GLI3-related Pallister-Hall syndrome (GLI3-PHS) is characterized by a spectrum of anomalies ranging from polydactyly, asymptomatic bifid epiglottis, and hypothalamic hamartoma at the mild end to laryngotracheal cleft with neonatal lethality at the severe end. Individuals with mild GLI3-PHS may be incorrectly diagnosed as having isolated postaxial polydactyly type A. Individuals with GLI3-PHS can have pituitary insufficiency and may die as neonates from undiagnosed and untreated adrenal insufficiency.

Typical Greig cephalopolysyndactyly syndrome (GCPS) is characterized by macrocephaly, widely spaced eyes associated with increased interpupillary distance, preaxial polydactyly with or without postaxial polydactyly, and cutaneous syndactyly. Developmental delay, intellectual disability, or seizures appear to be uncommon manifestations (~<10%) of GCPS and may be more common in individuals with large (>300-kb) deletions that encompass GLI3. Approximately 20% of individuals with GCPS have hypoplasia or agenesis of the corpus callosum.

Bardet-Biedl syndrome-19 (BBS19) is an autosomal recessive ciliopathy characterized by obesity, impaired intellectual development, polydactyly, renal failure, retinitis pigmentosa, and hypogonadism (Aldahmesh et al., 2014). For a general phenotypic description and a discussion of genetic heterogeneity of Bardet-Biedl syndrome, see BBS1 (209900).

Synpolydactyly (SPD), or syndactyly type II, is defined as a connection between the middle and ring fingers and fourth and fifth toes, variably associated with postaxial polydactyly in the same digits. Minor local anomalies and various metacarpal or metatarsal abnormalities may be present (summary by Merlob and Grunebaum, 1986). In some families with SPD, the foot anomalies are characterized by preaxial as well as postaxial polydactyly, and appear to be fully penetrant. The more severe features of classic SPD, involving 3/4 synpolydactyly in the hands and 4/5 synpolydactyly in the feet, also occur, but at reduced penetrance. This foot phenotype is not seen in patients with classic SPD due to HOXD13 polyalanine tract expansions (Goodman et al., 1998). Malik (2012) reviewed the syndactylies, noting that the extreme phenotypic heterogeneity observed in SPD families consists of approximately 18 clinical variants that can be 'lumped' into 3 categories: typical SPD features, minor variants, and unusual phenotypes. Genetic Heterogeneity of Synpolydactyly See also SPD2 (608180), caused by mutation in the fibulin-1 gene (FBLN1; 135820) on chromosome 22q13, and SPD3 (610234), which has been mapped to chromosome 14q11.2-q12.