Postaxial polydactyly refers to polydactyly where the additional digit is on the ulnar margin of the hand, or lateral to the 5^th (little) toe.

Epidemiology

Postaxial polydactyly is more common than preaxial polydactyly, with an estimated incidence of 1 in 3000 ⁵.

Associations

• trisomy 13

• Bardet-Biedl syndrome

• Meckel Gruber syndrome

• McKusick-Kaufman syndrome

• following oral-facial-digital syndromes (OFDS)

  • oral-facial-digital syndrome (OFDS) type II (Mohr syndrome)

  • oral-facial-digital syndrome (OFDS) type VI

• certain skeletal dysplasias

  • chondroectodermal dysplasia (Ellis-van Creveld syndrome)

  • asphyxiating thoracic dysplasia (Jeune syndrome)

• Smith Lemli Opitz syndrome

• certain short rib polydactyly syndromes: e.g. types I and III

Pathology

The majority of cases are idiopathic. Associations with mutations to the GLI3 gene on chromosome 7 as well as a second locus mapped to chromosome 13 have been implicated in postaxial polydactyly ⁵.

Classification

Several different classification systems exist for postaxial polydactyly of the hands, including the Pritsch, Rayan and Al-Qattan, and Duran classification systems.

Duran et al. introduced their classification system for post-axial polydactyly in 2015 ⁶. This classification was created as it was felt that the previous systems in use were not comprehensive for coverage of the different post-axial forms.

• type I: skin nub without nail or bone elements

• type II: hypoplastic proximal phalanx

• type III: proximal phalangeal malformations

  • IIIa: bifid proximal phalanx

  • IIIb: duplicated proximal phalanx

• type IV: metacarpal malformations

  • IVa: fusion of the metacarpal

  • IVb: bifid metacarpal

  • IVc: duplication of the metacarpal

• type V: (complicated type) with triplication of the small finger, polysyndactyly, or both