Synpolydactyly
Overview
Synpolydactyly (SPD) is a rare genetic disorder characterized by the presence of extra digits in the hands or feet, and the fusion of certain fingers or toes. This condition is a subtype of polydactyly, a term that refers to the occurrence of more than the usual number of digits. Synpolydactyly is distinct from other forms of polydactyly in that it involves both the duplication of digits (polydactyly) and their fusion (syndactyly), hence the name "synpolydactyly".
Genetics
Synpolydactyly is primarily caused by mutations in the homeobox D13 (HOXD13) gene. This gene is part of the homeobox gene family, which plays a crucial role in the formation of body structures during early embryonic development. The HOXD13 gene, in particular, is essential for the normal development of the limbs, fingers, and toes.
Mutations in the HOXD13 gene can lead to the production of an abnormal HOXD13 protein that cannot function properly. This can disrupt the normal development of the limbs, leading to the characteristic features of synpolydactyly. However, the exact mechanism by which these mutations cause synpolydactyly is not fully understood.
Types
There are two main types of synpolydactyly, known as Type I and Type II.
Type I
Type I synpolydactyly, also known as classic synpolydactyly, is characterized by the presence of an extra digit between the third and fourth fingers or toes, which are also typically fused together. This extra digit is usually a fully formed finger or toe, but in some cases, it may be a small, skin-covered bone.
Type II
Type II synpolydactyly, also known as synpolydactyly with foot anomalies, is a more severe form of the condition. In addition to the features of Type I, individuals with Type II synpolydactyly also have abnormalities in the bones of the feet and the lower legs. These may include extra toes, fused toes, and abnormal growth of the tibia and fibula.
Diagnosis
The diagnosis of synpolydactyly is typically based on a physical examination and a review of the individual's medical and family history. In some cases, imaging tests such as X-rays may be used to confirm the diagnosis and to assess the extent of the abnormalities.
Genetic testing can also be performed to identify mutations in the HOXD13 gene. However, this is not typically done as part of the initial diagnostic process, but may be used to confirm the diagnosis or to provide information for genetic counseling.
Treatment
The treatment of synpolydactyly primarily involves surgical correction of the abnormalities. This typically involves the removal of the extra digits and the separation of the fused fingers or toes. In some cases, additional surgeries may be needed to correct bone abnormalities or to improve the function of the hand or foot.
Physical therapy is often recommended following surgery to help improve strength and flexibility. In some cases, assistive devices such as splints or braces may be used to support the affected limb and to promote normal movement.
Prognosis
The prognosis for individuals with synpolydactyly varies depending on the severity of the condition and the success of surgical treatment. With appropriate treatment, most individuals with synpolydactyly are able to lead normal lives. However, some may experience ongoing difficulties with hand or foot function, particularly if the condition is severe or if there are additional bone abnormalities.