Equis ISSN 2398-2977

Musculoskeletal: polydactylism

Synonym(s): Supernumerary digits

Contributor(s): Simon Curtis, Graham Munroe

Introduction

  • Rare but the most commonly reported congenital phalangeal anomaly in the horse.
  • 80% of cases occur in the forelimb.
  • Cause: teratological or ? heredity. Transmission to offspring not proven.
  • Signs: partial or complete duplication of the digit, self-evident, interference injury to opposite limb; may be of more cosmetic than functional importance.
  • Diagnosis: signs, radiography.
  • Treatment: none; surgical removal.
  • Prognosis: teratological: poor; atavistic: good.

Pathogenesis

Etiology

  • Teratological: unknown cause.
  • Atavistic: mode of inheritance unknown: ? autosomal dominant gene with incomplete penetrance (humans).

Pathophysiology

  • Congenital defect. 
  • Usually an isolated defect in the horse, cf humans, other domestic species.
  • Teratological:
    • Duplication distal to fetlock.
    • 2 separate digits articulate with MC3/MT3.
    • Distal MC3/MT3 may or may not be divided.
    • Occasionally fusion between digits (P1).
  • Atavistic:
    • Most commonly on medial aspect of forelimb.
    • Most commonly the third metacarpal is duplicated, complete with functional metacarpal joint and phalanges, vascular and nerve supply, flexor/extensor tendons, sesamoid bones, navicular bone and hoof + second metacarpal bone is atrophied or present over entire length of duplicated digit.
    • Usually no direct connection between the two fetlock joints.
    • Second metacarpal bone is less commonly duplicated. It may be duplicated along its entire length terminating in either a complete or an atrophied digit.
  • Self-induced trauma is common due to interference from the supernumerary digit with the opposite limb.

Diagnosis

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Treatment

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Prevention

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Outcomes

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Further Reading

Publications

Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Stanek C & Hantak E (1986) Bilateral atavistic polydactyly in a colt and its dam. Equine Vet J 18, 76-79 PubMed.

Other sources of information

  • Welsh R D (1999) Polydactylism. In: Equine Medicine and Surgery. 5th edn. Eds: Colahan P T, Merritt A M, Moore J N & Mayhew I G. Mosby, USA. pp 1586.


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