Polydactyly (less commonly called hyperdactyly) refers to the situation where there are more than the usual number of digits (five) in a hand or foot. It can be broadly classified as:
- preaxial polydactyly: extra digit(s) towards the thumb/hallux (radially)
- postaxial polydactyly: extra digit(s) towards little finger/toe (ulnar)
- central polydactyly: middle three digits are involved
Estimated incidence is different for pre and postaxial polydactyly :
- postaxial: ~1 in 3000
- preaxial: ~1 in 7000
Central polydactyly is the rarest encountered.
In addition, there may be a greater prevalence in individuals of African descent (particularly for postaxial polydactyly) .
A large proportion of polydactyly is isolated although they can be associated with an immense amount of anomalies which include:
- aneuploidic syndromic
- trisomy 13: tends to give postaxial polydactyly
- non-aneuploidic syndromic
- Bardet-Biedl syndrome: often postaxial
- Carpenter syndrome
- fetal valproate syndrome
- hydrolethalus syndrome
- Joubert syndrome
- Juberg-Hayward syndrome
- Lhermitte duclos disease
- Meckel Gruber syndrome: tends to be postaxial
- McKusick-Kaufman syndrome: postaxial
- megalencephaly, polymicrogyria, polydactyly, and hydrocephalus (MPPH) syndrome
- oral-facial-digital syndromes
- oral-facial-digital syndrome (OFDS) type II - Mohr syndrome: postaxial
- oral-facial-digital syndrome (OFDS) type VI: postaxial
- Pallister-Hall syndrome
- short rib polydactyly syndrome(s)
- skeletal dysplasias
- Smith-Lemli-Opitz syndrome
- VACTERL association
- non-aneuploidic, non-syndromic
If it is an isolated anomaly it is incidental and not of concern but if associated with another anomaly it then carries a vastly variable prognosis dependent on the rest of the syndrome.
Other than describing polydactyly based on the position of the accessory digit, it can also be described by the number of total digits e.g. hexadactyly (six digits), or even, heptadactyly (seven digits). Indeed the presence of seven individual digits is exceedingly rare but has been reported twice .
Whilst radiology has little role in the diagnosis of polydactyly it is important in two ways:
History and etymology
The term "polydactyly" is derived from the Greek words "πολύς - polus" (many) and "δάκτυλος - daktulos" (finger).