Urethral obstruction in cats: Catheters and complications (Proceedings)
PathophysiologyThe most common causes of voiding disorders in cats include: obstruction with urethral plug, urolith, or blood clot; urethrospasm resulting in functional obstruction (usually post blockage); bladder dysfunction (detrusor atony) following blockage; and neurologic lesions leading to detrusor atony. Other causes include urethral stricture, neoplasia of the bladder neck or urethra, and extraluminal obstruction.
With lower urinary tract obstruction, the urinary bladder distends beyond its usual accommodating size. As intravesicular pressure increases to threshold levels, stretch receptors are activated and afferent information is relayed to higher centers. In an animal with an intact neurological system, these messages will trigger attempts to void. In an animal who cannot eliminate urine, the increased intravesicular pressure continues to build and is transmitted "upstream" to the ureter and ultimately to the kidney. Increased intratubular pressure opposes the forces driving glomerular filtration; ultimately RBF and GFR decline. Tubular concentrating ability and other tubular functions are also affected, leading to impaired sodium and water reabsorption, and impaired excretion of acids and potassium. Uremia, acidosis, hyperkalemia and volume depletion result. Inflammatory cells infiltrate urinary tract tissue and over time, mucosal damage or primary renal failure may occur. Acute complete urinary obstruction causes uremia within 24 to 48 hours and death within 60 – 75 hours. Following relief of obstruction, renal tubular impairment continues for some time, presumably to resolve the dramatic water and electrolyte aberrations that have developed. Renal tubular damage or overall nephron loss also contributes to the dramatic polyuria observed post-obstruction.
Diagnostic Approach to Micturition Disorders in Cats
The diagnostic approach includes a problem-specific history and a physical examination that includes neurological evaluation (especially tail and anal tone, perineal sensation and hindlimb function). In addition, observation of voiding is important to determine how the animal postures, quality of urine stream, bladder expressibility, and whether the bladder is emptied fully after voiding. Observation of voiding can be difficult in cats in the hospital, so a detailed client history is imperative. A urinalysis is recommended to rule out iatrogenic or secondary UTI. Other components of a minimum data base (CBC, biochemical panel) can be selected based on the cat's general condition and history.
Imaging with survey radiography is recommended in all cases because urolithiasis is fairly common in cats. Ultrasonography can be more sensitive for the detection of small uroliths or soft tissue masses in the urinary tract. Small uroliths can move back and forth between the urinary bladder and urethra, making their detection more difficult. Excretory urography or contrast cystourethrography may be indicated to rule out or characterize anatomical abnormalities in young cats, to completely rule out anatomical obstruction, or following traumatic injuries or surgery. Additional imaging procedures may be necessary to rule out or characterize spinal lesions. Cystoscopy is increasingly applied in order to visualize the urinary bladder and urethra and may help confirm small uroliths or inflammatory/neoplastic lesions. In female cats, rigid cystoscopy allows good visualization of the urinary bladder and urethra and introduction of biopsy or other instruments. Small flexible or semi-flexible scopes are required in male cats, however, and offer limited visualization and instrumentation options.
Often, transurethral urinary catheterization is used to rule in/out mechanical obstruction, because catheterization will be difficult or "rough" at the obstructed site. If a catheter advances quite easily, the obstruction may be functional rather than anatomical. However, a flexible catheter may be passed retrograde through an incomplete intraluminal or extraluminal obstruction fairly easily (especially soft tissue masses) while spontaneous antegrade voiding remains difficult or impossible for the animal.
In rare cases, specialized urodynamic tests that evaluate lower urinary tract function are used to further characterize a micturition disorder. Urethral pressure profiles are occasionally used to document urethrospasm or to pinpoint focal obstruction. These tests are available at some referral teaching hospitals and are more commonly performed in cats in research settings. Urodynamic testing can clarify dysfunction in complex cases, congenital incontinence, or cases that fail to respond to treatment.