Symphalangism was first described by Harvey Cushing in 1916, when he identified a family in which 27% (84/313) of those examined exhibited dominantly inherited ankylosis of the IP joints of the hand. Symphalangism is a rare, congenital, autosomal dominant disorder of the middle phalanx characterized by ankylosis of either the proximal interphalangeal (PIP) or distal interphalangeal (DIP) joints of the finger(s), with PIP involvement being more common. Because many patients are either not severely affected or able to adapt to functioning with symphalangism, ongoing observation and conservative interventions like physical and occupational therapy are most commonly recommended. Surgery may be performed in rare cases when the condition causes extreme functional impairments or for cosmetic purposes; however, surgical outcomes are predominantly poor.1,2,3
Pathophysiology
- Symphalangism is typically transmitted as an autosomal dominant trait by either sex, and it occurs due to failure of differentiation of the interphalangeal (IP) joint during the embryonic stage2
- The abnormal genes responsible for causing symphalangism are localized on chromosome 17q221
- Mutations in at least two genes—NOG and GDF5—have been identified as underlying causes of proximal symphalangism4
- Symphalangism may also occur spontaneously in some cases
- Infants with symphalangism may present with clinical fusion of the affected joint(s) early in life, but radiographic evidence of ankylosis is not seen until later childhood3
Related Anatomy
- Symphalangism is typically classified into 3 types:
- True symphalangism: involved digits are rigid but have normal length
- Symbrachydactylism: affected digits are short and stiff
- Symphalangism with associated anomalies (eg, Apert’s syndrome and Poland’s syndrome)5
- Another classification system groups symphalangism into 3 separate grades based on the level of anatomical involvement:
- Grade I: fibrous symphalangism
- Grade II: cartilaginous symphalangism
- Grade III: bony symphalangism6
- Fusion between the proximal and middle phalanges at the PIP joint is the most common manifestation of symphalangism, although fusion at the DIP joint is also possible; involvement of the metacarpophalangeal (MP) joint is extremely rare5,6
- The little finger is the digit most frequently affected by symphalangism, followed by the fourth, long, and index fingers; there have also been some reports of involvement of the thumb and hallux
- Proximal symphalangism has been observed to always affect the second through fourth digits, while the distal type predominantly affects the fourth and fifth digits3
- The index finger is generally only affected when other fingers are also involved5
Incidence
- True symphalangism accounts for 0.03% of all congenital anomalies of the upper extremity7
- Symphalangism occurs most commonly in Caucasians
- Many additional skeletal anomalies have been reported in association with symphalangism, but there is great variability in their expression, which is indicative of the genetic heterogeneity of the disease3
Related Conditions
- Proximal symphalangism is part of a spectrum of disorders that cause joint fusion, with multiple synostoses syndrome being at the most severe end of this spectrum4
- Brachydactyly, camptodactyly, clinodactyly, syndactyly
- Syndromes: Apert's, Nievergelt-Pearlman, Poland's, Herrmann, multiple synostoses syndrome
- Conductive hearing loss
- Taloscaphoid or radiohumeral fusion
- Carpal and metacarpal anomalies (particularly carpal synostosis)
- Pes planus
- Bilateral hip dislocation
- Tarsal coalitions (particularly talonavicular synostosis)
- Congenital fusion of the cervical or thoracic spine
Differential Diagnosis
