Anencephaly

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. Infants 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.

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Presentation

Infants born with anencephaly are usually blind, deaf, unconscious, and unable to feel pain. 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.^[1] Reflex actions such as breathing and responses to sound or touch may occur.

Diagnosis

Anencephaly can often be diagnosed before birth through an ultrasound examination. The maternal serum alpha-fetoprotein (AFP screening)^[2] and detailed fetal ultrasound^[3] can be useful for screening for neural tube defects such as spina bifida or anencephaly.

There are many false diagnoses for anencephaly, as it is not a common diagnosis, often confused with exencephaly or microcephaly. Also, sometimes a false prognosis stating that an anencephalic baby can live for years is given, but this cannot occur because the brain is open, meaning that infection sets in rapidly. The anencephalic brain is also usually very disorganised on a cellular level.

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.

Rate of occurrence

In the United States, approximately 1,000 to 2,000 babies are born with anencephaly each year. Female babies are more likely to be affected by the disorder.

Causes

The cause of anencephaly is unknown. Neural tube defects do not follow direct patterns of heredity, though there is some indirect evidence of inheritance^[4], and recent animal models indicating a possible association with deficiencies of the transcription factor TEAD2.^[5] Studies show that a woman who has had one child with a neural tube defect such as anencephaly, has about a 3% risk to have another child with a neural tube defect.^{[citation needed]}

It is known that women taking certain medication for epilepsy and women with insulin dependent diabetes have a higher chance of having a child with a neural tube defect. Genetic counseling is usually offered to women at a higher risk of having a child with a neural tube defect to discuss available testing.

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%.^[6] 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.

References

1. ^ Merker B (2007). "Consciousness without a cerebral cortex: a challenge for neuroscience and medicine". The Behavioral and brain sciences 30 (1): 63–81; discussion 81–134. doi:10.1017/S0140525X07000891. PMID 17475053.
2. ^ Joó JG, Beke A, Papp C, et al (2007). "Neural tube defects in the sample of genetic counselling". Prenat. Diagn. 27 (10): 912–21. doi:10.1002/pd.1801. PMID 17602445.
3. ^ Cedergren M, Selbing A (2006). "Detection of fetal structural abnormalities by an 11-14-week ultrasound dating scan in an unselected Swedish population". Acta obstetricia et gynecologica Scandinavica 85 (8): 912–5. doi:10.1080/00016340500448438. PMID 16862467.
4. ^ Shaffer LG, Marazita ML, Bodurtha J, Newlin A, Nance WE (1990). "Evidence for a major gene in familial anencephaly". Am. J. Med. Genet. 36 (1): 97–101. doi:10.1002/ajmg.1320360119. PMID 2333913.
5. ^ Kaneko KJ, Kohn MJ, Liu C, Depamphilis ML (2007). "Transcription factor TEAD2 is involved in neural tube closure". Genesis 45 (9): 577–87. doi:10.1002/dvg.20330. PMID 17868131.
6. ^ anencephaly at NINDS