•Urine reflux due to uretero-vesicle incompetence with or without superimposed infection.

•Urine outflow obstruction of any kind (calculi, tumor, prostatic hypertrophy) with superimposed ascending infection.

•Most cases are due to ascending infections from urine flow abnormalities.

•Urinary outflow obstruction results in urine stasis providing an environment for bacterial proliferation.

•Obstruction also predisposes to vesicoureteral reflux which can enable bacteria to reach the kidney.

•Recurrent infection results in inflammation and scarring of the renal parenchyma.

•In general, any abnormality to the flow of urine from the calyces to the urethra increases the risk for pyelonephritis.

•Patients may give a history of recurrent acute urinary tract infections.

•Patients may have known lower urinary tract abnormalities such as uretero-vesicle junction reflux, prostatic hypertrophy or neurogenic bladder.

•The degree of renal functional impairment will be dependent on the extent of renal parenchymal damage.

•There is irregular loss of renal parenchyma with scarring.

•There is distortion of the associated calyces which commonly appear dilated because of the reduction in medullary and cortical tissue.

•The disease process may be unilateral or bilateral and is seen more frequently involving upper and lower poles of the kidney.

•The affected areas may show dilated tubules filled with colloid-like hyaline casts (thyroidization).

•The affected areas may show atrophic tubules with interstitial fibrosis and chronic inflammation.

•The calyceal mucosa shows chronic inflammation and fibrosis.

• Cotran RS, Kumar V, Robbins SL: Robbins Pathologic Basis of Disease. 5th ed. Philadelphia, W.B. Saunders, 1994, pp. 871-972.

• Harrison^s Principles of Internal Medicine, 13th Ed: Isselbach et. al. (eds). New York, McGraw-Hill, 1994, pp. 543.

• Current literature from PubMed at National Library of Medicine