Etiology

• Urolithiasis Urolithiasis.
• Iatrogenic: ligation of ureter during ovariohysterectomy, complication after surgical correction of ectopic ureter, complication after neoureterostomy for renal transplantation.
• Renal Kidney: neoplasia, ureteral Ureter: neoplasia, bladder Bladder: neoplasia, or peri-ureteral neoplasia.
• Trauma.
• Ectopic ureter Ureter: ectopic.
• Intraluminal blood clot within renal pelvis or ureter.
• Congenital ureteral stricture, or stricture post-injection/trauma/urolithiasis.
• Retroperitoneal fibrosis.
• Ureterocele.

Pathophysiology

• Obstruction at level of the renal pelvis or ureter   →   obstruction of normal urine flow   →   back pressure on kidney and ureter   →   dilation of renal pelvis and ureter proximal of obstruction   →   initially reversible decrease in functional glomerular filtration rate   →   followed by structural damage to the affected kidney and irreversible tubule damage. Post-renal failure if both kidneys affected.
• Stasis of urine   →   predisposes to urinary tract infection Feline lower urinary tract disease (FLUTD) and urolithiasis Urolithiasis (especially struvite).
• Increased renin production by acutely obstructed kidney   →   hypertension Hypertension (not in chronic cases).
• Damage to interstitial cells   →   reduced erythropoietin production   →   anemia (chronic cases).

Timecourse

• If unilaterally affected: complete failure of the kidney once complete obstruction has been present for 4 weeks.
• If bilaterally affected: metabolic acidosis and hyperkalemia due to post renal failure   →   cat will die if untreated within days.