
    • It’s associated with midline defects of the face
  • What is microcephaly, and how does it relate to brain development?
    • A malformation where the cranial vault is abnormally small
    • It occurs due to decreased brain development