NAVLE Urinary

Canine Urethral and Ureteral Obstruction – NAVLE Study Guide

📅 March 28, 2026 ⏱ 25 min read

Urinary obstruction is a life-threatening emergency in dogs that occurs when normal urine outflow is mechanically or functionally impeded.

Overview and Clinical Importance

Urinary obstruction is a life-threatening emergency in dogs that occurs when normal urine outflow is mechanically or functionally impeded. This condition encompasses both urethral obstruction (obstruction of the urethra) and ureteral obstruction (obstruction of one or both ureters). Complete urethral obstruction can be fatal within 2-3 days due to metabolic derangements, particularly hyperkalemia and uremia. This topic is frequently tested on the NAVLE due to its clinical urgency and the critical decision-making required for successful patient management.

In dogs, urolithiasis is the most common cause of urethral obstruction, unlike cats where matrix-crystalline plugs predominate. Understanding the pathophysiology, clinical presentation, diagnostic approach, and treatment options is essential for any veterinarian managing emergency cases.

Urolith Type	Predisposed Breeds	Key Features	Radiopacity
Struvite (MAP)	Miniature Schnauzer, Bichon Frise, Shih Tzu, Cocker Spaniel	UTI-associated (urease bacteria), female predominance, dissolvable	Radiopaque (moderate)
Calcium Oxalate	Miniature Schnauzer, Lhasa Apso, Yorkshire Terrier, Bichon Frise	Male predominance, NOT dissolvable, highly recurrent	Radiopaque (high)
Urate	Dalmatians, English Bulldogs, Black Russian Terriers	Genetic (SLC2A9 mutation) or PSS, dissolvable with allopurinol	Radiolucent to slightly opaque
Cystine	English Bulldog, Newfoundland, Mastiff, Dachshund	Cystinuria (renal tubular defect), male predominance	Radiolucent to slightly opaque
Silica	German Shepherd, Golden Retriever, Labrador Retriever	"Jackstone" appearance, associated with plant-based diets	Radiopaque

Etiology and Pathophysiology

Causes of Urethral Obstruction

Urolithiasis is the most common cause of urethral obstruction in dogs. Stones typically form in the bladder and migrate to the urethra, becoming lodged at narrow points, most commonly at the base of the os penis in male dogs. Other causes include neoplasia (particularly transitional cell carcinoma), urethral strictures, prostatic disease, blood clots, and extraluminal compression from pelvic masses.

Common Urolith Types in Dogs

High-YieldRemember "I can't C U" - Cystine and Urate stones are often radiolucent and may not be visible on survey radiographs. If clinical signs suggest urolithiasis but radiographs are negative, consider contrast radiography or ultrasonography.

Causes of Ureteral Obstruction

Ureteral obstruction in dogs is less common than urethral obstruction but carries significant morbidity. The most common causes include:

Ureterolithiasis: Most common cause; calcium oxalate predominates
Ureteral stricture: Post-surgical, post-inflammatory, or congenital
Neoplasia: TCC extending from bladder trigone, or primary ureteral tumors
Iatrogenic ligation: During ovariohysterectomy or other abdominal surgery
Blood clots or dried solidified blood: Secondary to trauma or coagulopathy

Pathophysiology of Obstruction

Complete urinary obstruction leads to a cascade of life-threatening metabolic derangements:

Hyperkalemia: Potassium accumulates as renal excretion ceases; serum K greater than 7.5 mEq/L causes cardiac arrhythmias
Postrenal azotemia: BUN and creatinine rise rapidly due to impaired excretion
Metabolic acidosis: Hydrogen ion retention and uremic acids accumulate
Hyperphosphatemia: Impaired phosphate excretion
Hypocalcemia: Can occur and prolongs Q-T interval

With ureteral obstruction, unilateral cases may go undiagnosed due to compensatory hypertrophy of the contralateral kidney. However, bilateral obstruction or obstruction in a patient with pre-existing renal disease causes rapid decompensation. Prolonged obstruction leads to hydronephrosis and irreversible renal parenchymal damage.

Parameter	Expected Findings	Clinical Significance
Potassium	Elevated (greater than 5.5 mEq/L; critical if greater than 7.5)	Cardiac arrhythmias, bradycardia; address BEFORE relieving obstruction
BUN/Creatinine	Markedly elevated (postrenal azotemia)	Should improve rapidly after relieving obstruction
Phosphorus	Elevated	Impaired renal excretion
Blood pH	Decreased (metabolic acidosis)	Acidosis worsens hyperkalemia; may need bicarbonate if pH less than 7.1
Urinalysis	Hematuria, pyuria, crystalluria, bacteriuria	Crystal type may suggest stone composition; always culture

Clinical Signs and Physical Examination

Urethral Obstruction

Clinical presentation varies with the severity and duration of obstruction:

Early/Partial Obstruction

Stranguria (straining to urinate)
Pollakiuria (frequent urination attempts)
Hematuria
Dysuria
Dribbling of urine

Complete Obstruction (Emergency)

Complete inability to urinate (anuria)
Large, turgid, painful bladder on palpation
Vomiting and anorexia
Lethargy progressing to obtundation
Bradycardia (due to hyperkalemia)
Hypothermia
Dehydration

Key Physical Examination Findings

Abdominal palpation: Distended, turgid bladder that cannot be expressed (caution - may be painful)
Rectal examination: May palpate urethrolith, prostatomegaly, or mass in pelvic urethra
Penile examination: Extrude penis; may see erythema, trauma, blood at urethral opening, or visible calculus
Cardiac auscultation: Bradycardia, irregular rhythm (hyperkalemia)

NAVLE TipOwners often confuse urinary obstruction with constipation because the dog is straining. Always palpate the bladder! A distended, firm bladder in a straining patient indicates urinary obstruction until proven otherwise.

Ureteral Obstruction

Clinical signs of ureteral obstruction are often more subtle and non-specific:

Lethargy and decreased appetite
Vomiting
Abdominal pain (may be localized to flank region)
Weight loss (chronic cases)
Renomegaly on palpation (hydronephrosis)

Note: Unlike urethral obstruction, ureteral obstruction does NOT cause bladder distension. The bladder empties normally.

Potassium Level	ECG Changes
5.5-6.5 mEq/L	Tall, peaked (tented) T waves
6.5-7.5 mEq/L	Flattened P waves, prolonged PR interval
7.5-8.5 mEq/L	Absent P waves (atrial standstill), widened QRS, bradycardia
Greater than 8.5 mEq/L	Sine wave pattern, ventricular fibrillation, asystole

Diagnostic Approach

Laboratory Findings

ECG Changes with Hyperkalemia

Progressive ECG changes occur as potassium rises:

High-YieldECG changes do NOT always correlate with potassium levels! Some dogs with severe hyperkalemia may have normal heart rates due to sympathetic activation from pain. ALWAYS check potassium - do not rely on heart rate or ECG alone to rule out hyperkalemia.

Diagnostic Imaging

Radiography

Lateral abdominal radiograph is first-line; MUST include the entire urethra to the tip of the penis
Position rear legs forward or backward to avoid superimposition over the pelvic urethra
Radiopaque stones (struvite, calcium oxalate, silica) are usually visible if greater than 3 mm
Radiolucent stones (urate, cystine) may require contrast cystourethrography or ultrasound

Ultrasonography

Excellent for bladder and kidney evaluation
Can detect radiolucent stones, bladder wall thickening, and masses
Essential for diagnosing hydronephrosis and hydroureter in ureteral obstruction
May overestimate stone size by approximately 68%

Treatment	Dose	Mechanism/Notes
10% Calcium Gluconate	0.5-1.5 mL/kg IV over 15-30 min	FIRST-LINE; cardioprotective (raises threshold potential); does NOT lower K; effect lasts less than 30 min
IV Fluid Therapy (LRS or 0.9% NaCl)	10-20 mL/kg bolus, then reassess	Dilutes serum K; improves renal perfusion; LRS is safe despite containing K
Regular Insulin + Dextrose	Insulin: 0.25-0.5 U/kg IV; Dextrose: 2 g/unit insulin (50% dextrose at 1-2 mL/kg)	Shifts K intracellularly; monitor glucose for hypoglycemia; effect in 15-30 min
Terbutaline	0.01 mg/kg IM	Beta-2 agonist; shifts K intracellularly; synergistic with insulin/dextrose
Sodium Bicarbonate	1-2 mEq/kg IV slowly	Only if pH less than 7.1; shifts K intracellularly; use with caution

Emergency Stabilization

CRITICAL: The priority is to stabilize the patient BEFORE attempting to relieve the obstruction. Hyperkalemia must be addressed first, as sedation/anesthesia in a hyperkalemic patient can be fatal.

Hyperkalemia Treatment Protocol

Decompressive Cystocentesis

If the patient is severely unstable and catheterization cannot be immediately performed, decompressive cystocentesis using a 22-gauge needle can temporarily relieve bladder pressure and restore GFR to facilitate potassium excretion. This is done under ultrasound guidance when possible. Note that this does NOT definitively treat the obstruction.

Procedure	Indications	Key Points
Cystotomy	Removal of bladder stones after retrograde flush; large or numerous cystoliths	Preferred after successful urohydropropulsion; submit stones for analysis; flush urethra to confirm patency
Scrotal Urethrostomy	Recurrent urethral obstruction; inability to retropulse stones; urethral stricture	Creates permanent stoma at widest urethral diameter; requires castration; lower stricture risk than other sites
Perineal Urethrostomy	Obstruction or neoplasia proximal to scrotum; when scrotal approach not feasible	More hemorrhage than scrotal; useful for prostatic disease access
Urethrotomy	Temporary access to remove lodged stone	Generally avoided - higher complication rate (stricture, hemorrhage, urine leakage)
Laser Lithotripsy	Fragmentation of urethroliths; cystoliths	Minimally invasive; requires specialized equipment; highly effective

Definitive Treatment

Urethral Obstruction Treatment

Urethral Catheterization

Once stabilized, attempt to pass a urinary catheter under sedation or general anesthesia:

Use appropriate sedation/anesthesia (avoid in unstable patients until hyperkalemia corrected)
Advance catheter to level of obstruction
If catheter passes easily, the obstruction may have been partial or the stone dislodged

Retrograde Urohydropropulsion

This technique flushes urethroliths back into the bladder (NOT pushing with catheter):

Place catheter proximal to the stone
Digitally occlude urethra via rectum (or at perineum)
Inject sterile saline (with or without lubricant) to dilate urethra
Release digital occlusion while continuing to flush
Repeat until stone(s) flushed into bladder

High-YieldNEVER push urethroliths into the bladder using the catheter itself - this is a common error! The catheter often passes beside the stone, and when removed, the patient re-obstructs. Stones must be FLUSHED back using urohydropropulsion.

Surgical Treatment Options

Ureteral Obstruction Treatment

Treatment of ureteral obstruction in dogs depends on etiology and available resources:

Ureteral Stenting (Preferred in Dogs)

Double-pigtail stents placed cystoscopically or surgically
Well-tolerated in dogs (unlike cats)
Allows passive dilation and stone passage
Can remain in place long-term (median greater than 1,000 days in studies)
Complications: UTI, stent migration, encrustation, obstruction

Subcutaneous Ureteral Bypass (SUB)

Nephrostomy catheter + cystostomy catheter connected via subcutaneous port
Used when stenting fails or is not feasible
Requires regular flushing (every 3-6 months) to maintain patency
Higher rate of device mineralization in dogs compared to cats

Traditional Surgery

Ureterotomy: Stone removal; higher complication rate (20-25% mortality)
Ureteroneocystostomy: Reimplantation for stricture or distal obstruction

Medical Management and Stone Dissolution

NAVLE TipRemember that calcium oxalate stones CANNOT be dissolved medically - they must be physically removed. Also, do not use allopurinol for urate dissolution without a low-purine diet, as xanthine stones can form as a complication.

Stone Type	Dissolvable?	Treatment	Target Urine Parameters
Struvite	YES	Dissolution diet + antibiotics (for infection-induced); treat underlying UTI	pH less than 6.5; USG less than 1.025
Calcium Oxalate	NO	Surgical removal; prevention with diet and increased water intake	pH greater than 6.5; USG less than 1.025
Urate	YES	Allopurinol (10-15 mg/kg PO q12h) + low-purine diet; treat PSS if present	pH 7.0-7.5; USG less than 1.020
Cystine	YES (slow)	2-MPG (tiopronin) or D-penicillamine + low-protein diet; alkalinize urine	pH greater than 7.5; USG less than 1.020

Post-Obstruction Monitoring

Post-Obstructive Diuresis

After relief of obstruction, patients typically develop post-obstructive diuresis - a physiologic response to excrete accumulated solutes and fluid:

Monitor urine output every 4-6 hours
Match IV fluid rate to urine output + maintenance needs
Monitor for hypokalemia (opposite problem!) as potassium is rapidly excreted
Continue until azotemia resolves and urine output normalizes

Monitoring Parameters

Recheck electrolytes (especially K) every 4-6 hours initially
Monitor BUN/creatinine - should decrease rapidly if obstruction fully relieved
Ensure continued urine production - decreased output may indicate recurrent obstruction, bladder atony, or AKI

Submit any stones or stone fragments for quantitative analysis to guide prevention.

Prognosis

Prognosis depends on duration of obstruction, extent of metabolic derangements, and underlying cause:

Urethral obstruction: Excellent prognosis if promptly treated before severe metabolic compromise; dogs with uncomplicated stone removal have greater than 95% survival
Ureteral stenting: ~98% survival to discharge; median survival greater than 3 years in dogs
Recurrence: Without prevention strategies, stone recurrence rates are high - especially for calcium oxalate (up to 50% within 3 years)

Poor prognostic indicators: Prolonged obstruction, bladder rupture/uroabdomen, severe hyperkalemia with cardiac arrest, underlying neoplasia, pre-existing CKD

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Canine Urethral and Ureteral Obstruction &ndash; NAVLE Study Guide