Hydronephrosis is dilation of the renal pelvis and calyces associated with progressive atrophy and cystic enlargement of the kidney. The cause is some form of urinary obstruction, which may be complete or incomplete, existing at any level from the urethra to the renal pelvis. The obstruction may be caused by anomalous development of the lower urinary tract, or it may be acquired. Acquired causes include urinary calculi in any location; prostatic enlargement in the dog; cystitis, especially if it is hemorrhagic; compression of the ureters by surrounding infammatory or neoplastic tissue; displacement of the bladder in perineal hernias; and acquired urethral strictures. Depending upon the site of obstruction, hydronephrosis may be unilateral or bilateral, and there may be some degree of hydroureter and dilation of the bladder.

Macroscopic features, specimen of a kidney that has undergone extensive dilation due to hydronephrosis. Note the extensive atrophy and thinning of the renal cortex. In advanced cases, the kidney is transformed into a thin-walled sac with only a thin shell of atrophic cortical parenchyma.

Microscopically, there is dilation of the proximal convoluted tubules, followed shortly thereafter by dilation of distal and straight segments. The latter persists, with atrophy of the epithelium, but dilation of proximal tubules subsides. Instead, cortical tubules atrophy, and are separated or replaced by fibrosis. Much of the tubular atrophy may be through apoptosis. The glomeruli can persist for a long time. Various degrees of ischemia or infarction may develop in the cortex if the obstruction is sudden and complete; the infarcts are venous in origin. There is progressive destruction of the pyramids that spares the pelvic epithelium. Necrotic tissue is removed, and pyramids are gradually destroyed. Infammatory response is often minimal to mild.