Fibrodysplasia ossificans progressiva

Children who have FOP appear normal at birth except for congenital malformations of the great toes (hallux valgus, malformed first metatarsal, and/or monophalangism). During the first decade of life, sporadic episodes of painful soft tissue swellings (flare-ups) occur which are often precipitated by soft tissue injury, intramuscular injections, viral infection, muscular stretching, falls or fatigue. If diagnosis of FOP is suspected, any invasive intervention (such as biopsy), which may lead to flare-ups, is contraindicated. These flare-ups transform skeletal muscles, tendons, ligaments, fascia, and aponeuroses into heterotopic bone, rendering movement impossible. Patients with atypical forms of FOP have been described. They either present with the classic features of FOP plus one or more atypical features (e.g. intercurrent aplastic anemia, craniopharyngioma, childhood glaucoma or growth retardation) (FOP plus), or present major variations in one or both of the two classic defining features of FOP (e.g., normal great toes or severe reduction deficits of digits) (FOP variants).