Congenital Brain Defects Health Article

Causes

Congenital brain defects may have genetic, infectious, toxic, or traumatic causes. In most cases, no certain cause can be identified.

GENETIC CAUSES. Some brain defects are caused by trisomy, the inclusion of a third copy of a chromosome normally occurring in pairs. Most trisomies occur because of improper division of the chromosomes during formation of eggs or sperm. Trisomy of chromosome 9 can cause some cases of Dandy-Walker and Chiari II malformation. Some cases of holoprosencephaly are caused by trisomy of chromosome 13, while others are due to abnormalities in chromosomes 7 or 18. Individual gene defects, either inherited or spontaneous, are responsible for other cases of congenital brain malformations.

DRUGS. Drugs known to cause congenital brain defects when used by the mother during critical developmental periods include:

• anticonvulsant drugs
• retinoic acid and tretinoin
• warfarin
• alcohol
• cocaine

OTHER. Other causes of congenital brain defects include:

• intrauterine infections, including cytomegalovirus, rubella, herpes simplex, and varicella zoster
• maternal diabetes mellitus
• maternal phenylketonuria
• fetal trauma

Symptoms

Besides the features listed above, symptoms of congenital brain defects may include:

• Chiari II malformation: impaired swallowing and gag reflex, loss of the breathing reflex, facial paralysis, uncontrolled eye movements (nystagmus), impaired balance and gait.
• Dandy-Walker malformation: symptoms of hydro-cephalus, lack of muscle tone or "floppiness," seizures, spasticity, deafness, irritability, visual impairment, deterioration of consciousness, paralysis.
• Lissencephaly: lack of muscle tone, seizures, developmental delay, spasticity, cerebral palsy.
• Hydranencephaly: irritability, spasticity, seizures, temperature oscillations.
• megalencephaly due to neurological or metabolic disease: mental retardation, seizures.

Diagnosis

Congenital brain defects are diagnosed either from direct physical examination or imaging studies including computed tomography scans (CT) and magnetic resonance imaging (MRI) scans. Electroencephalography (EEG) may be used to reveal characteristic abnormalities.

Prenatal diagnosis of neural tube defects causing anencephaly or meningomyelocele is possible through ultrasound examination and maternal blood testing for alpha-fetoprotein, which is almost always elevated. Ultrasound can also be used to diagnose Dandy-Walker and Chiari II malformations. Amniocentesis may reveal trisomies or other chromosomal abnormalities.

Treatment

Meningomyelocele may be treated with surgery to close the open portion of the spinal cord. Surgery for encephalocele is possible only if there is a minimal amount of brain tissue protruding. Malformations associated with hydrocephalus (Dandy-Walker, Chiari II, and some cases of hydranencephaly) may be treated by installation of a drainage shunt for cerebrospinal fluid. Drugs may be used to treat some symptoms of brain defects, including seizures and spasticity.

Prognosis

Most congenital brain defects carry a very poor prognosis. Surgical treatment of meningomyelocele and encephalocele may be successful, with lasting neurological deficiencies that vary in severity. Early treatment of hydrocephalus may prevent more severe brain damage.

Prevention

Some cases of congenital brain defects can be prevented with good maternal nutrition, including folic acid supplements. Folic acid is a vitamin that has been shown to reduce the incidence of neural tube defects. Pregnant women should avoid exposure to infection, especially during the first trimester. Abstention from drugs and alcohol during pregnancy may reduce risk. Genetic counseling is advisable for parents who have had one child with anencephaly, since the likelihood of having another is increased.

BOOKS

Bradley, W. G., et al. Neurology in Clinical Practice. 2nd ed. Butterworth-Heinemann, 1996.

Fenichel, G. M. Clinical Pediatric Neurology. 3rd ed. W. B. Saunders Co., 1997.

Norman, M. G., et al. Congenital Malformations of the Brain. Oxford University Press, 1995.

Richard Robinson

KEY TERMS

Amniocentesis—Removal of fluid from the sac surrounding a fetus for purposes of diagnosis.

Cerebrospinal fluid—Fluid produced within the brain for nutrient transport and structural purposes. CSF circulates through the ventricles, open spaces within the brain, and drains through the membranes surrounding the brain.

Congenital—Defect present at birth.

Fetus—The unborn human, developing in a woman's uterus, from the eighth week after fertilization to birth.