- Polydactyly of the hand is inherited. This may be a sporadic genetic mutation or may have an autosomal dominant inheritance pattern with variable penetrance.
- Sonic hedgehog (SHH) / zone of polarizing activity pathways regulate anterior / posterior (ulnoradial) development.
- Currently 6 genes have been identified as contributing to polydactyly of the hand: ZNF141, GLI3, MIPOL1, IQCE, PITX1, and GLI1.
Prevalence: There is a male prevalence for both preaxial (thumb duplication) and postaxial (ulnar digit) polydactyly.
Risk factors:
- Preaxial polydactyly has a higher prevalence in White individuals.
- Postaxial polydactyly has a 10 times higher prevalence in Black individuals with an autosomal dominant inheritance pattern. White patients with postaxial polydactyly need genetic work-ups.
- The limb bud forms from ectoderm with underlying mesoderm. Limb development takes place in weeks 4-8 of gestation.
- The zone of polarizing activity is an area of mesenchyme beneath the apical ectodermal ridge on the posterior aspect of the limb bud.
- Via secretion of SHH, there is development of the limb in the ulnoradial dimension.
- Oversecretion of SHH results in postaxial polydactyly. Ectopic SHH expression or disruption of this signaling pathway leads to preaxial polydactyly.
- Postaxial polydactyly –
- Type A: well-formed digit
- Type B: rudimentary digit
- Preaxial polydactyly (thumb duplication) – Wassel classification
- Type I: bifid distal phalanx
- Type II: duplicated distal phalanx
- Type III: bifid proximal phalanx
- Type IV (most common): duplicated proximal phalanx
- Type V: bifid metacarpal
- Type VI: duplicated metacarpal
- Type VII: triphalangism