RheumatologyTier 2Disease (DEADMAN)

IgG4-related disease

Description

IgG4-related disease - a fibro-inflammatory condition characterised by tissue infiltration with IgG4-positive plasma cells, causing tumour-like mass lesions and fibrosis across multiple organs - increasingly recognised as a unifying diagnosis for previously separately-named organ-specific conditions

Affects predominantly middle-aged to older men; can involve nearly any organ, with the pancreas (autoimmune pancreatitis), biliary tree, salivary/lacrimal glands, retroperitoneum, and kidneys among the more commonly affected sites

Characterised histologically by a dense lymphoplasmacytic infiltrate rich in IgG4-positive plasma cells, storiform (swirling) fibrosis, and obliterative phlebitis - the combination of these three features on biopsy is relatively specific for the diagnosis, distinguishing it from other causes of organ fibrosis/mass lesions
The exact immunopathogenic mechanism remains incompletely understood, though it is considered a distinct immune-mediated fibro-inflammatory process rather than a classic autoantibody-driven or T cell-mediated autoimmune disease

Presents as tumour-like mass lesions or organ enlargement (pancreatic mass mimicking pancreatic cancer, salivary/lacrimal gland swelling, retroperitoneal fibrosis) - frequently mistaken for malignancy on imaging alone, a key diagnostic trap
Elevated serum IgG4 supports but is neither sensitive nor specific enough to confirm the diagnosis alone (can be normal in confirmed IgG4-related disease, and mildly elevated in other conditions) - tissue biopsy with the characteristic histological triad remains the diagnostic gold standard
Often affects multiple organs simultaneously or sequentially - recognising a multi-organ pattern (rather than treating each affected organ as an independent, unrelated diagnosis) is key to arriving at the unifying diagnosis

Corticosteroids are highly effective first-line treatment, typically producing rapid clinical and radiological improvement - a notably good initial response that itself provides diagnostic support in an ambiguous case
Rituximab is increasingly used for relapsing/refractory disease or as a steroid-sparing strategy, given B cell/plasma cell involvement in pathogenesis
Long-term monitoring is needed given the relapsing nature of the disease and the risk of progressive fibrosis (potentially irreversible organ damage) if inadequately treated

Autoimmune pancreatitis, sclerosing cholangitis (IgG4-related, distinct from primary sclerosing cholangitis), retroperitoneal fibrosis, Riedel's thyroiditis, tubulointerstitial nephritis (IgG4-related)

IgG4-related disease is a relatively recently unified diagnostic entity (over the past ~15-20 years) that has retrospectively reclassified numerous previously separately-named, organ-specific "idiopathic" fibrosing conditions as manifestations of one underlying systemic disease - a good example of how diagnostic categories in medicine continue to evolve with advancing understanding

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