Ovariohysterectomy (OHE; spay) is a common procedure in feline medicine used for behaviour modification, population control, and prevention of multiple health issues. Complications are uncommon, but many have been described including ureteral ligation, hemorrhage, infection, and suture reaction. A phenomenon has been anecdotally described of mild, transient hyphema (bleeding inside the eye) after sterilization. In anecdotal reports this occurs primarily in juvenile cats and resolves without issue.
The purpose of this study was to describe post sterilization hyphema and estimate the incidence and factors contributing to its occurrence. The study was designed as a combined retrospective and prospective observational study with a retrospective survey component. The survey was sent to the Association of Shelter Veterinarians to determine the incidence of post sterilization hyphema in their practice. Medical records of a humane society were examined for a 14-month period retrospectively, and all sterilizations prospectively for an 8 week period.
Only cats that were negative for FeLV and FIV were included in the prospective arm. Cats received vaccines, deworming, and other preventative healthcare as per standard protocol. Animals were fasted, premedicated with buprenorphine, and induced with either tiletamine-zolazepam or ketamine-midazolam. Animals received meloxicam post operatively. Ocular exams were performed post operatively by a veterinary student and board-certified ophthalmologist.
Twenty surveys were returned, with nine veterinarians having observed post-operative hyphema. All respondents had witnessed this in cats < 1 year of age, and 2 of 9 reported cats > 1 year of age. Eight of nine reported the incidents to be unilateral and transient.
The review of medical records included 1204 cats and identified 2 cases of hyphema (0.2% incidence). The prospective arm included 195 cats, of which one developed bilateral hyphema. This cat had no change in intraocular pressure (IOP) post operatively and signs resolved within 20 hours with no residual change to the eyes.
All cats affected in the prospective and retrospective arm received tiletamine-zolazepam induction, as did 7 of 9 of the survey responses. While this could support a role of this drug combination in the development of hyphema, it is also possible this is a simple coincidence due to the common use of this drug combination. All cats in the clinical portion of the study were male, however the small sample size means assessing the significance of this is not possible.
The authors suggest that the hyphema may have occurred as a result of retrograde flow of blood through the iridiocorneal angle, though they do not have a proposed mechanism for why this occurs.
While reasonable first attempt at description of this condition, this study did not include a large enough number of cats for accurate characterization of such a rare occurrence. As such, larger, prospective studies are needed to properly describe post sterilization hyphema. All surveys carry an inherent response bias, which may have led to an over-estimate of the incidence of this condition in the survey (ie 45% of survey respondents vs 0.2% of clinical patients).
While further work is needed to characterize and determine the factors influencing the occurrence of post-sterilization hyphema, this paper demonstrates that, while rare, it is a condition veterinary clinicians should be aware of. (MRK).