Anencephaly

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief:

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Anencephaly
The side view of the head of an anencephalic fetus
ICD-10	Q00.0
ICD-9	740.0
OMIM	206500
DiseasesDB	705
MeSH	C10.500.680.196

Overview

Anencephaly is a cephalic disorder that results from a neural tube defect that occurs when the cephalic (head) end of the neural tube fails to close, usually between the 23rd and 26th day of pregnancy, resulting in the absence of a major portion of the brain, skull, and scalp. Children with this disorder are born without a forebrain, the largest part of the brain consisting mainly of the cerebral hemispheres (which include the isocortex, which is responsible for higher level cognition, i.e., thinking). The remaining brain tissue is often exposed - not covered by bone or skin.

Pathophysiology

The cause of anencephaly is disputed.

Etiology and Risk factors

Folate deficiency

Studies show that most of the neural tube defects are caused by folic acid deficiency. This inadequate folate could be due to less oral intake, decreased intestinal absorption, or due to abnormal folate metabolism due to gene mutation. Some drugs antagonize the effect of folic acid resulting in folic acid deficiency and hence NTD. Most important ones are anti-epileptic drugs such as valproic acid and carbamazepine. Also, methotrexate, which is an antineoplastic drug also used for the treatment of ectopic pregnancy, has been linked with increased risk of NTD.

Genetics

Neural tube defects do not follow direct patterns of heredity, though there is some indirect evidence of inheritance, and recent animal models indicating a possible association with deficiencies of the transcription factor TEAD2. The motivation behind studying the genetic patterns is following:

1. NTDs are consistently prevalent among monozygotic twins as compared to dizygotic twins
2. There is a high recurrence rate within families. Statistics show the recurrence risk of 1/20 if one previous pregnancy is affected and 1/10 if two pregnancies are affected in a family.
3. Due to predilection towards female fetuses as compared to male

Syndromes

Anencephaly is associated with

1. Trisomy 13 or 18
2. Meckel-Gruber syndrome
3. Roberts syndrome
4. Jarcho-Levin syndrome
5. HARD (hydrocephalus, agyria and retinal dysplasia)
6. OEIS complex (omphalocele, exstrophy of the cloaca, imperforate anus and spinal defects)
7. Limb-body wall complex (LBWC)

Fever/hyperthermia

If pregnant mother’s core body temperature elevates from baseline it could lead to congenital anomalies such neural tube defects, including anencephaly. The risk is more profound if this happens during the first trimester as this is the time during which organogenesis takes place. The National Birth Defects Prevention Study (NBDPS) infers that the risk of birth defects due to maternal infection-related fever can be reduced by the usage of acetaminophen.

Amniotic bands

If a pregnant mother develops amniotic bands, it could effect the normal development and growth of the central nervous system. It could result in neural tube defects including anencephaly

Pregestational diabetes

If a woman has uncontrolled diabetes mellitus before conception, it could result in neural tube defects including anencephaly. Therefore, close monitoring of periconceptional glycemic level is essential to prevent neural tube defects and other congenital anomalies.

Maternal Obesity

Obesity doubles the risk of neural tube defects. Also, it provides a limitation to fetal imaging. Folate acid supplementation does not decrease this risk.

Toxins

Anencephaly and other physical and mental deformities have also been blamed on high exposure to such toxins as lead, chromium, mercury, and nickel.

Differentiating Anencephalopathy from other Disorders

There are many false diagnoses for anencephaly, as it is not a common diagnosis, often confused with exencephaly or microcephaly.

Epidemiology and Demographics

Incidence

In the United States, approximately 1,000 to 2,000 babies are born with anencephaly each year. In 2001, the National Center for Health Statistics reported 9.4 cases among 100,000 live births

Demographics

The highest prevalence has been seen among the Hispanic population. Female babies, whites and children born to mothers who are at extreme of ages are more likely to be affected by the disorder. Worldwide, Ireland and British Islands has higher prevalence as compared to Asia and Africa which has a lower prevalence rate.

Recurrence rate

Like any other neural tube defect, the recurrence rate of anencephaly is 2-4% if one sibling is affected and 10 percent if two siblings are affected. This familial tendency is due to genetics, environmental factors, or both.

Natural History, Complications, Prognosis

There is no cure or standard treatment for anencephaly and the prognosis for affected individuals is poor. Most anencephalic babies do not survive birth, accounting for 55% of non-aborted cases. If the infant is not stillborn, then he or she will usually die within a few hours or days after birth from cardiorespiratory arrest.

In almost all cases anencephalic infants are not aggressively resuscitated since there is no chance of the infant ever achieving a conscious existence. Instead, the usual clinical practice is to offer hydration, nutrition and comfort measures and to "let nature take its course". Artificial ventilation, surgery (to fix any co-existing congenital defects), and drug therapy (such as antibiotics) are usually regarded as futile efforts. Clinicians and medical ethicists may view the provision of nutrition and hydration as medically futile. Occasionally some may even go one step further to argue that euthanasia is morally and clinically appropriate in such cases.

Prenatal screening and diagnosis

American College of Obstetricians and Gynecologists (ACOG) and the American College of Medical Genetics and Genomics (ACMG) advocates universal screening of all the pregnant women for the NTD. This results in early diagnosis which helps the couple to plan either pregnancy termination or otherwise prepare them for the birth.

Ultrasound

Anencephaly can often be diagnosed before birth through an ultrasound examination. The maternal serum alpha-fetoprotein (AFP screening) and detailed fetal ultrasound can be useful for screening for neural tube defects such as spina bifida or anencephaly.

Clinical features

Infants born with anencephaly are usually blind, deaf, unconscious, and unable to feel pain.

Physical Examination

Neurologic

Reflex actions such as breathing and responses to sound or touch may occur. Although some individuals with anencephaly may be born with a rudimentary brainstem, which controls autonomic and regulatory function, the lack of a functioning cerebrum is usually thought of as ruling out the possibility of ever gaining consciousness, even though it has been disputed specifically.

Management

So far, prevention is the best management of anencephaly.

Surgery

At this point in time there are no surgical options available.

Prevention

Recent studies have shown that the addition of folic acid to the diet of women of child-bearing age may significantly reduce, although not eliminate, the incidence of neural tube defects. Therefore, it is recommended that all women of child-bearing age consume 0.4 mg of folic acid daily, especially those attempting to conceive or who may possibly conceive, as this can reduce the risk to 0.03%. It is not advisable to wait until pregnancy has begun, since by the time a woman knows she is pregnant, the critical time for the formation of a neural tube defect has usually already passed. A physician may prescribe even higher dosages of folic acid (4 mg/day) for women who have had a previous pregnancy with a neural tube defect.

Pregnancy management

Anencephaly has a poor pregnancy outcome. Majority of the fetuses are either stillborn or die soon after birth, however, few infants do manage to live up to 28 days. Therefore, mothers are counselled regarding the termination soon after the prenatal diagnosis. Vaginal delivery is the preferred mode unless there are any maternal indications

References

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