Feline urethral obstruction (UO) is a very common emergent complication of feline lower urinary tract disease, seen almost exclusively in male cats, yet clear evidence-based information regarding optimal care of these patients is lacking. Both functional (secondary to urethral spasm and edema) and/or physical (secondary to substances such as mucus plugs or urethral calculi, and less commonly strictures or neoplasia) etiologies are reported for feline UO. Most patients (90-95%) treated with relief of the obstruction, usually through urethral catheterization, and correction of metabolic, electrolyte, and cardiovascular derangements, survive the episode, but UO may recur in 15-40% of these animals.
Feline UO appears to be frequently associated with feline interstitial cystitis (FIC). In cats with FIC there is an imbalance between the sympathetic nervous system and the hypothalamic-pituitary-adrenal axis engendered by stressful events. Impaired blood flow and the release of inflammatory mediators secondary to this imbalance cause edema, smooth muscle spasm, and pain within the lower urinary tract. The pain further potentiates urethrospasm and inflammation, resulting in a vicious cycle. The presence of any potentially obstructive materials such as mucus plugs, crystal rafts, or cystoliths or urethroliths in conjunction with FIC may increase the probability of occurrence of UO. Various studies have reviewed possible predisposing factors for feline UO, and none have been in complete agreement regarding these risk factors, which include increased body weight, decreased water intake, predominantly indoor status, exclusively dry diet, use of a litter pan, residence in a multicat household, and conflict with a housemate cat.
Although intravenous fluid therapy is a mainstay of UO treatment and is essential for correction of metabolic and electrolyte derangements that accompany this condition, controversy exists as to the optimal type of isotonic crystalloid to use. Based on a review of the available literature, the author concludes that the type of fluid used does not seem to matter. There are also no real guidelines for choosing a fluid rate. Criteria for selecting a fluid rate include need for resuscitation if cardiovascular collapse is present, replacement of dehydration deficit, maintenance requirements, potential for postobstructive diuresis, and presence of intercurrent disease such as cardiomyopathy which may require more cautious fluid therapy than is frequently used.
Another significant controversy in the management of feline UO is the use of decompressive cystocentesis prior to urethral catheter placement. While there is actually no evidence that decompressive cystocentesis may facilitate easier placement of a urethral catheter by relieving back-pressure against the obstruction, it can allow for immediate relief of pressure and pain, as well as more rapid resumption of glomerular filtration, which stops progression of renal injury. In addition, decompressive cystocentesis can be performed with minimal sedation, which may be beneficial for patients with cardiovascular compromise. On the other hand, this procedure could result in tearing or rupture of a distended and possibly friable urinary bladder, leading to uroabdomen. Recent evidence cited by the author indicates that the risk of clinically significant uroabdomen with decompressive cystocentesis is low, especially if the bladder is kept decompressed by subsequent placement of a urethral catheter.
Decompressive cystocentesis may be especially valuable if a management protocol not involving urethral catheterization is employed. One study evaluated such a protocol utilizing acepromazine/buprenorphine, decompressive cystocentesis every 8 hours as needed, and a low-stress environment for up to 4 days. This treatment regimen was developed for cats without significant physical examination or electrolyte derangements whose owners were faced with potential euthanasia of their pets due to financial constraints. In this study 11/15 cats were treated successfully, attaining spontaneous urination and survival to discharge and having undergone up to 10 cystocenteses. It is questionable, however, whether or not this protocol actually is significantly less expensive than traditional catheterization, especially if it is unsuccessful and conventional management must be implemented. Another low-cost alternative is to place a temporary urethral catheter, flush the bladder on an outpatient basis, and then send the cat home with pain medication and home care instructions. At this time there is no evidence to indicate how often this latter approach is successful.
A variety of urethral catheter types and sizes are available for cats with UO. The author reviews the uses, risks, and benefits of each type. The polytetrafluroethylene and polyurethane catheters are firmer at room temperature, which facilitates unblocking, but then soften as they reach body temperature, which means they can be left in place, unlike polypropylene catheters, which are very rigid and may cause urethral irritation if left in place. Most urinary catheters used in cats are either 3.5 Fr or 5 Fr, and while the larger size (5 Fr) catheter would be less likely to kink or become obstructed, in one study cats with 5 Fr catheters had an increased risk of reobstruction within 24 hours compared to those with 3.5 Fr catheters. In another study no association was found between catheter size and risk of reobstruction. There is no evidence to specify optimal duration of catheterization, so the length of time a catheter is present should be based on clinical criteria, such as resolution of biochemical abnormalities and any postobstructive diuresis, as well as gross appearance and character of the urine.
The use of antimicrobials in UO is likewise controversial. It is well documented that the use of antimicrobials does not prevent development of catheter-associated urinary tract infections (UTIs) in dogs and cats. At presentation most cats with UO are very unlikely to have a UTI, which means that it is generally unnecessary to obtain a urine culture or initiate use of antimicrobials at the beginning of treatment. The risk of UTI acquired during treatment of UO is very low, especially when the catheter is aseptically placed. The author suggests that the optimal way of evaluating these patients for UTI is to obtain a urine culture at the time of catheter removal or having the patient return 3 days later for a cystocentesis. Culturing the catheter tip is not recommended, as there is significant potential for contamination of the catheter tip during its removal.
The use of antispasmodics, especially in the postobstructive period, is widespread in the care of patients with UO, as urethral spasm and increased smooth muscle tone may play a role in the development of the initial obstruction and in reobstruction. Phenoxybenzamine, acepromazine, and prazosin have all been used in this regard due to their function as α-1 antagonists, which cause smooth muscle relaxation. Phenoxybenzamine appears to be less effective than other medications and its effects may be delayed for up to one week. Acepromazine may be useful for its sedative effects, causing reduced stress responsiveness, as well as its α-1 antagonist effects. Prazosin is the most desirable of these three commonly used agents as it has a much faster onset than phenoxybenzamine and does not have the sedative effects of acepromazine while still (according to one study) causing a decrease in central sympathetic outflow. Despite their common use, the author notes that there is little evidence to support a beneficial impact of urethral relaxants in UO patients; further studies are needed.
Reobstruction is a major reason for euthanasia of cats with previous UO. Several studies have reported a reobstruction rate of 35-40%, while others have reported reobstruction rates of 15-24%. Most reobstructions occur within 4 days of the initial episode, but there is also risk for reobstruction later on, with a significant number of cats reobstructing within 30 days of the first obstruction. Factors which may be associated with reobstruction include size (5 Fr associated with a higher rate of reobstruction than 3.5 Fr) or duration of indwelling urethral catheters, patient age, lower urine pH at the time of presentation, use of phenoxybenzamine rather than prazosin in the post-obstructive period, and indoor lifestyle. Factors such as patient weight, presence of UTIs, administration of antimicrobials, magnitude of azotemia, type of diet (wet or prescription diet vs. dry diet), presence of crystalluria, urine pH, and urine specific gravity do not appear to influence recurrence of UO. The evidence currently available supports increased water intake and environmental modification (such as increasing the number of litterboxes and change in husbandry) as being factors that are significant in reducing the risk of reobstruction. Prospective studies are required to further elucidate best practices in initial management and postobstructive care in order to increase treatment success and reduce the risk of reobstruction in these patients. [PJS]