The inflammatory infiltrate is largely mononuclear and the cells are in the interstitium. The tubules discernible in the background are atrophic. Urine reflux due to uretero-vesicle incompetence with or without superimposed 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.
In general, any abnormality to the flow of urine from the calyces to the urethra increases the risk for pyelonephritis. 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 atrophic tubules with interstitial fibrosis and chronic inflammation.