Macrocephaly and Megalencephaly

Abstract This chapter reviews isolated and syndromic causes of macrocephaly and megalencephaly. The two terms are usually but not always interchangeable. The differential diagnosis of macrocephaly includes familial macrocephaly, chromosome duplications and deletions, metabolic disorders and complex somatic mosaic conditions including CLOVES, MCAP and MPPH. Single gene disorders with macrocephaly include those in the Noonan syndrome spectrum (RASopathies). Syndromes with hemimegalencephaly, including linear nevus sebaceous and tuberous sclerosis are briefly reviewed. Many syndromes with macrocephaly have associated overgrowth and are covered in the Overgrowth chapter as well, including Weaver, Sotos and Malan syndromes. A clinical case presentation features an infant with basal cell nevus syndrome.