Specific Imaging Findings Microcephaly is characterized by a reduced (at least <30%) cranio-facial ratio compared with age-matched normocephalic children. Primary congenital microcephaly refers to a small head size with relatively preserved brain architecture. Microcephalywith simplified gyral pattern shows a reduced number of gyri and shallow sulci (one-quarter to one-half of normal depth) without thickening of the cerebral cortex (cortical thickness <4 mm). There is a correlation between the degree of microcephaly, the volume of white matter and the presence of simplified gyral pattern. Reduced white matter bulk and enlarged extra-axial spaces are frequent accompanying features. Other developmental brain anomalies are often associated, including callosal agenesis, dysgenesis or hypoplasia, periventricular nodular heterotopias, pontocerebellar hypoplasia, and abnormal myelination. Cortical malformation such as lissencephaly and polymicrogyria may also be present. Secondary microcephaly may present white matter T2 hyperintensities (consistent with gliosis, cavitations, demyelination), cortical malformations, and calcifications. Pertinent Clinical Information Microcephaly is a clinical observation defined as a head circumference over 2 standard deviations below the mean for age and gender; severe microcephaly is characterized by a head circumference over 3 standard deviations below the mean. Microcephaly may be present at birth, or it may become evident in the first few years of life when the head fails to grow while the face continues to develop at a normal rate, producing a child with a small head, a relatively large face, and slanted forehead. Patients may present with mental retardation (50%), epilepsy (40%), cerebral palsy (20%), and ophthalmologic disorders (20-50%).
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