Pathophysiology of urethral obstruction
Urethral plugs are the most common cause of obstruction in male cats. In one series (Krueger 1991), urethral plugs occurred
in 60%, no cause was found in 30%, uroliths alone were documented in 10% (struvite exclusively) and uroliths with bacterial
urinary tract infection were observed in 2%. Occasionally stricture and rarely neoplasia are the causes of obstruction. Urethral
obstruction due to calcium oxalate urethroliths is a phenomenon of the 1990's that was not encountered in the 1980's.
Figure 1. Underlying causes and mechanisms of urethral obstruction
Urethral plugs consist of proteins and embedded constituents of the urine in varying proportion. They contain little internal
structure, and most commonly form in the penile urethra. Lower urinary tract inflammation, caused by either idiopathic cystitis/urethritis
or bladder stones, precedes the formation of urethral plugs. Struvite still comprises the major crystal in plugs, despite
the emergence of calcium oxalate crystalluria. Inflammatory exudates of white blood cells and proteins, red blood cells from
hemorrhage, sloughed epithelial cells, fibrin, calcium oxalate crystals, and calici-like viral particles may also become trapped
if present in urine at the time of plug formation.
In addition to intraluminal obstruction, lesions of the urethral wall may also contribute to obstruction. Edema, hemorrhage,
and inflammation contribute to urethritis and can decrease the diameter of the urethral lumen. Functional decreases of the
urethral lumen diameter also may result from inflammation and pain (see spasm below). Cats with chronic urethritis or recurrent
urethral obstruction may become obstructed secondary to urethral stricture. Extramural causes (prostatic or urethral tumor)
of urethral obstruction are exceedingly rare.
Most plugs cause obstruction within the penile urethra, but obstructions can also occur at more proximal sites. The predominant
mineral composition in most plugs is magnesium ammonium phosphate (struvite). Secondary components can contribute to plug
formation including inflammatory exudate ( WBC and proteins), red blood cells, cellular debris, sloughed tissue (epithelial
cells), struvite crystals and combinations. Virus-like particles resembling calicivirus and bacteria have also been observed
within urethral plugs examined by transmission electron microscopy. Primary inflammatory changes ( exudates, blood, and edema)
or changes within the urethral wall secondary to intraluminal urethral plugs may contribute to the obstructive process. These
changes may be magnified following instrumentation with catheters and back-flushing solutions used in therapeutic endeavors.
Diagnostics and management
Urethral obstruction is diagnosed by the finding of an enlarged bladder in a male cat with signs of urinary urgency, difficulty
in manually expressing urine, and by resistance encountered during the passage of a urethral catheter. It may not be obvious
what is causing the urethral obstruction. Diagnostics and management of urethral obstruction are performed simultaneously.
The degree of uremia, electrocardiographic stability, and the magnitude of bladder distention will dictate how quickly and
in what order treatment must be performed. Those cats with uremic crisis and those with very large hard bladders are in need
of prompt attention.
Figure 3. Approach to the Moribund Cat with Advanced Urethral Obstruction