Microcephaly is a medical condition in which the circumference of the head is smaller than normal because the brain has not developed properly or has stopped growing. Microcephaly can be present at birth or it may develop in the first few years of life. It is most often caused by genetic abnormalities that interfere with the growth of the cerebral cortex during the early months of fetal development. It is associated with Down’s syndrome, chromosomal syndromes, and neurometabolic syndromes. Babies may also be born with microcephaly if, during pregnancy, their mother abused drugs or alcohol, became infected with a cytomegalovirus, rubella (German measles), or vermicelli (chicken pox) virus, was exposed to certain toxic chemicals, or had untreated phenylketonuria (PKU). Babies born with microcephaly will have a smaller than normal head that will fail to grow as they progress through infancy.
In many cases, microcephaly may not be evident by ultrasound until the third trimester, and, therefore, may not be seen on ultrasounds performed earlier in pregnancy. The diagnosis of microcephaly may be made at birth or later in infancy. The baby's head circumference is much smaller than normal. During the physical examination, the physician obtains a complete prenatal and birth history of the child. In older babies and children, the physician may also ask if there is a family history of microcephaly or other medical problems. The physician will also ask about developmental milestones since microcephaly can be associated with other problems, such as mental retardation.
Infants with microcephaly are born with either a normal or reduced head size. Subsequently the head fails to grow while the face continues to develop at a normal rate, producing a child with a small head and a receding forehead, and a loose, often wrinkled scalp. As the child grows older, the smallness of the skull becomes more obvious, although the entire body also is often underweight and dwarfed. Development of motor functions and speech may be delayed. Hyperactivity and mental retardation are common occurrences, although the degree of each varies. Convulsions may also occur. Motor ability varies, ranging from clumsiness in some to spastic quadriplegia in others.
Causes of Microcephaly
Microcephaly can occur alone or in association with other health problems, and may occur from inheritance of an autosomal recessive, or rarely, an autosomal dominant gene. Acquired microcephaly may occur after birth due to various brain injuries such as lack of oxygen or infection.
Microcephaly may be inherited but the gene is considered to be extremely rare. In the general population, microcephaly due to genetic factors occurs in 1 in 30,000 to 50,000 live births, and in 1 per 10,000 births due to other causes. In some populations, frequency may be as high as 1 in 2,000 births.
Microcephaly can also be caused if the mother has an infection while pregnant or if she is exposed to harmful substances during early pregnancy, such as x-rays (a type of energy) or chemicals. Another cause of microcephaly is if the bones of the skull come together too early. For genetic (inherited) causes of microcephaly, see the next section.
Microcephaly is either caused by exposure to harmful substances during the fetal development, or it may be associated with genetic problems or syndromes that may have a tendency to run in families.
Microcephaly most often occurs because of failure of the brain to grow at a normal rate. Skull growth is determined by brain expansion, which takes place during the normal growth of the brain during pregnancy and infancy.
