equis - Articles
Musculoskeletal: polydactylism
Synonym(s): Supernumerary digits
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.
Presenting signs
- Polydactylism is defined as the occurrence of more digits than typical for a certain species.
- Diagnosis is self-evident.
Age predisposition
- Neonate.
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.