Isolated Medial Ankle Tenosynovitis of the Flexor Digitorum and Posterior Tibialis Tendons: A Case Report of the Role of Early Musculoskeletal Ultrasound in Identifying Anti-citrullinated Protein Antibody (ACPA)-Positive At-Risk Rheumatoid Arthritis.

Tenosynovitis is a recognized feature of rheumatoid arthritis (RA), yet it typically presents alongside symmetric polyarticular synovitis. Isolated focal tenosynovitis as the primary and initial manifestation of RA is uncommon and frequently leads to diagnostic delays. We present the case of a 62-year-old female who presented with localized right ankle pain in the absence of systemic joint involvement or elevated inflammatory markers. While the rheumatoid factor was negative and both the erythrocyte sedimentation rate and C-reactive protein levels were within normal limits, musculoskeletal ultrasound revealed focal tenosynovitis of the flexor digitorum and posterior tibialis tendons, accompanied by possible early erosive changes of the medial malleolus. Markedly elevated titers of anti-cyclic citrullinated peptide (112 U) and mutated citrullinated vimentin (171 U) antibodies supported the diagnosis. The patient responded rapidly to prednisone and was subsequently transitioned to methotrexate. This case emphasizes that isolated tenosynovitis can be the early presenting feature of seropositive RA. It underscores the critical role of advanced serologic testing and high-resolution imaging in identifying early-stage disease when traditional clinical markers are absent, allowing for timely intervention to prevent irreversible structural damage.