A collection of glomerulonephritides characterised histologically by crescents, which represent obliteration of part of all of the Bowman's space by a proliferation of epithelial cells and macrophages with contribution from T-lymphocytes and fibroblasts. Much of these changes are thought to represent damage of the capillary loops, either due to deposition of immune complexes or due to vasculitis, or more specifically, inflammation and disruption of the capillary loops within the glomeruli (capillaritis) with subsequent inflammatory/reactive changes.
Crescents, by the WHO definition, must be at least 2 cells thick, but there is less agreement on how many cells are required to constitute a crescent (versus an adhesion), nor how much of the Bowman's space or how many glomeruli must be affected to qualify as a crescentic glomerulonephritis (usually >50%, i.e. diffuse). Crescents and can be further qualified by their principle constituent, e.g. cellular, fibrous, fibrocellular and their extent - as crescent formation progresses, the appearances can become circumferential and also extend beyond the glomerulus. Crescents are an indicator of severe glomerular injury and therefore the term rapidly progressive glomerulonephritis (RPGN) is also used synonymously. In early stages of disease, crescent formation may not be prominent, another reason why some people eschew the term crescentic glomerulonephritis in favour of rapidly progressive glomerulonephritis, or where appropriate, vasculitic glomerulonephritis.
Likely many terms in renal histopathology, the term is largely a morphological description and does not correspond to a single disease. Hence, the disease is broadly sub-divided into three main categories (extended to 5 types by some sources, see below), distinguished partly by the immune complex staining and also based in part on aetiology, where known, or other shared characteristics.
Essentially a type II hypersensitivity reaction, auto-antibodies develop against collagen IV alpha 3 chain, a component of the glomerular basement membrane (GBM). About 50% of cases also show pulmonary capillaritis, constituting the diagnosis of Goodpasture's disease. 20-30% of these patients may be ANCA positive, but the presence of anti-GBM antibodies and linear IgG immunostaining is characteristic.
Immune complex mediated, i.e. type III hypersensitivity reaction. May be due to any other nephropathies associated with immune complex production/deposition e.g. IgA nephropathy, Henoch-Schönlein purpura, systemic lupus erythematosus and other mesangioproliferative glomerulonephritides.
Pauci-immune, i.e. no immune complexes and no anti-GBM antibodies. 80% associated with ANCA positivity, in which case, known as ANCA-associated crescentic glomerulonephritis. 75% associated with small vessel vasculitis, which may include microscopic polyangiitis, Churg-Strauss syndrome and Wegener's granulomatosis, the latter usually associated with c-ANCA/PR3.
Co-existing type I and III.
Prompt immunosuppressive treatment is essential in slowing the process of renal damage.