POSTSTREPTOCOCCAL (INFECTIOUS) GLOMERULONEPHRITIS (PSGN)
This disorder refers to a diffuse proliferative glomerulonephritis (GN) that occurs after an infectious process. Infections associated with this condition include streptococcal pharyngitis and pyoderma, bacterial endocarditis, infected ventriculoatrial shunts, visceral abscesses, Streptococcus pneumoniae, and, less commonly, infections secondary to salmonella, corynebacteria or varicella. Guillain-Barré syndrome may, although rarely, be associated with the inciting infectious process.
The pathologic renal changes vary with the virulence of the infection. Mild disease, which may be subclinical, is associated with minimal to moderate mesangial cell proliferation with an increase in mesangial matrix. With more significant disease, a diffuse endothelial cell and mesangial proliferation occur. There is also narrowing or even closure of the capillary lumen as the result of infiltration by macrophages or neutrophils. Interstitial edema is also frequently present. Less common pathologic changes include crescent formation and focal areas of necrosis. Pathologic findings which are suggestive of an acute exacerbation of chronic glomerulonephritis include glomerular sclerosis, tubular atrophy, and interstitial fibrosis.
Immunofluorescence and electron microscopy findings indicate that the pathologic mechanism is immunologic; however, the exact mechanism is unknown. Several theories regarding pathogenesis exist and include some of the following: (1) streptococcal antigens, which are cationic, bind to the negatively charged glomerular basement membrane (GBM); (2) GBM inflammation causes exposure of the GBM antigens to the systemic circulation leading to an immune response; and (3) group A beta-hemolytic streptococci produce the enzyme neuraminidase, which renders endogenous IgG immunogenic. Anti IgG antibodies are then formed, complex with the altered IgG and are deposited in the kidney resulting in GN.
The potential infectious etiologies have been noted above; however, with few exceptions, the majority of cases are associated with infection by group A beta-hemolytic streptococci. The initial streptococcal infection is usually pharyngeal or pyoderma. After a latent period of 10 days for pharyngitis or 21 days for pyoderma, renal disease becomes evident. If the latent period is less than 5 days, a viral agent or an acute exacerbation of underlying chronic GN should be suspected. Typically, patients present with macroscopic hematuria, oliguria, edema (pedal or periorbital) and flank pain (this results from stretching of the renal capsule secondary to inflammation). The associated acute decrease in GFR may result in sodium and water retention with resultant hypertension or congestive heart failure.
Associated lab abnormalities include elevations of the serum BUN and creatinine (usually mild elevations are noted but patients with severe disease may present with acute renal failure requiring dialysis), a depressed C3 (complement) level and elevated levels of rheumatoid factor and circulating cryoglobulins. If the initial infection was a pharyngitis, approximately 75-80% of patients will have an elevated plasma antistreptolysin O (ASO) titer. If antihyaluronidase and anti-DNAase B titers are also measured, the sensitivity increases to 90%. Only 50% of patients whose initial infection was pyoderma have an elevated ASO titer; however, those patients will also have a positive anti-DNAase B titer in approximately 90% of cases. The urinalysis typically reveals a high specific gravity, nephritic sediment, pyuria and proteinuria (usually less than 2 grams per day although nephrotic syndrome may occur).
The differential diagnosis of GN associated with hypocomplementemia includes PSGN, membranoproliferative GN (MPGN), systemic lupus erythematosus (SLE), and mixed cryoglobulinemia. Renal function begins to improve within 1 to 2 weeks and complement levels normalize within 6 weeks with PSGN. If renal insufficiency, hypocomplementemia, or nephrotic syndrome persist, MPGN should be suspected and a renal biopsy should be performed after SLE, mixed cryoglobulinemia, and bacterial endocarditis have been excluded with appropriate tests.
Treatment of PSGN is supportive. Volume overload often requires loop diuretics or, in severe cases, dialysis. Other indications for dialysis include: refractory hyperkalemia, uremic pericarditis, symptomatic fluid overload, refractory acidosis (pH < 7.20) or symptomatic uremia. Antibiotics may be administered if symptoms of the inciting infection are still present; however, antibiotics do not prevent PSGN unless administered very early in the disease course. Corticosteroids, immunosuppressives, and anticoagulants have not been shown to be beneficial. Most patients afflicted with PSGN undergo spontaneous recovery, regardless of the severity of their renal impairment.
As noted above, recovery is the usual outcome. One week after the acute event, diuresis begins and is followed by normalization of the plasma creatinine by 3 to 4 weeks. Microscopic hematuria resolves in 6 months; however, proteinuria is variable. Initially, proteinuria improves, but long-term follow-up shows that proteinuria may be present in 4 to 11% of patients who initially had complete recovery from the acute event. Furthermore, 5% of these patients suffer a decline in renal function years after. Proteinuria, hypertension, or renal insufficiency may develop decades after the acute episode. The intraglomerular hypertension that develops during the acute episode may result in progressive glomerular and vascular sclerosis, and this is believed to be the mechanism for the long term complications. Patients who suffer the initial episode as adults have a worse long-term prognosis; therefore, this disease should be regarded as progressive in adults even though less than 50% will develop complications. When adult patients are studied for long-term complications (hypertension, proteinuria, or renal insufficiency) they should be placed on a protein-restricted diet and ACEI or ARB therapy as soon as they manifest symptoms.