Fibrodysplasia Ossificans Progressiva

This chapter discusses the clinical presentation, radiologic findings, laboratory findings, etiology and pathogenesis, and treatment of fibrodysplasia ossificans progressiva (FOP). FOP is a rare heritable disorder of connective tissue characterized by congenital malformations of the great toes and progressive heterotopic endochondral ossification (HEO) in characteristic anatomic patterns. Skeletal anomalies and soft-tissue ossification are characteristic radiologic features of FOP. Early preosseous FOP lesions consist of an intense aggregation of mononuclear inflammatory cells including lymphocytes, macrophages, and mast cells in the perivascular spaces of edematous muscle. High-dose glucocorticoids have limited use in the management of the early inflammatory phase of flare-ups. The disorder's rarity, variability, and fluctuating clinical course pose substantial uncertainties when evaluating experimental therapies. Despite widespread heterotopic ossification and severe disability, some patients live productive lives into their seventh decade. Research on FOP led to the discovery of progressive osseous heteroplasia (POH), a distinct developmental disorder of heterotopic ossification.