Cutaneous Nodules and Inflammatory Arthritis: Two Illustrative Cases of Rare Mimics of Rheumatoid Arthritis.

Rheumatoid arthritis is a relatively common rheumatic disease that can present with inflammatory arthritis and subcutaneous nodules. Multicentric reticulohistiocytosis and fibroblastic rheumatism are rarer entities that also present with these features. Two cases, one of each of fibroblastic rheumatism and multicentric reticulohistiocytosis, are described highlighting characteristic clinical, radiographic, and histologic findings. A narrative review of the literature on these rarer conditions, compared with rheumatoid arthritis, is provided with a focus on articular and cutaneous findings, available information on disease presentations, and key contrasting features that can aid in diagnosis. Radiographic erosion distribution and joint space narrowing, clinical nodule distribution and characteristics, and nodule histology can differ between these diseases. Multicentric reticulohistiocytosis and fibroblastic rheumatism should be considered in the evaluation of seronegative rheumatoid arthritis, especially in cases that do not respond predictably to standard therapies, and cutaneous nodule biopsy can aid in differentiating these three conditions.