megalencephaly

the diagnosis is suspected in the presence of macrocephaly (head circumference in the transthalamic plane > 2 standard deviations above the mean), and is one of exclusion. several other intracranial anomalies will give rise to macrocephaly, and these include hydrocephalus, holoprosencephaly, hydranencephaly, porencephaly, and intracranial tumours; in contrast to these, with benign familial megalencephaly the intracranial architecture will appear normal, although there may be borderline ventriculomegaly. unilateral megalencephaly may be suspected if there is asymmetry of the cerebral hemispheres and displacement of the falx in association with unilateral ventriculomegaly. hemihypertrophy of the body may also be present. due to the large number of associated syndromes a careful systemic survey should be undertaken. for example, thanatophoric dysplasia (micromelia) and hypochondroplasia (rhizomelia) are both associated with megalencephaly. for the same reason, karyotyping is recommended due to the associations with klinefelterís syndrome (47, xxy), fragile x, and occasional partial trisomies (chromosome 1[q42-qter] trisomy). in the absence of any sonographic or karyotypic abnormality, the prognosis is favourable, although developmental studies have suggested an increased risk of learning disability despite average intelligence.