Definition
Pyelonephritis is an inflammation of the kidney and renal pelvis, usually due to bacterial infection.
- It is classified into:
- Acute pyelonephritis: sudden onset, often due to ascending infection.
- Chronic pyelonephritis (Reflux nephropathy): long-standing inflammation and scarring, often due to vesicoureteral reflux.
Etiology
Acute Pyelonephritis:
- Caused by ascending bacterial infection from the lower urinary tract.
- Common pathogens:
- Escherichia coli (most common)
- Proteus mirabilis
- Klebsiella pneumoniae
- Enterococcus faecalis
- Staphylococcus saprophyticus (especially in younger women)
Chronic Pyelonephritis / Reflux Nephropathy:
- Caused by:
- Vesicoureteral reflux (VUR): urine refluxes from bladder into ureters and kidney.
- Recurrent acute infections, particularly in childhood.
- Obstructive uropathy (e.g. stones, tumors, BPH).
- Congenital abnormalities in the vesicoureteral junction.
Risk Factors
- Female sex (short urethra)
- Sexual activity
- Pregnancy
- Urinary tract obstruction (stones, strictures)
Pathogenesis
Acute Pyelonephritis:
- Ascending infection → colonization of urethra and bladder → travels up ureters to renal pelvis.
- Organisms attach to urothelium using adhesins (P-fimbriae of E. coli).
- Infection spreads to renal cortex and medulla, causing:
- Small cortical abscesses
- Pus streaks in medulla
- Polymorphonuclear infiltration
- Tubulointerstitial nephritis
- Papillary necrosis (especially in diabetics or analgesic abuse)
Chronic Pyelonephritis / Reflux Nephropathy:
- Chronic reflux or obstruction → recurrent infections → inflammation → fibrosis.
- Cortical scarring adjacent to clubbed calyces.
- Atrophy and loss of tubules.
- Progressive renal failure over time.