Microcephaly is a neurodevelopmental disorder in which the circumference of the head is more than two standard deviations smaller than average for the person's age and sex. Microcephaly may be congenital or it may develop in the first few years of life. The disorder may stem from a wide variety of conditions that cause abnormal growth of the brain, or from syndromes associated with chromosomal abnormalities. Two copies of a loss-of-function mutation in one of themicrocephalin genes causes primary microcephaly.
In general, life expectancy for individuals with microcephaly is reduced and the prognosis for normal brain function is poor. The prognosis varies depending on the presence of associated abnormalities.
Microcephaly is a type of cephalic disorder.
A genetic factor may play a role in causing some cases of microcephaly. Relations have been found between autism, duplications of chromosomes and macrocephaly on one side. On the other side a relation has been found between schizophrenia, deletions of chromosomes and microcephaly
"Microcephaly" means "small head". "Microencephaly" means "small brain". Because the size of the head is mostly determined by the size of the brain, microencephaly is implied when discussing microcephaly
Microcephaly can also be associated with other conditions that are only indirectly associated with the nervous system:
- alcoholism (which can result in the fetal alcohol syndrome disability)
- varicella zoster virus (Chickenpox)
- rubella (German measles)
- CMV (human cytomegaovirus)
After the dropping of atomic bombs on Hiroshima and Nagasaki, several women close to ground zero who had been pregnant at the time gave birth to children with microcephaly. A total of seven of the in utero children at Hiroshima were affected.Microcephaly prevalence was 7 out of a group of 11 pregnant women who held the distinction of surviving the blast at an distance of ≈1 km from ground zero. Due to their proximity to the bomb, the pregnant women's in utero children received a biologically significant radiation dosage that was relatively high due to the massive neutron output of the lower explosive-yielding Little Boy.Microcephaly is the only proven malformation, or congenital abnormality, found in the children of Hiroshima and Nagasaki