Anencephaly

Anencephaly arises from failure of closure of the rostral neuropore around day 24-26 of gestation, resulting in absence of the cerebral hemispheres and calvarium, with exposed, degenerated neural tissue. The condition is part of the neural tube defect spectrum, which also includes spina bifida (failure of caudal neuropore closure) and encephalocele. Risk factors include folate deficiency, maternal diabetes, certain antiepileptic drugs (e.g., valproic acid, carbamazepine) which inhibit folate metabolism, and maternal hyperthermia. Folic acid supplementation (400 mcg/day) before conception and during early pregnancy significantly reduces the risk of neural tube defects including anencephaly. Diagnosis is often made prenatally via elevated maternal serum/amniotic fluid AFP and acetylcholinesterase, along with characteristic ultrasound findings (absence of cranial vault, 'frog-like' appearance of the face). Polyhydramnios occurs due to impaired fetal swallowing. Anencephaly is uniformly fatal, with most cases resulting in stillbirth or death within hours to days after birth due to absence of the brainstem and higher brain structures necessary for survival. It is more common in females and has geographic/ethnic variation in incidence. Associated with other neural tube defects and sometimes craniorachischisis (complete failure of neural tube closure). Key distinguishing feature from encephalocele: in anencephaly, there is no skin or bony covering over the defect, whereas encephalocele has skin-covered herniation of brain tissue through a skull defect.