Page 187 - Problem-Based Feline Medicine

11 – THE CAT STRAINING TO URINATE 179

Recurrence of signs within 6–12 months occurs in a ● Acid–base status of the urine is important in crystal
high percentage of cats. formation, with alkaline urine favoring struvite for-
mation and acid urine favoring oxalate formation.
● The role of viruses is controversial.
OBSTRUCTED IDIOPATHIC LUTD**
Occasionally urethral obstruction is from a urolith,
Classical signs rather than a plug. Calcium oxalate urate and cystine
uroliths are more common causes of urethal obstruction
● Straining with little or no urine passed
than struvite uroliths. The mechanism for urolith for-
(stranguria/dysuria).
mation is different from plug formation, and is covered
● Frequently seen trying to urinate
on page 184 in Urolithiasis.
(pollakiuria).
● Cat may cry out during urination. Urethral obstruction can result in post-renal renal
failure (obstructive nephropathy).
● The severity of the renal failure depends on the
Pathogenesis
degree and duration of the obstruction.
Historically, diets or components of diets, such as mag- ● Post-renal renal failure can be biochemically
nesium and phosphate, have been implicated in the detected by 24 hours after complete obstruction,
pathogenesis of some forms of idiopathic LUTD in cats. and by 48 hours clinical and metabolic changes can
become life threatening.
Although the etiology is unknown, urethral plugs are
● Post-renal renal failure is invariably reversible with
formed causing mechanical urethral obstruction that
appropriate treatment.
can lead to bladder distention, post-renal renal failure
and death. Bladder over-distention may result in ischemia,
● It occurs mainly in male cats possibly because their edema, hemorrhage and loss of epithelium. The ure-
urethra is longer, narrower and less distensible. thra, proximal to the obstruction, will have similar
● The urethral plugs are composed of colloidal injuries with desquamation of epithelium, edema and
matrix into which are embedded crystals, usually infiltration of neutrophils around blood vessels, lamina
struvite crystals (magnesium ammonium phos- propria and muscle layers.
phate).
Post-obstructive diuresis may occur following relief
– The ratio of matrix to crystalline material varies
of the obstruction and last for several days.
with some urethral plugs (about 5%) lacking
● The diuresis is possibly due to the clearance of
crystalline material and are known as non-
accumulated metabolites and a temporary loss of
crystalline matrix plugs.
concentrating ability by the renal tubules.
– Other crystals that occur less commonly
● Dehydration and possibly hypokalemia can occur
include ammonium acid urate, and calcium
during this polyuric phase.
phosphate and calcium oxalate.
– Some urethral plugs may also contain erythro-
cytes.
Clinical signs
– The exact chemical composition and origin
of the matrix are unknown although Tamm- Almost always seen in male cats under 5 years of age.
Horsfall protein of renal origin has been impli-
History of excessive licking around the perineal area
cated.
and the penis is often seen extruded from the prepuce.
● The factors(s) involved in the formation of urethral
plugs are unknown and may be different for the dif- Acute onset of dysuria.
ferent plugs.
The cat may cry out during urination.
● Since minerals other than struvite may be found
in some urethral plugs, suggesting matrix per se Close inspection of the penis may show a whitish,
may play an important primary role in the forma- chalky material blocking the urethral orifice and the
tion of some plugs. tip of the penis traumatized due to excessive licking.