Anencephaly is a term that refers to the incomplete development of the brain, skull, and scalp and is part of a group of birth defects called neural tube defects (NTD)where a sac containing brain/meninges/cerebrospinal fluid forms outside the skull due to a bone defect. The structure which will become the neural tube is supposed to fold and close together (to form a tube) during the third and fourth weeks of pregnancy. From this neural tube, the brain and spinal cord of the embryo develop. Neural tube defects happen when the neural tube does not close as expected. Anencephaly occurs when the end of the neural tube that would have developed into the brain does not close properly, failing the development of major portions of the brain, skull, and scalp. Other neural tube defects, such as spina bifida, form when the neural tube does not close properly in a different part of the neural tube.
Anencephaly is a pathology of development characterized by a fetus that has no calvarium, with a lack of most or all of the fetus’ brain tissue.[rx] Anencephaly belongs to a collective group known as neural tube defects (NTD) and is a result of the neural tube failing to close in its rostral end during fetal development.[rx] While the central nervous system (CNS) is developing in a fetus, the neural plate becomes folded and fused, creating the neural tube. Any disturbance to the process of neural tube closure can result in structural abnormalities collectively called neural tube defects. Anencephaly is one of the two main types resulting from the failure of closure of the rostral end of the neural tube.[rx] The central nervous system consists of the brain and spinal cord formed by folding of dorsal part neural plates under the influence of underlying notochord and prechordal mesoderm and closure of anterior (cranial) and posterior (caudal) neuropores by a process called neurulation that begins as early as 3 and 4 weeks of conception.
Infants with anencephaly are born without the front part of the brain, (forebrain) and the thinking and coordinating part of the brain (cerebral hemispheres and cerebellum). Most of the time the remaining brain tissue may be exposed, without a skull or scalp to cover and protect it. Although reflex actions such as breathing and responses to touch or sound may occur, gaining consciousness is not possible. Usually, infants with anencephaly do not survive more than a few days or weeks.
Meroanencephaly and holoanencephaly are terms that refer to the extent of the cranial defect, however, they typically are not used in clinical descriptions and are not predictive of the severity of the condition. The term acrania has been used interchangeably with anencephaly in some parts of the world but that practice is discouraged as it confuses two very different conditions.
Anencephaly can be classified as meroacrania if the foramen magnum is not involved, holoacrania if the defect goes beyond the foramen magnum, and holoacrania with rachischisis if it is associated with spina bifida [rx].
Anencephaly is a serious birth defect in which a baby is born without parts of the brain and skull. It is a type of neural tube defect (NTD). As the neural tube forms and closes, it helps form the baby’s brain and skull (upper part of the neural tube), spinal cord, and back bones (lower part of the neural tube).
Symptoms
Pregnancies affected by anencephaly show high levels of a specific fetal protein (called alpha fetoprotein) in tests done on the mother’s blood or amniotic fluid. The presence of excess fluid in the amniotic sac that contains the fetus (polyhydramnios) may be noted during an ultrasound.
Anencephaly may also be seen before birth using any one of several imaging techniques. The condition is characterized by the absence of the skull and parts of the brain (cerebral hemispheres and cerebellum). Abnormalities of facial features secondary to the absent skull are common and vary depending on the development of each fetus. Anencephaly usually (~80%) occurs without other birth defects.
Causes
Not all of the causes of anencephaly are understood. Anencephaly can be a multifactorial condition meaning that multiple genes are involved in interacting with environmental agents and chance events to cause the condition. Anencephaly can also be a feature of some chromosomal disorders such as trisomy 18 which are usually sporadic and not familial (inherited). Women who don’t get enough folic acid (vitamin B9) when they’re pregnant have a higher risk of having a baby with anencephaly. Women should take a prenatal vitamin with 400 micrograms (mcg) of folic acid before and during pregnancy.
Most times anencephaly occurs in a pregnancy where there is no family history of neutral tube defects. However, research has shown that once a woman has had one pregnancy that resulted in a fetus with a neural tube defect, any additional pregnancies have an increased risk that a similar defect will occur again (recurrence risk). This recurrence risk is estimated to be 3-4% compared to the background risk of much less than 1%. Researchers also expect that if a subsequent fetus has a neural tube defect, the defect, whether anencephaly or spina bifida, tends to be in a location similar to the prior pregnancy.
While not all the causes of anencephaly are known, many epidemiologic studies have demonstrated that folic acid supplementation before conception and during the first trimester can reduce the birth prevalence of spina bifida and anencephaly by at least 50%.
Isotretinoin, which is used as a dermatological drug, and selective serotonin reuptake inhibitors are associated with an increased risk of giving birth to an anencephalic fetus [rx]. Exposure to nitrates doubles the odds of neural tube defects in women whose occupations include working with cleaning products, spray paints or paint thinners [rx]. Also, nitrate exposure is found in farming communities, where the private wells might be contaminated by fertilizers and animal waste [rx]. Exposure to pesticides at home, at work, or in the community was associated to a higher risk of having a child with neural tube defects, especially when it comes to anencephaly [rx]. Organic solvents, such as aliphatic hydrocarbons, aromatic hydrocarbons, and their halogenated derivatives, halogenated hydrocarbons, aliphatic alcohols, glycols, and methoxy ethanol are associated with the development of anencephaly in fetuses whose mothers were exposed in the last five years before birth [rx].
Changes in dozens of genes in individuals with anencephaly and in their mothers may influence the risk of developing this type of neural tube defect. The best-studied of these genes is MTHFR, which provides instructions for making a protein that is involved in processing the vitamin folate (also called vitamin B9). While a shortage (deficiency) of this vitamin is an established risk factor for neural tube defects, there are many factors that can contribute to folate deficiency. Changes in other genes related to folate processing and genes involved in the development of the neural tube have also been studied as potential risk factors for anencephaly. However, no genes appear to play a major role in causing the condition.
Organic solvents have been used in the industry of varnishes and paints, production of colorants and explosives, production of natural and synthetic rubber, leather and shoe industry, enamels, adhesives, lacquers, and putty, production of polymers, and also during the cleaning and degreasing of industrial machinery [12].
Treatment
Prevention
The U.S. Public Health Service advises women of childbearing age to take 0.4 mg of folic acid daily, either through supplements or fortified foods, even if those women do not think they are likely to become pregnant. This is recommended because the neural tube forms early, often before women realize they are pregnant.
Women are urged not to take more than 1.0 mg of folic acid daily unless advised by a physician because high doses of folic acid may mask the diagnosis of severe B12 deficiency (pernicious anemia).
Some recommendations should be taken into account by neurologists to avoid neural tube defects, such as folic acid supplements or monitoring Lamotrigine, Carbamazepine, and Phenytoin levels. Valproate is considered the most teratogenic and therefore should never be given as the first line of treatment [rx]. More than that, Isotretinoin, which is used as a dermatological drug, and selective serotonin reuptake inhibitors are associated with an increased risk of giving birth to an anencephalic fetus [rx].
Women who have had a previous pregnancy affected by a neural tube defect are advised to consume 4 mg of folic acid beginning 30 days before conception through the first trimester under the care of their physician.
Palliative
Infants with anencephaly usually do not survive more than a few days or weeks. The role of healthcare providers is to provide a supportive environment that will enable the family to accept the diagnosis and make preparations for their loss.
References
