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.
Al Qattan (2011) reported two families with Synpolydactyly Type II with variations of expression. In the first family, all three children born to unrelated parents were affected. The mother was also affected, and had a family history for Synpolydactyly Type II for several generations, while the father was phenotypically normal. The feet were normal in all affected members. One of the affected children showed intrafamilial variability in the form of synpolydactyly of the little finger in the left hand, and syndpolydactyly of the third web in the right hand. In the second family, four of the six children were affected. The parents were first-cousins, and both had little finger clinodactyly. There was a family history of Synpolydactyly II going back several generations. The two affected boys had severe bilateral hand brachydactyly with hypoplasia of the middle phalanges, bilateral accessory carpal bones and bilateral thumb clinodactyly, bilateral feet involvement with brachydactyly and hypoplasia of the middle phalanges, and some of the metacarpals and metatarsals losing their shape and appearing polygonal. The two affected girls showed similar features with one hand showing the classis synpolydactyly and the other showing syndactyly, but no polydactyly. There was no involvement of the feet in their case.