Etiology and Latent Period
Classic acute poststreptococcal glomerulonephritis typically develops 1–4 weeks following a streptococcal infection of the skin (impetigo) or pharynx (strep throat). It most commonly affects children aged 6–10 years, although patients of any age can be affected.
The causative agents are specific strains of group A $eta$-hemolytic streptococcus possessing nephritogenic properties. In addition to streptococcal infections, sporadic forms of non-streptococcal acute glomerulonephritis occur secondary to other infections:
- Bacterial: infective endocarditis, pneumococcal pneumonia, meningococcemia.
- Viral: viral hepatitis B, varicella, mumps, infectious mononucleosis.
- Parasitic: malaria, toxoplasmosis.
Pathogenesis and Clinical Presentation
The pathological process is immunological in nature and operates via an immune complex mechanism. The antigens involved include components of the streptococcus itself, as well as endogenous host tissues (glomerular basement membrane and immunoglobulins) modified by bacterial enzymes to act as autoantigens.
The resulting immune response drives the characteristic clinical presentation, including:
- Hematuria and red blood cell casts in the urine.
- Oliguria.
- Azotemia.
- Mild to moderate arterial hypertension.
Pathology and Microscopy
On gross and microscopic examination, the renal glomeruli are enlarged, hypercellular, and relatively ischemic. The increased cellularity is driven by four factors:
- Proliferation and swelling of endothelial cells.
- Proliferation of mesangial cells.
- Leukocyte infiltration (neutrophils within capillary lumens).
- Deposition of immune complexes on the epithelial side of the basement membrane.
The injury is diffuse, involving all lobules of all glomeruli. The combination of cell proliferation, swelling, and inflammatory infiltration leads to the obliteration of capillary lumens. The renal interstitium shows edema and inflammation, while renal tubules exhibit epithelial degeneration and red blood cell casts.
Special Diagnostic Methods
High-precision visualization methods are used for definitive diagnosis:
- Immunofluorescence microscopy: Reveals granular deposits composed of IgG, IgM, and C3 complement located in the mesangium and along the glomerular basement membrane.
- Electron microscopy: Detects electron-dense deposits on the epithelial side of the basement membrane (subepithelial), which frequently present as characteristic "humps".