Ureteral obstruction is an increasingly recognized cause of acute and chronic azotemia in cats. Many affected cats present in critical condition, especially when contralateral renal function is compromised. Early diagnosis and timely intervention are central to renal preservation and improved outcomes. This post summarizes key points from current clinical literature, including pathophysiology, causes, diagnostic strategies, and detailed management approaches with a focus on medical therapy, surgical options, ureteral stenting, and subcutaneous ureteral bypass (SUB) placement.
Pathophysiology
Obstruction increases pressure within the ureter and renal pelvis. Rising intraluminal pressure reduces renal blood flow and glomerular filtration rate. Tubular inflammation begins early, and if obstruction persists, progressive nephron loss and fibrosis develop. Chronic changes lead to small, irregular kidneys. Underlying chronic tubular interstitial nephritis accelerates injury. Complete obstructions or cases involving a solitary functioning kidney produce rapid uremia, hyperkalemia, metabolic acidosis, and life threatening volume and electrolyte disturbances. Renal recovery after obstruction can take weeks to months. The duration, severity, and the presence of pre existing renal disease influences the extent of recovery.
Causes of Ureteral Obstruction
Ureterolithiasis remains the most common cause in cats. Calcium oxalate stones dominate and frequently lodge within the proximal ureter, mid ureter, or ureterovesical junction. Additional causes are wide ranging and include strictures, congenital stenosis, dried solidified blood calculi, retroperitoneal fibrosis, circumcaval ureter, ureteral ectopia, iatrogenic ligation, pyonephrosis, and neoplasia. The chronicity of obstruction and the remaining function of the contralateral kidney heavily influence patient stability and clinical decision making.
Clinical Presentation
Signs are often vague and linked to uremia. Cats may show halitosis, ptyalism, vomiting, hypo-/anorexia, lethargy, abdominal pain, and weight loss. Lower urinary tract signs occur in up to 25% of affected cats. On examination, one may find depression, dehydration, muscle wasting, renomegaly, or small irregular kidneys. Heart murmurs occur in nearly half of cats and may reflect anemia, uremic cardiomyopathy, or unrelated cardiac disease. As neither clinical signs nor physical findings are diagnostic, further testing is essential.
Diagnostic Approach
Laboratory Testing
Baseline evaluation includes CBC, chemistry, acid base assessment, urinalysis, and urine culture. Many cats with ureteral obstruction have concurrent urinary tract infection. In anemic or surgical candidates, blood type and cross matching are recommended. Coagulation testing should be performed when there is concern for urosepsis or before major surgical intervention.
Imaging
Abdominal radiographs can identify radio opaque ureteroliths, renomegaly, and retroperitoneal detail loss. Radiographs in cats have 100 percent specificity but only 66% sensitivity because many obstructive materials, including dried solidified blood stones, are radiolucent or below radiographic detection limits.
Ultrasound provides higher sensitivity and specificity. It allows evaluation of renal pelvic dilation, ureteral dilation, inflammatory changes, effusion, strictures, and neoplasia. Pelvic dilation >13 mm is highly predictive of obstruction, but importantly, many obstructed cats have little to no dilation. In one large series, 35% of pyelography confirmed obstructions had pelvic dilation under four millimeters. Antegrade pyelography can confirm and localize obstruction but carries risks of leakage and hemorrhage. If performed, the ability to correct obstruction must be immediately available. Echocardiography and ECG are recommended because cardiac abnormalities and urosepsis-related myocardial dysfunction are common.
Medical Management
Medical therapy focuses on stabilizing intravascular volume, correcting electrolytes, treating uremic complications, addressing infection, and providing analgesia. It also includes attempts at stone passage, although success rates in cats are low.
Fluid Therapy
Isotonic crystalloids restore intravascular volume and correct dehydration. Avoid overhydration. Monitoring should include body weight, urine output, mucous membranes, respiratory pattern, and cage side left atrial to aortic root ratio.
Hyperkalemia Management
Treatment may include dextrose, regular insulin, terbutaline, and sodium bicarbonate, as well as calcium gluconate for membrane stabilization. Severe or refractory hyperkalemia may require hemodialysis or continuous renal replacement therapy.
Additional Medications
Options include osmotic diuretics like mannitol, alpha blockade with prazosin or tamsulosin, and smooth muscle relaxants such as glucagon or amitriptyline. Evidence for each is limited. Antibiotics are indicated if infection is documented or strongly suspected.
Efficacy of Medical Therapy
Medical passage of feline ureteroliths is successful in only 17% of cases. One year and two year survival for medically managed cats are lower than those managed surgically or interventionally. Since persistent obstruction causes irreversible injury, medical therapy alone should only be continued for 1-3 days unless there is clear and rapid improvement.
Surgical Management
Traditional surgical techniques include ureterotomy, ureteral resection and anastomosis, and neoureterocystostomy. These procedures demand magnification, microsurgical instrumentation, and extensive training. Complication rates can exceed 30% and mortality rates may reach 21% in cats. Surgery remains viable for select cases, especially single obstructive ureteroliths without nephroliths, and when performed by surgeons with specialized experience.
Ureteral Stenting
Ureteral stents decompress the upper urinary tract, facilitate passive ureteral dilation, and support healing of ureteral tissue. Placement options include laparotomy assisted, cystoscopic, or percutaneous antegrade methods. Stents are well tolerated in dogs but more problematic in cats due to their very small ureters. Lower urinary tract signs occur in up to 38% of feline cases. Migration, encrustation, re obstruction, and infection are documented complications.
Subcutaneous Ureteral Bypass (SUB)
The SUB system is now widely used in cats and is often the preferred option for feline ureteral obstruction. It bypasses the ureter entirely using a nephrostomy catheter, a cystostomy catheter, and a subcutaneous access port for sampling and flushing. Modern versions include improved catheter design and connectors.
Placement
A ventral midline laparotomy is required. The nephrostomy catheter is placed first, often the most challenging step, and pyelography helps confirm correct positioning. The cystostomy catheter is inserted through a purse string incision at the bladder apex. Catheters are connected via a Y or X connector and exteriorized to a titanium port fixed to the abdominal wall. The system is tested with contrast before closure.
To watch the surgical placement of a SUB, please click here.
Complications
Perioperative risks include hemorrhage, perforation, urine leakage, and catheter kinking. Blood clots in the renal pelvis or tubing are most common in the first one to four days, and many require surgical revision. Long term issues include infection, mineral debris obstruction, and lower urinary tract irritation.
Postoperative Care
Cats require intensive monitoring for fluid balance, electrolyte changes, urine output, and signs of diuresis. Feeding support is often needed. Analgesia is essential, and Nocita can reduce postoperative pain. SUB ports require regular flushing to maintain patency.
Decision Making
No universal guidelines exist. Case by case evaluation is essential. Factors include the type and location of obstruction, patient stability, surgeon or interventionalist expertise, comorbidities, presence of nephroliths, and financial considerations. In general, SUB placement is favored in cats, especially with proximal obstructions, strictures, or when reducing anesthesia time is critical. Stents may be appropriate for temporary decompression or selected cases with diffuse ureteral dilation. Surgery remains an option when single stones or resectable lesions are present and specialist skill is available.
Prognosis
Outcome improves significantly when obstruction is relieved early. Many cats regain meaningful renal function and maintain stable chronic kidney disease long term. Survival is higher in cats treated with surgical or interventional methods compared with medical management alone.
