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:
- pre-axial polydactyly: extra digit(s) towards the thumb/hallux (radially)
- post-axial polydactyly: extra digit(s) towards little finger/toe (ulnar)
- central polydactyly: middle three digits are involved
Estimated incidence is different for pre and post-axial polydactyly 6:
- post-axial: ~1 in 3000
- pre-axial: ~1 in 7000
Central polydactyly is the rarest encountered.
In addition, there may be a greater prevalence in individuals of African descent (particularly for post-axial polydactyly) 7.
A large proportion of polydactyly is isolated although they can be associated with an immense amount of anomalies which include:
- aneupliodic syndromic
- trisomy 13: tends to give post-axial polydactyly
- non-aneuploidic syndromic
- Bardet-Biedl syndrome: often post-axial
- Carpenter syndrome
- fetal valproate syndrome
- hydrolethalus syndrome
- Joubert syndrome
- Juberg-Hayward syndrome
- Lhermitte duclos disease
- Meckel Gruber syndrome: tends to be post-axial
- McKusick-Kaufman syndrome: post-axial 5
- megalencephaly, polymicrogyria, polydactyly, and hydrocephalus (MPPH) syndrome 2
- oral-facial-digital syndromes
- Pallister-Hall syndrome
- short rib polydactyly syndrome(s) 3
- 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 8.
While radiology has little role in the diagnosis of polydactyly it is important in two ways:
- assessment of the remainder of the skeleton (if appropriate) for other skeletal anomalies, and as such aiding in the diagnosis on an underlying syndrome (which in turn may point to additional unsuspected anomalies, and allow for genetic counseling etc.)
- assess the local anatomy to aid in surgical planning. Of particular importance is the anatomy of the 'normal' digits and the relationship of the extra digit to the adjacent bones and joints.
History and etymology
The term "polydactyly" is derived from the Greek words "πολύς - polus" (many) and "δάκτυλος - daktulos" (finger).
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- 5. Son SH, Kim YJ, Kim ES et-al. A case of McKusick-Kaufman syndrome. Korean J Pediatr. 2011;54 (5): 219-23. doi:10.3345/kjp.2011.54.5.219 - Free text at pubmed - Pubmed citation
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- 8. Piette N, Zambelli PY, N'Dele D. Isolated heptadactylia: A case report of central polydactyly of the foot. (2017) Medicine. 96 (42): e8324. doi:10.1097/MD.0000000000008324 - Pubmed