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microcephaly
Microcephaly is a neurodevelopmental disorder. It is an important neurologic sign, but no uniformity exists in its definition. It is usually defined as a head circumference (HC) more than two standard deviations below the mean for age and gender. Some academics advocate defining it as head circumference more than three standard deviations below the mean for the 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. A homozygous mutation in one of the ''microcephalin'' 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.
==Signs and symptoms==
Affected newborns generally have striking neurological defects and seizures. Severely impaired intellectual development is common, but disturbances in motor functions may not appear until later in life.
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 intellectual disability 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.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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