Isolated hemihypertrophy/hemihyperplasia (IH) refers to asymmetric regional body overgrowth secondary to abnormal cell proliferation in individuals without any other underlying diagnosis. IH should be distinguished from hemiatrophy and from asymmetrical overgrowth that can be associated with Beckwith–Wiedemann syndrome (BWS), proteus syndrome, neurofibromatosis type 1, mosaic trisomy 8, and disorders associated with vascular malformations such as Klippel–Trenaunay syndrome. IH may be associated with a significant risk of learning disability, genitourinary abnormalities, and neoplasia (Viljeon et al. 1984). The most frequently associated neoplasm is Wilms Tumours, but there are also reported associations with adrenal cortical Tumours and hepatoblastoma (Viljeon et al. 1984).
Estimates of the Tumours risks associated with IH are variable (e.g., risk of Wilms Tumours 1 to 3 % (Viljeon et al. 1984; Dempsey-Robertson et al. 2012)) and may be influenced by ascertainment bias. In a prospective study of 168 children for 10 years, the Tumours incidence was ~6 % (6 Wilms and 2 adrenal cortical carcinomas out of the total of 10 Tumours) (Hoyme et al. 1998).
There is overlap between the molecular abnormalities described in BWS (e.g., paternal uniparental disomy for chromosome 11p15.5, imprinting center 1 epimutations) and those detectable in a subset of patients with IH (Grundy et al. 1991). However, it has been suggested that the results of such testing cannot reliably distinguish between those children who will and will not develop an embryonal Tumours and that Tumours surveillance (abdominal ultrasound every 3 months until 7 years and serum alpha-fetoprotein measurement every 3 months until 4 years) should be offered to all children with IH (Clericuzio and Martin 2009). In contrast, Scott et al. (2006) recommended that abdominal ultrasound screening should only be offered to children with hemihypertrophy with paternal uniparental disomy 11p15 or isolated H19 hypermethylation.