Acute unilateral hip pain in fibrodysplasia ossificans progressiva (FOP)

Background: Flare-ups of the hips are among the most feared and disabling complications of fibrodysplasia ossificans progressiva (FOP) and are poorly understood. In order to better understand the nature of hip flare-ups in FOP, we evaluated 25 consecutive individuals with classic FOP (14 males, 11 females; 3–56 years old, median age, 17 years old) who presented with acute unilateral hip pain. Results: All 25 individuals were suspected of having a flare-up of the hip based on clinical history and a favorable response to a four day course of high-dose oral prednisone. Ten individuals (40%) experienced rebound symptoms of pain and/or stiffness within seven days after discontinuation of prednisone and all ten subsequently developed heterotopic ossification (HO) or decreased mobility of the affected hip. None of the 14 individuals who experienced sustained relief of symptoms following a course of oral prednisone experienced HO or decreased mobility. Incidental radiographic findings at the time of presentation were multifactoral and included osteochondromas of the proximal femur (18/25; 72%), degenerative arthritis (17/25; 68%), developmental hip dysplasia (15/25; 60%), previously existing heterotopic ossification (12/25; 48%), intra-articular synovial osteochondromatosis (8/25; 32%) or traumatic fractures through pre-existing heterotopic bone (1/25; 4%). Conclusions: Developmental joint pathology may confound clinical evaluation of hip pain in FOP. The most useful modality for suspecting an ossification-prone flare-up of the hip was lack of sustained response to a brief course of oral prednisone. Evaluation of soft tissue edema by ultrasound or magnetic resonance imaging showed promise in identifying ossification-prone flare-ups and warrants further analysis in prospective studies.