An encephalocele is a defect characterized by the herniation of brain tissue and membranes through an opening in the cranium.
Description
Encephlaoceles are classified as neural tube defects, which are a group of disorders occurring due to the failure of closure of the neural tube at about week four of fetal development.
Other neural tube defects include anencephaly and spina bifida. Anencephaly results from failure of closure of the cranial end of the neural tube. This is a lethal condition. Spina bifida results from failure of neural tube closure in the spine. Spina bifida is a variable condition that is usually not lethal, but causes problems with bladder and bowel control and ambulation. It is usually associated with hydrocephalus (water on the brain) which can be treated with a shunt to drain the fluid into the body cavity. Encephalocele is the most rare neural tube defect.
Encephaloceles are classified according to their location. Occipital (arising at the back of the head where the head meets the neck) encephaloceles occur in 75% of cases, parietal encephaloceles in 10%, and anterior encephaloceles (arising from the base of the nose) in 15%. Anterioposterior encephaloceles have a poorer prognosis.
Genetic profile
The genetics of neural tube defects, including encephalocele, are not well understood.
Most encephaloceles are sporadic, following a multi-factorial pattern (genetic and environmental factors involved) of inheritance. It is known that there is a genetic basis to encephaloceles and other neural tube defects, and it is believed that neural tube defects may be caused by different genetic factors in different subsets of families. Proof that genetic factors contribute to encephaloceles is that it is
known to run in families, and it has been seen in association with some chromosome abnormalities. The number of genes and their location is still not known.
Occipital encephaloceles are associated with several single gene syndromes, including Meckle syndrome, dyssegmental dwarfism, Knobloch syndrome, Warburg syndrome, cryptophthalmos, and Voss syndrome. Anterior encephalocele may occur with frontonasal dysplasia. Encephalocele can also be seen in the amniotic band syndrome.
Demographics
The frequency of encephalocele has been reported to be between one in 2,000 to one in 5,000 live births. Anterior encephalocele is more common in Africa, Thailand and India. Females outnumber males for occipital encephalocele but not other types.
The incidence of all neural tube defects is different in different parts of the world. It is highest in northern Europe, specifically the British Isles and especially South Wales. In the United States, it is higher on the East Coast than the West Coast.
The rate of sporadic neural tube defects in the general population is about one in 1,000. The rate is higher in areas with higher incidence. The chance for a recurrence of a neural tube defect after having an affected child is 2%. After two affected children the risk is 10%.
The chance for an affected person to have an affected child is 4%. The chance for a second degree relative to have an affected child is 0.5%. Third degree relatives do not have an increased risk. Recurrence risks are given for neural tube defects as a group. A family with a previous child with anencephaly could have a child with spina bifida or encephalocele (the types do not "breed true" in families).
Care must be taken to be sure that the neural tube defect in the family was sporadic and not associated with a genetic syndrome, which would have a higher risk of recurrence.
The size of the cerebral and skull abnormalities associated with encephaloceles are variable. Large encephaloceles are usually associated with microcephaly (abnormally small head). Microcephaly is usually associated with mental retardation.
Occipital encephalocele may be asymptomatic. If the ventricles are involved, hydrocephalus may occur. Anterior encephalocele may progress in size and may be solid, cystic, or both. There may be microcephaly and/or hydrocephaly, ocular hypertelorism (widespaced eyes), and cleft palate. There may be problems with vision, breathing, and feeding in patients with anterior encephaloceles. Many patients have mental retardation.
Diagnosis
Encephalocele can be diagnosed by ultrasoundexamination. Ultrasound examination is a screening test, the quality of which is affected by many factors including the machine used, skill of the operator, size and location of the lesion, and position of the fetus.
It is not likely that maternal serum alpha-fetoprotein testing (AFP) or amniocentesis would detect encephalocele. Alpha fetoprotein is a normal serum protein produced by the fetal liver. The AFP normally stays within the fetus, with a small amount present in the amniotic fluid from the fetal urine. When there is an "open" neural tube defect, there is a high amount of AFP in the amniotic fluid and the maternal serum. Although encephalocele is a neural tube defect, AFP testing on maternal blood or amniotic fluid only detects open neural tube defects. Encephaloceles are closed neural tube defects, meaning they are covered by a thick covering. This covering does not allow the AFP to leak into the maternal blood or the amniotic fluid in increased amounts that would be detected by the aforementioned tests. Pregnancies in which an encephalocele is diagnosed should be offered an amniocentesis and amniotic fluid biochemistry to better understand the cause of the disorder.
CT scan can be used to determine the contents of the encephalocele once the baby is born. Some centers offer fetal MRI to attempt to classify the encephalocele prior to deliver. This is usually done at 22 weeks gestation.
Treatment and management
Nutrition, specifically deficiency of folic acid, has been implicated as causing an increased risk for neural tube defects. All women of childbearing age should take 0.4 mg of folic acid to reduce the risk of birth defects. Women with a previous child with a neural tube defect should take 4.0 mg of folic acid. This amount has been shown to reduce the recurrence risk for neural tube defects by 50%.
Prognosis
Size, location, and contents of the encephalocele determine the outcome for the child. Anterior encephaloceles have a much better prognosis than posterior. Mortality due to occipital encephalocele is reported as about 30% if hydrocephalus is present, and 2% if it is not. For all types of encephalocele with hydrocephalus, the mortality rate is 60%. Most patients with parietal encephalocele have associated brain malformations, and mental retardation occurs in 40%. Massive occipital encephalocele with microcephaly have a mortality rate of nearly 100%. Patients with encephaloceles that contain a single frontal lobe are more likely to have normal intelligence without hydrocephalus. Posterior have a poorer prognosis if they contain large amounts of the contents of the posterior fossa (an area of the brain at the back of the head), especially the brain stem. Complications such as hemorrhage or air embolism (stroke) can occur.
BOOKS
Goodman, Richard M., and Robert J. Gorlin. Encephalocele. New York: Oxford University Press, 1983.
ORGANIZATIONS
Association of Birth Defects in Children. 930 Woodcock Rd., Suite 225, Orlando, FL 32803. (407) 895-0802. <http://www.biethdefects.org>.
March of Dimes Birth Defects Foundation. 1275 Mamaroneck Ave., White Plains, NY 10605. (888) 663-4637. resourcecenter@modimes.org. <http://www.modimes.org>.
