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determining what skeletal elements are present. Two forms of polydactyly are usually classified in horses: the most common (80% of cases) is the ‘atavistic’ type, which is characterised by an extra-medial digit that articulates with a second metacarpal bone; the second, more rare, is the ‘teratogenic’ form, which is characterised by the duplication of bones distal to the fetlock joint, producing a cloven hoof (McGavin and Leipold 1975; Palmer 1993). Surgical removal of the supernumerary digits is possible in
LM
the ‘atavistic’ form and is provided to improve cosmetic appearance and prevent injury to the distal limb (Evans et al. 1965; McGavin and Leipold 1975), but should not destabilise the involved joint (Evans et al. 1965; McGavin and Leipold 1975; Carstanjen et al. 2007). The ‘teratogenic’ form more severely affects musculoskeletal function and quality of life of the animal and prognosis is usually poor (Nascimento et al. 2012). In the present case report, we consider that pseudo-
polydactyly caused an overload on the medial portion of the solar surface and, as a consequence, lateral deviation of the digit which reasonably contributed to bone changes, as well as to worsen the lameness. The hoof lesion described in the present paper does not
Fig 2: Right forelimb. Imaging diagnostic investigations rule out the presence of supernumerary bone. Axial deviation of the digit is evident with internal rotation of the MP and lateral deviation of the DP. Computed tomography, 3D rendering. L = lateral; M = medial.
main digital blood vessels and nerves of both forelimbs, were also grossly evaluated with no apparent anomaly detected.
Diagnosis On the basis of clinical signs, diagnostic imaging study and gross examination, a diagnosis of pseudo-polydactyly was proposed.
Discussion
‘True’ polydactyly is a localised developmental disturbance of limbs which consists of an increased number of digits. Polydactyly has been described in man, as well as in many animal species (Palmer 1993). In man, polydactyly is common and can occur as a simple, isolated malformation or as part of a pleiotropic developmental anomaly syndrome. The aetiology and genetic bases of human polydactyly are complex, a wide range of genes having been potentially implicated (Biesecker 2011). In domestic animals, polydactyly has been reported in cats, dogs, horses and cattle (Palmer 1993) and inherited forms have been previously described in Australian sheepdogs (Sponenberg and Bowling 1985) and Simmental cattle (Johnson et al. 1981). Although the occurrence of polydactyly is rare in horses,
this is the most frequent congenital malformation of the phalanges (Stanek and Hantak 1986; Nascimento et al. 2012). Radiographic examination of the digit is beneficial in
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fulfil the aforementioned diagnostic criteria of equine polydactyly, since post mortem imaging and pathological investigations ruled out the presence of supernumerary digits. As a consequence, we consider more appropriate the definition of pseudo-polydactyly, which has been proposed herein. The aetiology and pathogenesis of equine true and/or pseudo-polydactyly is still largely unknown and is well beyond the scope of the present case report. Although in absence of anamnestic data, we reasonably consider that pseudo-polydactyly described herein likely results from a traumatic event. In this respect, the uneven axial margins of the cleaved hoof might result from a normal layer of the epithelium which migrated to cover a defect during the healing process, while the ‘lateral hoof’ might represent the outcome of a partial, albeit remarkable hoof avulsion (Fig 1b). Alternatively and less likely, the cloven hoof might result from the congenital failure in the development of the coronary band. In conclusion, to the best of our knowledge, the features of
the present case report are unique, thus widening the range of lesions observed at the level of the hoof.
Authors’ declaration of interests No conflicts of interest have been declared.
Acknowledgements We thank Dr Livio Giammaria for reporting this case.
Manufacturers’ addresses
1Philips Electronics, Amsterdam, 1070 MX, NV. 2KitWare, 28 Corporate Drive, Clifton Park, New York, USA.
References
Baxter, G.M. and Stashak, T.S. (2011) Examination for lameness. In: Adams and Stashak’s Lameness in Horses, 6th edn., Ed: G.M. Baxter, Wiley-Blackwell, Ames, lowa. pp 475-558.
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