Rare genetic systemic or rheumatologic disease

Orphanet code 271870 corresponds to IgG4-related disease (IgG4-RD), also known as IgG4-related systemic disease or hyper-IgG4 disease. This is a rare immune-mediated fibroinflammatory condition characterized by the infiltration of IgG4-positive plasma cells into affected tissues, leading to organ enlargement, tissue destruction, and fibrosis. The disease can affect virtually any organ system, but commonly involves the pancreas (autoimmune pancreatitis), salivary and lacrimal glands (formerly known as Mikulicz disease), bile ducts, kidneys, retroperitoneum, aorta, lungs, thyroid, and lymph nodes. Elevated serum IgG4 levels are frequently observed but are not universally present. Key clinical features depend on the organs involved and may include painless organ swelling, obstructive jaundice, kidney dysfunction, orbital pseudotumor, and retroperitoneal fibrosis. The disease tends to affect middle-aged to older adults, with a male predominance in many organ manifestations. Diagnosis is based on a combination of clinical, serological, radiological, and histopathological findings, including storiform fibrosis, obliterative phlebitis, and dense lymphoplasmacytic infiltration rich in IgG4-positive cells. Treatment typically involves glucocorticoids as first-line therapy, which often produce a rapid clinical response. However, relapses are common upon steroid tapering. Steroid-sparing immunosuppressive agents such as azathioprine, mycophenolate mofetil, and methotrexate are used for maintenance therapy. More recently, B-cell depletion therapy with rituximab has shown significant efficacy in refractory or relapsing cases. Early diagnosis and treatment are important to prevent irreversible organ damage from fibrosis.

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