In myositis ossificans progressiva, average age of onset for ossification is 5 years, with a reported onset range of birth to 25 years. The condition rarely is asymptomatic and may be diagnosed from radiographs obtained for unrelated problems. The history of trauma causing the condition may be difficult to elicit. Other sites include intercostal spaces, erector spinae, pectoralis muscles, glutei, and the chest. The condition produces painful lumps and stiffness in the adjoining joint.
Lumps decrease in a few weeks, but joint mobility reduction persists.
Exacerbating factors for ossifications at new sites include venipuncture, biopsy of lumps, IM injections, dental treatments, and excision of masses. The most common sites are the sternocleidomastoid muscle, paraspinal muscles, the jaw's masticatory muscles, and shoulder and pelvic girdle muscles. Spared are the abdominal muscles, extraocular muscles, and GI tract and tongue muscles. Ossification progresses from proximal to distal and cranial to caudal. Routine laboratory results may be within reference ranges.