Progressive Soft Tissue Swelling in a Pediatric Patient Leading to the Diagnosis of Fibrodysplasia Ossificans Progressiva: A Case Report.

Fibrodysplasia ossificans progressiva (FOP) is a rare genetic disorder characterized by progressive heterotopic ossification (HO) and congenital malformation of the great toes. This case describes a 5-year-old Caucasian girl who initially presented with painless neck and back swelling as well as severe limitation of movement in the neck and shoulder girdle, who was diagnosed as FOP following tissue-based genetic testing revealing the ACVR1 (p.Arg206His) pathogenic variant. The diagnostic process, which included imaging, skin biopsy, and a thorough infectious and autoimmune workup, emphasized the challenges in identifying FOP due to its rarity; however, bilateral congenital hallux valgus, often present in FOP, can be a clue for early diagnosis. While corticosteroids provide temporary relief, long-term management now includes palovarotene, a novel therapy to suppress HO.