Exencephaly is a rare and severe medical condition. This article provides clear information about exencephaly, including its nature, contributing factors, prenatal detection, typical progression, and prevention strategies.
What Is Exencephaly?
Exencephaly is a rare and severe type of neural tube defect (NTD) affecting fetal development. It occurs when the neural tube, which forms the early brain and spinal cord, fails to close properly during the first few weeks of pregnancy. This failure leads to the absence of the fetal skull bones, leaving the developing brain tissue exposed outside the skull.
This condition is distinct from other neural tube defects like spina bifida, where the spinal cord is affected. While both are neural tube defects, exencephaly specifically involves the head and brain, resulting in a distinct physical presentation. The absence of the skullcap, also known as acrania, allows the brain to remain unprotected.
Causes and Risk Factors
The development of exencephaly, like other neural tube defects, is generally considered multifactorial, meaning it arises from a combination of genetic and environmental influences rather than a single cause. In many instances, the specific reason for its occurrence remains unidentified. Genetic predispositions can play a role, as changes in various genes in both the fetus and the mother might influence the risk. However, it is not typically a simple inherited trait.
Environmental exposures during early pregnancy also contribute to the risk. Certain medications, especially some anti-seizure drugs, have been associated with an increased likelihood of neural tube defects. Maternal hyperthermia, resulting from high fevers or excessive heat exposure like saunas, during the early stages of gestation can also elevate the risk. Furthermore, underlying maternal health conditions, such as unmanaged diabetes, are recognized as risk factors.
A significant contributing factor is a deficiency in folate, a B vitamin. Folate is naturally found in foods, while folic acid is its synthetic form used in supplements. Insufficient folate levels in the mother’s system around the time of conception and early pregnancy are strongly linked to an increased risk of neural tube defects, including exencephaly.
Diagnosis During Pregnancy
Exencephaly is typically identified during prenatal screening, often through routine ultrasound examinations. This condition can be visible as early as the late first trimester or early second trimester. A sonographer would observe the absence of the skullcap, a condition referred to as acrania, with disorganized brain tissue visible outside the fetal head.
In some cases, an elevated level of alpha-fetoprotein (AFP) in maternal serum screening can serve as an early indicator. This blood test can prompt further investigation, leading to a more detailed ultrasound to assess for neural tube defects. If elevated AFP levels are found, amniocentesis may also be performed, which can detect increased AFP in the amniotic fluid.
Prognosis and Progression to Anencephaly
Exencephaly is a condition that is not compatible with life after birth. The prognosis for infants born with this malformation is exceptionally poor, with most not surviving for long, often only hours or minutes. A diagnosis typically allows for parental counseling to discuss options, which may include carrying the pregnancy to term with a focus on palliative care planning.
The condition is considered an embryological precursor to anencephaly, a more severe form of neural tube defect. The exposed and unprotected brain tissue in exencephaly is progressively damaged throughout gestation. This damage occurs due to direct exposure to the amniotic fluid, as well as mechanical and chemical trauma within the intrauterine environment. This degenerative process leads to the near or complete absence of brain tissue by the time of birth, resulting in the diagnosis of anencephaly. This explains why a fetus initially diagnosed with exencephaly may later be described as having anencephaly.
Prevention Strategies
Prevention strategies for neural tube defects, including exencephaly, primarily focus on ensuring adequate folic acid intake. Folic acid supplementation can significantly reduce the risk of these conditions. Health organizations recommend that all women of childbearing age consume 400 micrograms (0.4 mg) of folic acid daily. This recommendation is particularly important because neural tube defects occur very early in pregnancy, often before a woman realizes she is pregnant.
For women who have had a previous pregnancy affected by a neural tube defect, a higher dosage of folic acid, typically 4,000 micrograms (4 mg) per day, is recommended. This increased supplementation should begin at least one month before conception and continue through the first trimester of pregnancy. While folic acid greatly reduces the risk, it does not eliminate it entirely, as other factors can also contribute to the development of these defects.