Congenital and Inherited Anomalies of the Musculoskeletal System in Small Animals
Few reports of apodia have appeared in the literature. It is likely that this condition occurs more frequently than is documented, but affected animals probably either die, are euthanized by breeders without being brought to a veterinarian, or are just not reported. Congenital absence of the forearm or hand is a relatively common abnormality in people. Most defects are transverse (amelia) and unilateral. Lower limb deformities are less common and also usually unilateral. No familial tendencies have been seen, and intrauterine amputation by amniotic bands is thought to be a common cause of amelia. Longitudinal limb deficiency, or hemimelia, probably results from defective development of the embryonic limb bud.
Few cases of hemimelia in dogs and cats have been reported. Radial agenesis appears to be the most common of these defects. Affected animals have medially directed angular deformities of the affected limbs. There may be complete or partial agenesis of either one or both radii with a compensatory increase in the diameter of the ulna. Tibial agenesis is less common. The fibulae are markedly enlarged in affected animals.
Maldevelopment of the central rays of the limbs may produce longitudinal splitting of the extremities. Split hand or foot may be sporadic, but autosomal dominant and recessive forms of this condition, sometimes termed the "lobster claw" defect, have been described. The defect also occurs as part of several syndromes. Splitting is most common between the first and second metacarpals, although all variations have been seen. Other abnormalities associated with this defect include digit contractures, digit aplasia, and metacarpal hypoplasia and fusion. No breed or sex predilection has been noted. Studies of this defect in cats indicate that it is inherited as an autosomal dominant with variable expressivity.
In dogs and cats, preaxial polydactyly is by far the more common form (an additional digit or digits on the medial side of the paw). In cats, it is inherited as an autosomal dominant trait with variable expressivity. A similar inheritance pattern appears to apply to the occurrence of multiple dewclaws in dogs, as is seen in Great Pyrenees. Lateral polydactylism occurs less frequently. There is no apparent clinical significance to these conditions, other than an increased propensity for traumatic injury of the partial supernumerary digits.