The most common of the DAs are Freeman-Sheldon syndrome (FSS),
Sheldon-Hall syndrome, trismus-pseudocamptodactyly syndrome, and
distal arthrogryposis with ophthalmoplegia. In addition to limb
contractures, each of these syndromes is also associated with contractures
of the facial or oculomotor muscles. For example, in individuals
with FSS, facial contractures result in a very small oral orifice,
often only a few millimeters in diameter at birth, along with puckered
lips, and an H-shaped dimple of the chin. Hence, FSS has also been
called whistling face syndrome. The natural history of each of these
disorders is different; therefore, distinguishing among them is important.
All of these disorders are caused by mutations in one or more genes
that encode components of the contractile apparatus of fast-twitch
myofibers (eg, myosin, troponin, tropomyosin).