Clinical characteristics: Fibrodysplasia ossificans progressiva (FOP) is characterized by congenital bilateral hallux valgus malformations and early-onset heterotopic ossification, which may be spontaneous or precipitated by trauma including intramuscular vaccinations. Painful, recurrent soft-tissue swellings (flare-ups) may precede localized heterotopic ossification. Heterotopic ossification can occur at any location, but typically affects regions in close proximity to the axial skeleton in the early/mild stages, before progressing to the appendicular skeleton. This can lead to restriction of movement as a result of ossification impacting joint mobility. Problems with swallowing and speaking can occur with ossification affecting the jaw, head, and neck, and restriction of the airway and breathing may lead to thoracic insufficiency syndrome.

Diagnosis/testing: The diagnosis of FOP is established in a proband with heterotopic ossification, hallux valgus malformations, and/or a heterozygous pathogenic variant in ACVR1 identified by molecular genetic testing.

Management: Targeted therapy: Palovarotene to reduce new heterotopic ossification in adults and children (females age ≥8 years and males age ≥10 years) with FOP.

Supportive care: Avoid intramuscular injections and arterial punctures. Fall prevention using household safety measures and ambulatory devices; use of protective headgear to reduce sequelae of falls; prompt medical attention after a fall with consideration of prophylactic corticosteroid use; management by a dietician for those with feeding difficulties; preventative dental care with precautions to avoid injury; orthodontic treatment with a practitioner with experience in FOP; consultation with an expert anesthetist with experience in FOP prior to elective anesthesia; use of singing, swimming, incentive spirometry; positive pressure ventilation when indicated for mechanical respiratory difficulties including thoracic insufficiency syndrome; anti-inflammatory medications for flare-ups; consider corticosteroids for flare-ups of the submandibular region or jaw, major joints, after significant soft-tissue trauma, and for prophylaxis prior to dental and surgical procedures. Conservative management for scoliosis. Consider bisphosphonates for corticosteroid-induced osteopenia; fractures should be managed by an expert in FOP; hearing aids and appliances for conductive hearing impairment; encourage hydration and avoidance of high protein and high salt intake to prevent renal stones; occupational therapy; warm water hydrotherapy for mobility difficulties; lower extremity elevation, DVT prophylaxis, and supportive stockings while avoiding traumatic compression for lymphedema. Psychological support.

Surveillance: Annual clinical evaluation including evaluation for scoliosis with orthopedist or geneticist familiar with FOP; annual nutrition evaluation and examination for jaw ankylosis; baseline pulmonary function assessment, sleep assessment, and echocardiogram before age ten years followed by annual clinical evaluation of respiratory status; annual evaluation for fracture risk; audiology assessment every 12 to 24 months; annual assessment for signs and symptoms of nephrocalcinosis, gastrointestinal complications, and skin integrity; dental examinations every six months; Doppler ultrasound if DVT is suspected.

Agents/circumstances to avoid: Avoid procedures that predispose to soft-tissue injury, including intramuscular injections such as vaccinations, arterial punctures, dental procedures, procedures related to anesthesia, biopsies, removal of heterotopic bone, and all nonemergent surgical procedures. Avoid contact sports, overstretching of soft tissues, muscle fatigue, and passive range of motion. Avoid falls. In individuals with thoracic insufficiency syndrome, avoid supplemental oxygen, which can suppress respiratory drive.

Genetic counseling: FOP is inherited in an autosomal dominant manner. The majority of affected individuals represent simplex cases (i.e., a single occurrence in a family) resulting from a de novo ACVR1 pathogenic variant. Rarely, an individual diagnosed with FOP has an affected parent. If a parent of the proband is affected and/or is known to have the pathogenic variant identified in the proband, the risk to sibs is 50%. Once the ACVR1 pathogenic variant has been identified in an affected family member, prenatal testing for a pregnancy at increased risk and preimplantation genetic testing are possible.