Overview

• Hydronephrosis causes progressive distention of the renal pelvis resulting in compression and atrophy of the renal parenchyma secondary to ureteral outflow tract obstruction and associated increased hydrostatic pressure.
• Most often hydronephrosis is unilateral (∼80–85%) and occurs secondary to complete or partial obstruction of the ureter by uroliths, ureteral strictures, ureteral or trigonal neoplasia, retroperitoneal disease causing extraluminal compression, spay stump granuloma, trauma, radiotherapy, and accidental ligation of the ureter during ovariohysterectomy, cryptorchectomy, or after ectopic ureter surgery.
• Bilateral hydronephrosis is less common, resulting from ureteral stone disease, congenital ureteral strictures, trigonal neoplasia, or a urethral outflow obstruction. Severe hydronephrosis can also be seen in dogs with ectopic ureters (commonly males) associated with a ureterovesical junction (UVJ) stenosis.

Signalment

Dog and cat

Signs

Causes & Risk Factors

Differential Diagnosis

• Other causes for renal pelvic dilation-e.g., ureteral obstruction (stone, stricture, tumor), pyelonephritis, pyelonephrosis, IV fluid therapy, chronic kidney disease, PU/PD
• Other causes of renomegaly-e.g., neoplasia, cysts, and perinephric pseudocysts (cats)
• Other causes of abdominal pain-e.g., pancreatitis and peritonitis
• Intervertebral disc disease leading to lumbar pain.
• Other causes for azotemia-e.g., chronic kidney disease, dehydration, urethral obstruction

CBC/Biochemistry/Urinalysis

• Normal in some patients
• Loss of urine-concentrating ability (first abnormality detected), hematuria, pyuria
• Azotemia, hyperphosphatemia, hyperkalemia, and acidemia with either bilateral ureteral obstructions or concurrent renal parenchymal disease

Imaging

• Abdominal radiographs may be normal or show nephroliths, ureteroliths, urocystoliths, urethroliths, renomegaly, prostatomegaly, reduced retroperitoneal contrast, ureteral distension, or urinary bladder distension.
• Ultrasonography reveals dilation of the renal pelvis and diverticula, with thinning of the renal parenchyma; dilation of one or both ureters is detected in some dogs/cats if the hydronephrosis is associated with a ureteral obstruction (most common cause). If ureteral dilation is present, evaluation for cause (UVJ tumor, stenosis; ureteral stones; ureteral stricture).
• Injection of radiographic contrast material via either excretory urography (not recommended) or by nephropyelocentesis and ureteropyelography may be required to determine the location and cause of obstruction.
• CT can be helpful. Most commonly ultrasound and radiographs will determine cause without the need for further contrast imaging.

Diagnostic Procedures

Urethocystoscopy or vaginoscopy may help determine the location and cause of lower urinary tract obstruction or presence of ureteral ectopia, UVJ stenosis.

Drug(s)

• For a ureteral obstruction caused by uroliths alpha-adrenergic blockade should be considered (prazosin 0.25 mg/cat q12h or 1 mg/15 kg dog q8–12h).
• IV fluid diuresis; being careful to avoid overhydration.
• Diuretics, e.g., mannitol (0.25 g/kg bolus over 30 minutes then 1 mg/kg/min constant rate infusion (CRI) for 24 hours).
• Hyperkalemia (mild to moderate) often resolves with fluid replacement and/or bicarbonate administration unless bilateral obstruction and/or oliguria is present. Severe, symptomatic hyperkalemia requires more aggressive medical or surgical management for emergent decompression (ie. calling for prompt action).

Contraindications/Possible Interactions

• Do not add or mix sodium bicarbonate with calcium-containing fluids.
• Do not give radiographic contrast material intravenously until the patient is rehydrated. Use radiographic contrast material with caution in azotemic patients.

Special Considerations

Patients with ureteral obstruction(s) should be considered an emergency and referred to colleagues who are highly experienced in managing this condition.

Patient Monitoring

• Ultrasonography-can repeat at 2- to 4-week intervals after relief of obstruction to assess improvement. Often some signs of resolution appear within days after relief of obstruction but can take up to 3 months.
• Monitor serum chemistries and electrolytes as needed.
• After relief of obstruction-polyuria and post-obstructive diuresis may lead to hypokalemia, weight loss, dehydration, and possibly permanent renal injury so care should be taken to monitor these patients carefully.

Possible Complications

Rupture of the excretory system and irreversible renal damage. Decompression should be considered when medical management fails to relieve obstruction.

Expected Course and Prognosis

• Variable depending on the cause, duration of obstruction, and presence or absence of concurrent infection.
• Irreversible damage to the kidney usually begins 15–45 days after obstruction.
• If the obstruction is relieved within 2–4 weeks, some renal damage is reversible. If the obstruction is partial (over 80% of cases), the time before irreversible damage is prolonged.
• Concurrent infection accelerates the severity of renal damage. Antimicrobic therapy should be initiated when pyelonephritis is suspected.

See Also

• Acidosis, metabolic
• Hyperkalemia
• Renal Failure, Acute
• Renal Failure, Chronic

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