Congenital fiber-type disproportion.

Congenital fiber-type disproportion is a form of congenital myopathy that may be best viewed as a syndrome rather than as a formal diagnosis. The central histologic abnormality is that type 1 fibers are consistently smaller than type 2 fibers by at least 35%-40%. Care is needed in diagnosing patients, as this histologic abnormality can occur in other congenital myopathies and in other neuromuscular disorders. Many of the genetic causes have been identified. Careful surveillance of respiratory function is required in all patients until the specific genetic cause is known and advice can be individualized.