| Anencephaly | |
|---|---|
| Classification and external resources | |
A front view of an anencephalic fetus |
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| ICD-10 | Q00.0 |
| ICD-9 | 740.0 |
| OMIM | 206500 |
| DiseasesDB | 705 |
| eMedicine | neuro/639 |
| MeSH | C10.500.680.196 |
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.[1] Strictly speaking, the translation of the Greek term to English is "no brain" (that is, totally lacking), but it is accepted that children with this disorder are born without a forebrain, the largest part of the brain consisting mainly of the cerebral hemispheres, including the neocortex, which is responsible for higher-level cognition, i.e. thinking. The remaining brain tissue is often exposed — not covered by bone or skin.[2] Most babies with this genetic disorder do not survive birth; whether they are technically born (if no brain is present) is an ethical and legal issue.
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The National Institute of Neurological Disorders and Stroke (NINDS) describes the presentation of this condition as follows: "A baby born with anencephaly is usually blind, deaf, unconscious, and unable to feel pain. Although some individuals with anencephaly may be born with a main brain stem, the lack of a functioning cerebrum permanently rules out the possibility of ever gaining consciousness. Reflex actions such as breathing and responses to sound or touch occur."[2]
The cause of anencephaly is disputed. Generally, neural tube defects do not follow direct patterns of heredity, though there is some indirect evidence of inheritance,[3] and recent animal models indicate a possible association with deficiencies of the transcription factor TEAD2.[4] Studies show that a woman who has had one child with a neural tube defect such as anencephaly has about a 3% risk of having another child with a neural tube defect.[5]
It is known that women taking certain medications for epilepsy and women with insulin-dependent diabetes have a higher risk of having a child with a neural tube defect. [6] 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,[2] 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.
Anencephaly and other physical and mental deformities have also been blamed on a high exposure to such toxins as lead, chromium, mercury, and nickel.[7]
Until recently, medical literature did not indicate a connection among many genetic disorders, both genetic syndromes and genetic diseases, that are now being found to be related. As a result of new genetic research, some of these are, in fact, highly related in their root cause despite the widely varying set of medical symptoms that are clinically visible in the disorders. Anencephaly is one such disease, part of an emerging class of diseases called ciliopathies. The underlying cause may be a dysfunctional molecular mechanism in the primary cilia structures of the cell, organelles which are present in many cellular types throughout the human body. The cilia defects adversely affect "numerous critical developmental signaling pathways" essential to cellular development and thus offer a plausible hypothesis for the often multi-symptom nature of a large set of syndromes and diseases. Known ciliopathies include primary ciliary dyskinesia, Bardet-Biedl syndrome, polycystic kidney and liver disease, nephronophthisis, Alstrom syndrome, Meckel-Gruber syndrome, and some forms of retinal degeneration.[8]
Anencephaly can often be diagnosed before birth through an ultrasound examination. The maternal serum alpha-fetoprotein (AFP screening)[9] and detailed fetal ultrasound[10] can be useful for screening for neural tube defects such as spina bifida or anencephaly.
There is no cure or standard treatment for anencephaly and the prognosis for patients is death. Most anencephalic fetuses 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.[2][11] There are longer-surviving examples, namely Baby K, who lived for 2 years 174 days.
In almost all cases, anencephalic infants are not aggressively resuscitated because 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. Other clinicians and medical ethicists may view even the provision of nutrition and hydration as medically futile.
In the United States, anencephaly occurs in about 1 out of every 10,000 births.[12] Research has suggested that, overall, female babies are more likely to be affected by the disorder.[13]
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