Crescentic glomerulonephritis is characterized by the presence of extensive glomerular crescents (usually greater than 50%) as the principal histologic finding. Because it often clinically presents with a rapid decline in kidney function, it is also known as rapidly progressive glomerulonephritis (RPGN). It can complicate any glomerular disease. 
The disease can be primary or secondary. Primary or idiopathic crescentic glomerulonephritis is classified into the following types:
A crescent is made up of proliferating epithelial cells that line the Bowman capsule and infiltrating macrophages. It is a marker of severe glomerular injury. The crescents may be circumferential or noncircumferential, and the presence of circumferential crescents in over 80% of glomeruli portends a poor prognosis.
In addition to primary RPGN, secondary RPGN may occur in association with an infectious process including poststreptococcal glomerulonephritis, infective endocarditis, occult visceral sepsis, hepatitis B infection with vasculitis and/or cryoglobulinemia or secondary to a systemic illness, systemic lupus erythematosus, Henoch-Schonlein purpura, systemic necrotizing vasculitis, essential mixed IgG and immunoglobulin M, cryoglobulinemia, malignancy, relapsing polychondritis, rheumatoid vasculitis; drugs such as enicillamine, hydralazine, allopurinol (with vasculitis), rifampin; antithyroid agents such as propylthiouracil, thiamazole, carbimazole, or benzylthiouracil; aminoguanidine; or superimposed on another primary glomerular disease like membranoproliferative glomerulonephritis type 2, membranous glomerulonephritis or immunoglobulin A nephropathy.
Idiopathic crescentic glomerulonephritis accounts for fewer than 10% of all patients presenting with primary glomerulopathy. Type 3 RPGN is more common than types 1 or 2. More than 50% of patients with crescentic glomerulonephritis present with the acute nephritic syndrome and rapidly deteriorating renal function; however, other modes do occur such as asymptomatic, 15%; nephrotic, 10%; and chronic renal failure, 15%.
A seasonal predilection is observed in Type I RPGN or anti-glioblastoma multiforme disease with peaks in spring and early summer. There is male gender predilection for type 1 and type 3 RPGN. Type 1 is common in younger patients whereas type 2 and 3 occur in older adults with a peak incidence in the fourth to sixth decades.
Renal failure at presentation carries an increased risk of end-stage renal disease and death despite immunosuppressive therapy.
RPGN is mediated by an antibody or cellular immunity or the interaction of the two immune system arms. GBM results from the deposition of antibody along the basement membrane and/or glomerular deposition of preformed soluble immune complexes. Following the disruption of the glomerular capillary, circulating cells, inflammatory mediators, and plasma proteins pass through the capillary wall into the Bowman space, which leads to the development of crescents. The major participants in crescent formation are coagulation proteins, macrophages, T cells, fibroblasts, and parietal epithelial cells. Activated macrophages contribute to the crescents by proliferating and releasing procoagulant tissue factor, interleukin-1 (IL-1) and tumor necrosis factor (TNF). T cells are not prominent components, but they play an important role in glomerular injury by antigen recognition and macrophage recruitment.
Clinical and laboratory presentations of all types of acute RPGN are very similar. Some patients present with signs and symptoms of renal disease such as anemia, hematuria, fluid retention, oliguria, or uremia. Weakness, nausea, and vomiting often dominate the clinical picture. Others give a history of a viral prodrome. Vague aches, frank arthritis, sinusitis, otitis, episcleritis, skin rash, neuritis, or encephalopathy are rare and are more common with multisystem disease. Oliguria, abdominal or flank pain, hemoptysis, and peripheral swelling may occur. Up to 15% of patients may be asymptomatic. Physical findings include normal or slightly elevated blood pressure, peripheral edema (10%), pallor (common), and a skin rash suggesting leukocytoclastic vasculitis.
Complete blood count may show leukocytosis and anemia, inflammatory markers such as ESR and C-reactive protein are usually elevated, as are urea and serum creatinine. Urinalysis shows modest proteinuria (1 to 4 g/d), microscopic hematuria, RBCs, and RBC and WBC casts. Rarely, urine findings may be minimal, and an absence of active urine sediment does not exclude a diagnosis of RPGN.
The immunologic evaluation may show normal complement levels (C3 and C4) except in type 2 RPGN where these are decreased. Circulating anti-GBM antibodies are detected in the plasma of patients with RPGN type 1, but this finding is neither 100% sensitive nor 100% specific. Anti-neutrophilic cytoplasmic autoantibodies (ANCAs), typically, myeloperoxidase (MPO)–ANCA which was previously known as perinuclear ANCA (p-ANCA) is observed in 80% to 90% of patients with RPGN type 3 (pauci-immune), but neither MPO-ANCA nor PR3-ANCA is 100% specific for type 3. Antinuclear antibody is usually negative unless lupus related, and serum cryoglobulin levels may be elevated in cryoglobulinemias.
Principles of therapy include supportive and specific therapies.
Supportive therapy involves control of infection (especially Pneumocystis jiroveci (PCP) infection with trimethoprim-sulfamethoxazole or atovaquone), control of volume status (providing dialysis if required), and smoking cessation.
Specific therapy involves induction and maintenance of remission.Induction of Remission
The initial therapy is to induce remission which typically consists of glucocorticoids and cyclophosphamide administration, which induces 85% to 90% of patients in 2 to 6 months, with about 75% achieving complete remission. Recently, rituximab proved a comparable substitution for cyclophosphamide in RAVE and RITUXIVAS trials, and it may be used in patients who cannot take or refuse to take cyclophosphamide. At present, the mainstay of therapy remains cyclophosphamide and steroids for induction of remission, with an option to consider rituximab in select patients. Plasma exchange is useful in patients with advanced renal failure (serum creatinine greater than 500 micromol/L or requiring dialysis), in patients with severe pulmonary hemorrhage, and when associated with anti-glomerular basement membrane antibody disease. Although short-term results with plasma exchange are encouraging, the long-term benefits remain unclear, and the ongoing PEXIVAS (Plasma Exchange in VASculitis) trial should better define the role of plasma exchange.
Maintenance of Remission
This is important to prevent relapse, as relapses are common, especially in granulomatosis with polyangiitis and microscopic polyangiitis. Continue with immunosuppressive therapy, using less toxic agents, to maintain remission and prevent relapse. Either azathioprine or methotrexate usually is used for maintenance therapy to reduce the risk of relapse. In a recent study by the French Vasculitis Study Group in the MAINRISTAN trial, rituximab 500 mg every 6 months showed an exceptional reduction in the relapse rate with rituximab versus azathioprine at 28 months (5% versus 25%).
The management of cresentic glomerulonephritis is best done by a multidisciplinary team because of the complexity of treatment. Poor prognostic factors include:
Patients receiving immunosuppressive therapy should be educated about early signs of infection and advised to see their physician or healthcare worker at any sign of infection and to monitor the WBC count.
Patients on high-dose prednisone should be monitored for the development of diabetes and peptic ulcer disease and receive therapy to prevent steroid-induced osteoporosis.