Some demonstrate immediate signs of neurologic dysfunction such as hypotonicity, hypertonicity, or an abnormal cry, whereas others function surprisingly normally for the first few months of life. Acquired microcephaly may result from perinatal infections such as herpes simplex, from intrapartum or neonatal hypoxic-ischemic insults, and from metabolic causes, including aminoacidurias and hypothyroidism. In these patients, head circumference is normal at birth, but the rate of brain growth is impaired, with resultant microcephaly. Anencephaly is a variable malformation of brain development in which the cerebral hemispheres are virtually absent and brain stem structures are variably affected. There is a variable inherited component to this disorder.
Antenatal diagnosis is aided by finding of elevated alpha-fetoprotein levels in amniocentesis.
Furthurmore, the accompanying absence of bony structures of the skull leads to a characteristic froglike appearance on antenatal ultrasound. There is no possibility for long-term survival in these patients. This is a feature of many syndromes including Mueller-Dieker syndrome. The clinical picture includes microcephaly, hypotonia, and seizures, most commonly infantile spasms. Pachygyria is characterized by relatively few broad gyri and shallow sulci.