Central nervous system birth defects

Birth defects of the central nervous system are known as neural tube defects (NTDs). They include spina bifida, anencephaly and encephalocele. All are present at birth and result from a problem in the development of the brain and/or spinal cord in the developing baby (fetus).

How the central nervous system develops#

The central nervous system consists of the brain and spinal cord. Both are wrapped in a thin lining called the meninges and surrounded by cerebrospinal fluid. The brain connects to the spinal cord through the brain stem at the base of the brain. The spinal cord runs the length of the backbone, protected by the bones (vertebrae) of the spine, and nerves branch off it to reach the rest of the body.

In a developing fetus, the central nervous system begins as a simple structure called the neural groove, which folds in to form the neural tube. The tube then develops into the spinal cord and brain. As it forms, the neural tube “zips up” along its length to close and protect these structures. By about day 28 after conception, the tube should be closed and fused.

If the neural tube does not close along part of its length, the baby will have a neural tube defect. The type of defect (spina bifida, anencephaly or encephalocele) depends on where the tube failed to close, leaving parts of the brain and/or spinal cord exposed. In many cases these defects can be diagnosed during pregnancy with ultrasound scans and, rarely, with other tests such as amniocentesis (analyzing a sample of amniotic fluid).

Risk factors and prevention#

Neural tube defects are thought to be caused by a combination of genetic and environmental factors. These include:

• Folate deficiency – if the mother lacks certain nutrients, especially the B-group vitamin folate (folic acid), the chance of having a baby with an NTD increases. If folate is taken before and during at least the first four weeks of pregnancy, around 7 out of 10 cases of NTDs can be prevented. Talk to your doctor about how much folate you should take if you are thinking of becoming pregnant.
• Genetics – the exact genetic link is unclear, but a woman is at increased risk if she has a close relative who has had a baby with the condition (a family history). A woman who has already had a child with a neural tube defect is also at increased risk in later pregnancies. A personal or family history can affect how much folate is needed to reduce the risk.
• Some medications – including certain medications used to treat and control epilepsy.

In some cases there is evidence that certain forms of neural tube defect are caused by specific genetic changes (mutations) not related to folate. Here the defect arises when the baby inherits faulty gene copies from both parents, which prevent the baby from making use of folate even when enough is present. In these cases, taking folate before and during pregnancy will not prevent the condition.

Spina bifida#

In spina bifida, the spinal cord and nerves protrude through a gap created where the neural tube failed to close. This can affect the nerves that spread from this area into the abdomen and legs. Spina bifida can occur anywhere along the spine but more commonly appears in the lower back. Nine out of 10 affected babies also have a build-up of cerebrospinal fluid inside the brain, a condition called hydrocephalus and sometimes referred to as “water on the brain.” The incidence of spina bifida is around one in every 1,235 births.

Spina bifida can be mild, moderate or severe, and is graded by the degree of the defect:

• Occulta – the vertebrae have not closed completely, but the spinal cord is unharmed. The characteristic soft lump may be absent, which is why this form is sometimes diagnosed later in life.
• Meningocele – the membrane (meninges) covering the spinal cord bulges out through the gap in the spine.
• Myelomeningocele – the meninges, spinal cord and blood vessels protrude through the gap.

Spina bifida is incurable. The main treatment is surgery to seal the gap. If the baby has hydrocephalus, a shunt is inserted into the brain to drain the excess cerebrospinal fluid.

Anencephaly#

When the neural tube does not close at the head, the uppermost brain tissue, the meninges, the top of the skull (calvarium) and the scalp may be partially or completely missing. For reasons that are unknown, anencephaly occurs twice as often in females as in males, and around one in 10 affected babies is one of a pair of twins. In some cases the baby also has other problems such as congenital heart disease and cleft palate. The incidence of anencephaly is around one in every 1,370 births. A baby with anencephaly cannot survive; most are stillborn or die within a few days of birth.

Encephalocele#

In this rare neural tube defect, the meninges and brain tissue bulge out through a gap in the skull. In severe cases the bundle of brain tissue covered by the meninges may be larger than the baby’s head. Infection can occur if the membrane breaks and exposes the brain tissue. Other associated problems can include cleft lip or palate, extra fingers (polydactyly) and abnormalities of the sex organs. The incidence of encephalocele is around one in every 6,667 births.

If a significant amount of brain tissue has been pushed out, surgery may not be possible. Treatment options include using a shunt to drain a fluid build-up (treating hydrocephalus) if necessary. The baby may experience a range of difficulties, including intellectual impairment, difficulty controlling muscles (spasticity) and fits (convulsions). Physiotherapy and anticonvulsant medication may help.

Key points#

• Neural tube defects are thought to be caused by a range of genetic and environmental factors working in combination.
• If folate (folic acid) is taken before and during at least the first four weeks of pregnancy, around 7 out of 10 cases of NTDs can be prevented.
• Some defects are caused by the baby inheriting faulty gene copies from both parents; in these cases taking folate will not prevent the condition.

Where to get help#