Definition of Microcephaly
Microcephaly is a condition in which an infant’s head circumference (the distance around the top of the head) is smaller than normal compared with other infants of the same age. Microcephaly is a head circumference greater than two standard deviations below the mean (i.e., less than the 3rd percentile).
Depending on the underlying cause, an infant may be born with microcephaly, or it can develop over time. According to the CDC, microcephaly occurs in about 2 to 12 times per 10,000 live births in the United States.
What Causes Microcephaly?
Microcephaly is generally the end result of processes that alter brain growth or cause abnormal brain development. Underlying causes can be divided into conditions specific to the infant and conditions or exposures that affected the expectant mother.
Conditions resulting in microcephaly which are specific to the infant include chromosomal abnormalities like Down Syndrome (trisomy 21) and abnormal growth of the skull bones (craniosynostosis).
Maternal exposures that can cause microcephaly include alcohol, drugs, and other toxic chemicals. Microcephaly can also occur when an expectant mother is exposed to infections such as toxoplasmosis, cytomegalovirus, rubella (German measles), or varicella (chicken pox). More recently, the mosquito-borne Zika virus has been linked to an increase in the occurrence of microcephaly in newborns.
Microcephaly may also occur when a mother has uncontrolled PKU (phenylketonuria), severe malnutrition, or any abnormality during pregnancy which results in loss of oxygen to the developing fetal brain.
What are the Associated Symptoms and Complications of Microcephaly?
Microcephaly may be isolated, meaning there are no other symptoms or problems, or it may be accompanied by other birth defects. Birth defects may be recognized early while other complications may become apparent over time. The associated symptoms of microcephaly vary depending on the underlying cause. Symptoms and complications may include:
Impairment in cognitive development/mental disability
Developmental delay (speech or motor difficulties)
Abnormalities of movement, balance, or coordination
Abnormalities of the face
Impairment of hearing or vision