Proximal symphalangism

Proximal symphalangism (SYM1), also known as Cushing's symphalangism or hereditary symphalangism, is a rare genetic skeletal disorder characterized by bony fusion (ankylosis) of the proximal interphalangeal (PIP) joints of the fingers and, in some cases, the toes. The condition results from failure of normal joint formation during embryonic development. Affected individuals typically present with stiff, straight fingers due to the absence of functional proximal interphalangeal joints, with the ring and little fingers most commonly involved. The hands may appear relatively normal externally, but the inability to bend the affected joints leads to reduced grip strength and impaired fine motor function. In addition to the hand involvement, proximal symphalangism frequently affects other joints and structures. Conductive hearing loss is a common associated feature, resulting from fusion of the ossicles (small bones) in the middle ear, particularly the stapes. Some patients may also have fusion of carpal (wrist) or tarsal (ankle) bones. The condition is caused by heterozygous mutations in the NOG gene (encoding noggin) on chromosome 17q22 or, less commonly, in the GDF5 gene. Proximal symphalangism is allelic with multiple synostoses syndrome, representing a milder end of the same phenotypic spectrum. There is no cure for proximal symphalangism. Management is primarily supportive and symptomatic. Hearing aids may be beneficial for individuals with conductive hearing loss, and surgical intervention on the middle ear ossicles has been attempted in some cases. Occupational therapy can help optimize hand function. Surgical attempts to restore joint mobility in the fingers have generally been unsuccessful due to the fundamental absence of joint structures. Genetic counseling is recommended for affected families given the autosomal dominant inheritance pattern with high penetrance but variable expressivity.

Common questions about Proximal symphalangism