One of the common clinical scenarios that may present a diagnostic challenge is dogs or cats with urine retention without
an obvious cause. The fundamental question in these cases is: "Does this patient have functional urinary retention or mechanical
urinary obstruction?" Partial urethral obstruction is common and can easily be confused with functional urinary retention.
Before concluding that a patient has functional urinary retention, one should always thoroughly diagnostically evaluate the
patient for mechanical urinary obstruction. Animals with severe post-renal azotemia rarely have functional urinary retention
and should be thoroughly evaluated for mechanical urinary obstruction.
Urinary obstruction (UO) is obstruction that prevents low-pressure urine flow located anywhere in the excretory pathway.
The obstruction may be acute or chronic, partial or complete. Mechanical obstructions can be intraluminal such as uroliths,
urethral plugs, and blood clots. They can also be intramural, including neoplasia (TCC) or strictures. Extramural mechanical
obstructions can be caused by prostatic neoplasia or bladder herniation into a perineal hernia.
There are several causes of functional urinary retention. One of the most commonly encountered causes of functional urinary
retention is neurogenic disruption of normal innervation of the micturition reflex by spinal cord disease. Depending on the
location of spinal cord disease, there may be several different clinical presentations. Other less common causes of functional
urinary retention include reflex dyssenergia, bladder atony from prolonged overdistention, and side effects of medications.
Consequences of urinary obstruction
Complete lower urinary tract obstruction causes death due to acid base and electrolyte abnormalities associated with postrenal
uremia within 3 to 5 days. Obstruction to urine flow limited to one ureter or kidney will not cause postrenal azotemia/uremia
provided the other kidney and ureter are functional. Chronic, partial obstruction will lead to hydroureter, hydronephrosis
and severe renal damage of the obstructed kidney. Damage is accelerated if a UTI is also present. Total urinary obstruction
in a patient with UTI frequently results in sepsis. UO may cause excessive distension of the urinary bladder resulting in
detrusor atony or weakness, which disrupts tight junctions between muscle cells. This is common in cats obstructed by urethral
plugs, and usually resolves over several days if the bladder is kept decompressed.
Diagnosis of urinary obstruction is by demonstration of obstruction to urine flow by physical exam, radiographically or cystoscopy.
Physical exam of dogs with urethral obstruction should include a rectal exam to rule out urethral uroliths or masses. Although
urethral catheterization may indicate obstruction in some animals, the ability to pass a urinary catheter does NOT rule out
urethral obstruction. Urethral obstructions are frequently missed if the sole means of ruling out urethral obstruction is
based on ability to pass urinary catheters. This is one of the most common errors in veterinary urology. Another potential
complication of passing urinary catheters to rule out uroliths is that rarely the catheter and urolith may become lodged together
in the urethra.
Diagnostic imaging is the most common means of confirming urinary obstruction. Ultrasonography an excellent screening tool
for detecting upper tract obstruction. A positive contrast urethrocystogram is useful for confirmation of lower urinary tract
obstruction. For upper urinary tract obstruction an intravenous urogram will often document ureteral obstruction. Antegrade
pyelography using ultrasonographic guidance also may be used to document ureteroliths. The most sensitive imaging technique
of detecting ureteral obstruction is to perform a CT scan before and after intravenous contrast.
Cystoscopy may also be used to document urethral obstruction. Urethral masses, strictures or urethroliths can easily be detected
during cystoscopy. If a urethral mass or urethral stricture causes the lumen to be too narrow to allow passage of the cystoscope,
a flexible tipped urologic guide wire may be passed through the strictured area under fluoroscopic guidance. In some animals,
the cystoscope may then be advanced over the guidewire. Alternatively, an open ended catheter or vascular access sheath may
be passed over the guide wire and retrograde contrast studies may be performed to delineate the extent and severity of urethral