Synpolydactyly (SPD) is a rare condition characterized by fusion of the middle and ring fingers and of the fourth and fifth toes, with partial or complete digit duplication within the webbings between these digits. SPD may also be associated with brachydactyly, camptodactyly, or clinodactyly of the fifth fingers, and variable syndactyly of the second to fifth toes with middle phalanx hypoplasia. There have been reports of cases of SPD without any polydactyly and with normal feet. On the other hand, there are also reports of severe phenotypes showing peculiar arrangements of all the digital elements, with the malformation reaching as far as the carpals or metatarsals. Thus, the phenotypic expression of SPD1 is heterogeneous.
Synpolydactyly is usually evident upon visual examination. However, an X-ray might be required to show the extra digits. The only physical disability associated with the condition is slight contractions resulting from the fusion. There are no other cranial or skeletal malformations. SPD1 has been reported in less than 30 families worldwide.
SPD1 is inherited as an autosomal dominant trait with reduced penetrance. SPD1 is caused by mutations in the HOXD13 gene, a homeobox transcription factor crucial for autopod development. The product of this gene is assumed to play a role in axial skeletal development and forelimb morphogenesis.