Microstomia ("micro-" a combining form meaning small + "-stomia" a combining form meaning mouth = (abnormally) "small mouth") is a clinical feature of many craniofacial syndromes, including Freeman-Sheldon syndrome and Sheldon-Hall syndromes (or distal arthrogryposis multiplex congenita). It may present with whistling-face feature, as well, as in Freeman-Sheldon syndrome. In this syndrome, it impairs alimentation and may require repeated oral surgeries (called commissurotomy) to improve function.

It can also be a feature of systemic scleroderma.

Freeman–Sheldon syndrome is a type of distal arthrogryposis, related to distal arthrogryposis type 1 (DA1). In 1996, more strict criteria for the diagnosis of Freeman–Sheldon syndrome were drawn up, assigning Freeman–Sheldon syndrome as distal arthrogryposis type 2A (DA2A).

On the whole, DA1 is the least severe; DA2B is more severe with additional features that respond less favourably to therapy. DA2A (Freeman–Sheldon syndrome) is the most severe of the three, with more abnormalities and greater resistance to therapy.

Freeman–Sheldon syndrome has been described as a type of congenital myopathy.

In March 2006, Stevenson et al. published strict diagnostic criteria for distal arthrogryposis type 2A (DA2A) or Freeman–Sheldon syndrome. These included two or more features of distal arthrogryposis: microstomia, whistling-face, nasolabial creases, and 'H-shaped' chin dimple.

There are little data on prognosis. Rarely, some patients have died in infancy from respiratory failure; otherwise, life expectancy is considered to be normal.