Prazosin in cats with urethral obstruction
The authors of the article “Initial treatment factors associated with feline urethral obstruction recurrence rate: 192 cases (2004–2010)”1 suggest that administration of prazosin to cats with urethral obstruction could reduce the likelihood of recurrence. They do warn that prospective randomized trials are needed to substantiate their conclusions; however, current research calls into question prazosin's usefulness for this purpose.
First, to our knowledge, urethral spasms have never been documented in cats. In a study2 of 6 cats with spontaneous urethral obstruction, for instance, urethral pressures were similar to or lower than values measured in healthy sexually intact male cats. Only 1 cat had an increased urethral pressure, and in that cat, pressure was high only in the postprostatic segment.
Second, although prazosin is an α1-adrenoceptor antagonist and α1-adrenoceptors can be found in the smooth muscle of the urethra, urethral smooth muscle is confined to the proximal 28% to 37% of the urethra in male cats,3 and radiographic imaging and clinical experience suggest that urethral plugs and idiopathic obstruction occur in the distal portion of the urethra. Prazosin should not affect the distal portion of the urethra, which is primarily composed of skeletal muscle.
Third, although the authors report significant differences in the percentages of cats with recurrent urethral obstruction 24 hours and 30 days after urinary catheter removal when cats were grouped on the basis of α1-adrenoceptor antagonist treatment (prazosin vs phenoxybenzamine vs neither), this likely reflects differences between cats that received prazosin and those that received phenoxybenzamine. Univariate analysis of data in Tables 2 and 3 of the report suggests that there were no significant differences 24 hours or 30 days after urinary catheter removal between cats that received prazosin and cats that received no α1-adrenoceptor antagonist treatment (P = 0.380 and 0.999, respectively [Fisher exact test]), between cats that received phenoxybenzamine and cats that received no α1-adrenoceptor antagonist treatment (P = 0.999 and 0.396, respectively), or between cats that received either medication and cats that received no α1-adrenoceptor antagonist treatment (P = 0.506 and 0.999, respectively).
Admittedly, the number of cats that received no α1-adrenoceptor antagonist treatment in this study (n = 6) was small, making it difficult to draw conclusions. But, without a physiologic explanation for the effects of α1-adrenoceptor antagonist treatment or a controlled study, we cannot really assume that prazosin was a cause or even a contributing factor in minimizing the rate of recurrence of urethral obstruction. Maybe it was the general anesthesia or the carefulness and experience of the veterinarians caring for the cats.
Jody Lulich, dvm, phd, dacvim
Carl Osborne, dvm, phd, dacvim
Minnesota Urolith Center
Department of Veterinary
Clinical Sciences
University of Minnesota
Saint Paul, Minn
1. Hetrick PF, Davidow EB. Initial treatment factors associated with feline urethral obstruction recurrence rate: 192 cases (2004–2010). J Am Vet Med Assoc 2013; 243:512–519.
2. Streater-Knowlen IM, Marks SL, Rishniw M, et al. Urethral pressure response to smooth and skeletal muscle relaxants in anesthetized adult male cats with naturally acquired urethral obstruction. Am J Vet Res 1995; 56:919–923.
3. Wang B, Bhadra N, Grill W. Functional anatomy of the male feline urethra: morphological and physiological correlations. J Urol 1999; 161:654–659.
The authors respond:
We thank Drs. Lulich and Osborne for their letter, which calls into question prazosin's usefulness in the treatment of cats with urethral obstruction.
The letter's first point is that urethral spasms have not been documented in cats. It is true that published research on urethral tone in male cats with urethral obstruction is limited, and the authors of the letter cite the only study1 we are aware of that examined this issue. In that study,1 urethral pressure was measured in 6 male cats with urethral obstruction, with all measurements performed after the cats had been anesthetized. One cat in that study1 did, in fact, have high pressures in all 3 urethral segments, compared with pressures in the other cats. That cat also had the largest reductions in urethral pressure in all 3 urethral segments following treatment with prazosin and a skeletal muscle relaxant. From these limited data, one could hypothesize that there is a subset of cats in which increased urethral tone plays a role in urethral obstruction. As stated in the discussion of the report of that study,1 “it seems that only some cats with urethral obstruction have urethral muscle spasms and that these cats could possibly benefit from muscle relaxants.”
Physiologically, prazosin has been demonstrated to reduce smooth muscle tone in the proximal half of the urethra in male cats.2 In a study3 of nonsedated male Beagles, it appeared to be more effective than phenoxybenzamine at reducing urethral tone.
In their letter, Drs. Lulich and Osborne suggest that in most cats, urethral obstructions occur in the distal portion of the urethra, which is composed primarily of skeletal muscle. However, it is important to consider that our study4 focused on recurrent urethral obstructions, not initial obstructions. The mechanism of recurrent urethral obstruction may be different than that of initial obstruction. In particular, it is possible that increased urethral smooth muscle tone occurs after prolonged urinary catheterization. Over three-quarters of recurrent obstructions in our study4 occurred within 48 hours after urinary catheter removal.
Finally, Drs. Lulich and Osborne focus on comparisons to a small group of 6 cats in our study4 that did not receive either prazosin or phenoxybenzamine. We agree that drawing conclusions from such a small group is difficult. Comparison of the much larger groups of cats that received one or the other α-adrenoceptor antagonist revealed significant differences in the rates of recurrent urethral obstruction between cats treated with prazosin and those treated with phenoxybenzamine both 24 hours (10/140 [7.14%] and 10/46 [21.74%], respectively; P = 0.006) and 30 days (20/110 [18.18%] and 18/41 [39.00%], respectively; P = 0.008) after urinary catheter removal.
With these points in mind, we believe that there is physiologic reasoning behind the use of prazosin in cats with urethral obstruction and suggest that findings in our study point to advantages in the use of prazosin versus phenoxybenzamine. We agree that prospective randomized trials are needed to substantiate these findings.
Peter Hetrick, dvm
Elizabeth Davidow, dvm, dacvecc
Animal Critical Care
and Emergency Services
Seattle, Wash
1. Streater-Knowlen IM, Marks SI, Rinshniw M, et al. Urethral pressure response to smooth and skeletal muscle relaxants in anesthetized adult male cats with naturally acquired urethral obstruction. Am J Vet Res 1995; 56:919–923.
2. Lefevre-Borg F, O'Connor SE, Schoemaker H, et al. Alfuzosin, a selective alpha 1 adrenoceptor antagonist in the lower urinary tract. Br J Pharmacol 1993; 109:1282–1289.
3. Fischer JR, Lane IF, Cribb AE. Urethral pressure profile and hemodynamic effects of phenoxybenzamine and prazosin in nonsedated male Beagle dogs. Can J Vet Res 2003; 67:30–38.
4. Hetrick PF, Davidow EB. Initial treatment factors associated with feline urethral obstruction recurrence rate: 192 cases (2004–2010). J Am Vet Med Assoc 2013; 243:512–519.
Diversity in veterinary medicine
Dr. Earl Strimple's letter,1 regarding diversity in veterinary medicine and his work with PAL (People. Animals. Love.), reminds us of the remarkable things that can be achieved when we create opportunities for underrepresented populations. Recruiting and preparing new generations of students to work in a profession that more truly reflects the society we serve is a priority of the highest order for the Association of American Veterinary Medical Colleges (AAVMC).
For more than 40 years, the AAVMC has convened the Dr. Iverson Bell Symposium, named in honor of the former (1971–1973) AVMA vice president in recognition of his outstanding leadership and promotion of diversity in veterinary medicine. This biennial symposium brings together students, educators, and leaders from throughout the profession to discuss achievements and best practices in diversity planning and programming.
In 2005, the AAVMC launched DiVersity Matters, a comprehensive initiative to recruit and retain underrepresented students and faculty members at the nation's veterinary medical colleges. The AAVMC also presents career fairs designed to inform and inspire underrepresented secondary school children and undergraduate students about careers in veterinary medicine. We participate in conferences such as the Society for Advancement of Chicano and Native Americans in Science that are specifically designed to promote the recruitment of underrepresented populations.
The AAVMC has partnered with the AVMA to conduct the DiVersity Matters Culture and Climate Initiative, a survey and analysis incorporating input from faculty, staff, and students to help our member institutions assess and improve diversity programming on their campuses. In addition, we are actively working to ensure opportunity for lesbian, gay, bisexual, and transgender students and faculty.
Eight years after the launch of DiVersity Matters, we can measure credible progress. The number of racially or ethnically underrepresented students enrolled in AAVMC member veterinary medical colleges has increased by 64%. This has occurred by raising awareness, creating metrics that are tracked across all veterinary colleges, and sharing best practices so colleges can learn from the success of others. There are many great achievements that have increased diversity at our member colleges, yet much remains to be done if we are to create an operating environment for veterinary medicine that is more reflective of society as a whole. Despite the hard work of many champions and visionaries, veterinary medicine remains one of the least diverse professions in the United States.
Our educational institutions represent the future of veterinary medicine and we recognize our responsibility to lead in this area. We will continue to gather data that helps us better understand where we are and where we could be, advocate for positive change, and convene forums where leaders and strategists can generate ideas for creating a more inclusive and more diverse profession in the future.
Andrew T. Maccabe, dvm, mph, jd
Executive Director
Association of American Veterinary
Medical Colleges
Washington, DC
1. Strimple EO. Veterinary applicants (lett). J Am Vet Med Assoc 2013; 243:618.
