Acrania is a rare congenital abnormality characterized by complete or partial absence of flat bones of cranial vault (skull bones) in the human fetus. However brain tissue of fetus develops completely but abnormally. The cranial vault, or calvarium, surrounds and encloses the brain, and is formed from several plates of bone like parietal, squamosal, occipital, temporal, and frontal bones above the supraciliary ridge. In the embryo, the vault bones develop through ossification of the ectomeninx, the outer membranous layer surrounding the brain. The normal migration of mesenchymal tissue under the calvarial ectoderm should occur by the 4th week of gestation. However the closing of anterior neuropore and lack of mesenchymal migration (responsible for cranium formation) in the fourth week of embryological age results in Acrania.
With the absence of the skull, brain floats in the amniotic fluid over the base of skull. Increased amount of amniotic fluid is usually observed. Without the bony structure, the overlying ectoderm, as a thin amnion-like membrane covers the brain tissue.The fetus will generally have normal facial structures.
Acrania is always associated with severe condition called Anencephaly where most of the brain portions are absent along with skull and scalp. In most cases, Acrania progresses to Anencephaly, because, without the cranium, the cerebral tissue is exposed and thus gets degenerated resulting in eventual loss.
Diagnosis of Acrania
Acrania can be diagnosed early in pregnancy through an ultrasound. This anomaly appears during the the fourth week of the fetus's development. However The fetal cranium is not fully calcified before 10–11 weeks; therefore, diagnosis must be confirmed only after 11th week.
There is no treatment for Acrania. The condition is termed as incompatible with life. If at all the fetus survives the term, babies are either still born or die within few hours of birth. Since Acrania is a fatal pathology, medical abortion, as soon as the confirmed diagnosis, is suggested. This will help minimize medical interventions and also reduce the emotional agony to the mother and the family.
Anencephaly is a rare birth defect in which major parts of the brain, scalp and skull of the fetus fail to develop completely. Anencephaly is a type of neural tube defect wherein the neural tube which is flat initially and later folds and closes together to form a tube, fails to close.
Infants with Anencephaly are born without cerebrum or cerebellum responsible for thinking and coordination. However they do have brain stem that allows them to breathe and their hearts to beat. Whatever brain tissue that develops, often is not covered by bone or skin. Babies born with Anencephaly are often still born or die within few hours or days of birth.
The exact causes of Anencephaly are not known. However it is proven that the deficiency of folic acid prior to and during pregnancy may increase the chances of neural tube defects. Pregnant women taking certain prescription drugs for diabetes and anti depressants are also at higher risk of giving birth to anencephalic babies. Some of the other factors that may put the expectant mother at risk are environmental pollution, exposure to household chemicals especially those containing teratogens substances, or other toxins. Anencephaly does not run in families; it results from a combination of genes inherited from both sides coupled with environmental factors.
Newborns with Anencephaly are born without bony covering over the back of the head and also around the front and sides of the head. Their ears are normally folded and their mouth has a cleft palate. Due to the absence of cerebrum, the baby stays in a vegetative state.
Anencephaly is diagnosed as early as at 11-14th week of pregnancy, with certainty by performing ultrasound scan. The condition is also suspected when a pregnant mother's serum test reveals elevated alphafetoprotein (AFP)levels. Higher levels of AFP are only indicative of neural tube diseases, but does not in any way, confirm the condition. Hence further tests such as amniocentesis along with high definition ultrasound are ordered to confirm the condition.
If the prenatal test, for any reason, is missed and the fetus survives the term, the very appearance of the infant allows the medical team to diagnose Anencephaly. A large portion of the skull of the infant will be absent and the scalp, which extends to the margin of the bone will also be missing with no recognizable cerebral hemispheres. Often acrania is another similar condition which is confused with Anencephaly.
There is no cure or treatment available for Anencephaly. These babies do not have chances of survival beyond few hours or days. Parents may require counseling services to cope up with the loss of their child.
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Collection of Pages - Last revised Date: September 19, 2021