Received: 23 December 2019
| Revised: 26 February 2020
| Accepted: 3 March 2020
DOI: 10.1111/vop.12760
B R I E F CO M M U N I CAT I O N
Feline post-sterilization hyphema
Alex P. Schenk1 | Ann M. McGrath2 | Cheryl A. Beatty2 | Jennifer L. Ottney2 |
Ellen K. Gratch2 | Sheilah A. Robertson1 | András M. Komáromy1
1
Department of Small Animal Clinical
Sciences, College of Veterinary Medicine,
Abstract
Michigan State University, East Lansing, Objective: To determine incidence and risk factors of post-sterilization hyphema in
MI, USA shelter cats.
2
Capital Area Humane Society, Lansing,
Animals studied: Retrospective medical record review of 1204 cats and prospective
MI, USA
screening of 195 cats.
Correspondence Procedures: The study consisted of three parts: (a) Survey responses were col-
András M. Komáromy, Michigan State
lected from 20 veterinarians, who perform high-quality high-volume spay-neuter
University, Veterinary Medical Center, 736
Wilson Road, Room D-208, East Lansing, (HQHVSN) in both shelter and public clinic settings; (b) medical records of 1204
MI 48824, USA. cats were analyzed retrospectively over a 14-month time period; and (c) ophthalmic
Email: komaromy@msu.edu
examinations, including tonometry, were performed prospectively on 195 cats before
Funding information and after sterilization surgery over 8 weeks.
National Eye Institute, Grant/Award Results: Nine of 20 surveyed veterinarians reported having witnessed hyphema in
Number: R01-EY025752; Michigan State
University Graduate School Fellowship cats following sterilization surgery. Retrospective review of 1204 medical record and
Funds prospective screening of 195 cats showed that three juvenile (<1 year of age) male
cats (<2 kg) developed hyphema within 1 hour following surgery (0.2% incidence).
In all three affected cats, anesthesia was induced with tiletamine/zolazepam (3 of 523
cats induced with this drug combination; 0.6% incidence), and hyphema resolved
within 20 hours. Mean intraocular pressures as measured by Icare® TonoVet were
(mean ± standard deviation) 11.5 ± 3.8 mm Hg and 21.7 ± 4.6 mm Hg for juvenile
(<1 year of age) and adult (>1 year of age) cats, respectively.
Conclusions: Survey responses and three observed cases confirm the existence of
feline post-sterilization hyphema with an estimated incidence of 0.2%. The underly-
ing mechanism for this occurrence remains unknown.
KEYWORDS
cat, hyphema, shelter medicine, sterilization, tiletamine, zolazepam
1 | IN T RO D U C T ION develop mild, transient hyphema post-operatively. This ob-
served blood in the anterior chamber of the eye appears to
Discussions among veterinarians, who perform high-qual- affect mostly juvenile cats under the age of 1 year, does not
ity high-volume spay-neuter (HQHVSN) in both shelter and cause any discomfort, and tends to resolve within 24 hours.
public clinic settings, have raised our awareness of a phe- There is some speculation that post-sterilization hyphema
nomenon where a few cats undergoing sterilization surgery may be a side effect of specific anesthetic drugs, such as the
© 2020 American College of Veterinary Ophthalmologists
Veterinary Ophthalmology. 2020;00:1–4. wileyonlinelibrary.com/journal/vop | 1
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| SCHENK et al.
combined use of tiletamine and zolazepam (Telazol®, Zoetis Inc). Adult cats received antiparasitic treatment by topi-
Inc, or Tilzolan®, Dechra Veterinary Products). It is our im- cal administration of imidacloprid-moxidectin combina-
pression that feline post-sterilization hyphema is largely un- tion (Advantage Multi® for Cats; Bayer HealthCare, LLC,
known to veterinary ophthalmologists. The purposes of our Animal Health Division) or selamectin (Revolution®; Zoetis
study were (a) to describe feline post-sterilization hyphema, Inc). Antiparasitic treatment of juvenile cats consisted of oral
(b) to estimate its incidence, and (c) to determine any associ- administration of pyrantel pamoate suspension (10 mg/kg;
ation with the use of specific anesthetic drugs. Apexa™, MWI Animal Health) and ponazuril (20 mg/kg;
compounded by Roadrunner Pharmacy). Fecal samples were
examined microscopically of cats with diarrhea or defeca-
2 | METHODS tion outside the litter box. Cats were treated if any parasites
were noted on the fecal flotation or Giardia snap test (IDEXX
2.1 | Survey Laboratories, Inc). Routine urinalysis was performed in any
cat with inappropriate urination outside the litter box and
A closed-community forum, organized by the Association of consisted of urine specific gravity, centrifugation and sedi-
Shelter Veterinarians (ASV) for veterinarians who perform ment examination by microscopy, and Multistix® Reagent
high-quality high-volume spay-neuter (HQHVSN) in both Strip (Siemens Medical Solutions USA, Inc). Only intact,
shelter and public clinic settings, was surveyed about the oc- healthy cats >2 months of age and >1 kg body weight under-
currence of post-sterilization hyphema. went sterilization surgery under general anesthesia.
In preparation for sterilization surgery, animals were
fasted for 2-4 hours before the procedure with access to
2.2 | Review of medical records water. All cats were premedicated by subcutaneous injection
of 0.006 mg/kg buprenorphine hydrochloride (Roadrunner
Medical records of cats undergoing sterilization surgery at Pharmacy) and 0.05 mg/kg acepromazine maleate (VetOne®,
the shelter of the Capital Area Humane Society (Lansing, MI) MWI Animal Health). In male juveniles with a body weight
from May 2016 to July 2017 were reviewed in order to esti- <2 kg, general anesthesia was induced by intramuscular in-
mate the incidence of post-operative hyphema as observed by jection of 6.6 mg/kg tiletamine hydrochloride-zolazepam hy-
the veterinary staff without detailed ophthalmic examination. drochloride combination (Tilzolan®). Female juvenile and
all adult cats received 5.5 mg/kg of intravenous ketamine
hydrochloride (Zetamine™, VetOne®, MWI Animal Health)
2.3 | Prospective evaluation of cats combined with 0.275 mg/kg midazolam hydrochloride
undergoing sterilization surgery (Hospira) for induction of general anesthesia. Anesthesia was
maintained with isoflurane (Fluriso™, MWI Animal Health)
The prospective part of our study was performed at the shel- and oxygen gas mixture administered either via facemask,
ter of the Capital Area Humane Society (Lansing, MI) from in male juveniles with a body weight <2 kg, or via endotra-
June 2016 through July 2016, and it was approved by the cheal tube, in all adults and female juveniles. The sterilization
Michigan State University Institutional Animal Care and surgeries consisted of routine orchiectomies and ovariohys-
Use Committee (IACUC). The routine admission protocol terectomies on males and females, respectively.1 All cats
for cats admitted to this shelter included a routine physical received a subcutaneous injection of 0.1 mg/kg meloxicam
examination. If the date of birth was unknown, age was es- (Metacam®; Boehringer Ingelheim Vetmedica, Inc) for pain
timated based on tooth eruption and weight. Blood was col- relief following surgery and were monitored closely until
lected to screen for the presence of feline immunodeficiency they had fully recovered from general anesthesia.
virus (FIV) antibodies and feline leukemia virus (FeLV) an- Ophthalmic examinations were performed on surgery
tigen (SNAP FIV/FeLV Combo Test, Idexx Laboratories, days by a specifically trained veterinary student (APS), and
or Viracheck® FeLV, Zoetis); only cats that tested negative findings were verified by a board-certified veterinary oph-
were included in the study. All cats received vaccinations and thalmologist (AMK). Both eyes of each cat were examined
antiparasitic treatment before sterilization surgery; typically, in the morning before surgery and again in the afternoons
these occurred within 1-2 days before surgery, with timing following full recovery from general anesthesia. No phar-
depending on an animal's age, body weight, and health status macologic mydriasis was induced. Anterior segments were
at admission to the shelter. Cats were vaccinated routinely examined with diffuse and focal illumination using porta-
with feline rhinotracheitis-calici-panleukopenia modified ble hand-held slit-lamp biomicroscopes (Kowa SL14; Kowa
live vaccine (Fel-O-Guard Plus 3, Boehringer Ingelheim Company). Fundic examinations were performed by wide-
Animal Health). Animals ≥4 months of age were also given view direct ophthalmoscopy (PanOptic™; Welch Allyn, Inc)
rabies vaccine (Rabvac 3, Boehringer Ingelheim Vetmedica, and portable binocular indirect ophthalmoscopes (Keeler All
SCHENK et al.
| 3
Pupil II; Keeler Instruments) and condensing lenses (Pan surgery. Of the 195 cats, 96 received tiletamine/zolazepam
Retinal 2.2D; Volk Optical). Intraocular pressures (IOP) were and 99 received ketamine/midazolam, and hyphema was
measured with a rebound tonometer (Icare® TonoVet; Icare only observed in 1 animal that received tiletamine/zolaz-
Finland Oy). The presence of pupillary light reflexes, menace epam. The affected animal was a 3-month-old, male domes-
responses, and dazzle reflexes was also examined. tic short-haired cat with no preexisting ocular abnormalities.
Although the hyphema was bilateral, the right eye was more
severely affected with more blood in the anterior chamber
2.4 | Data processing (Figure 1). Prior to surgery, the cat's IOPs were 13 mm Hg
in both eyes. Following surgery, IOPs were essentially un-
The hyphema incidence data were processed by descriptive changed at 12 and 14 mm Hg in left and right eye, respec-
statistics. Intraocular pressures were compared between ju- tively. There were no signs of trauma and no loss of ocular
venile (<1 year of age) and adult cats (>1 year of age) by reflexes. The hyphema was completely resolved in both eyes
two-tailed unpaired t test (Excel; Microsoft® Corporation). by 20 hours following surgery with no visible lasting ocular
abnormalities such as aqueous flare and cells.
Table 1 summarizes all the cats examined with their base-
3 | R E S U LTS line IOPs. Mean IOPs were significantly lower in juvenile
(<1 year of age) compared to adult cats (>1 year of age):
3.1 | Survey 11.5 ± 3.8 vs 21.7 ± 4.6 mm Hg (P < .001).
A total of 20 survey responses were collected from veteri-
narians, who perform high-quality high-volume spay-neuter 4 | DISCUSSION
(HQHVSN) in both shelter and public clinic settings. Out
of the 20 responses, nine indicated having witnessed post- Survey responses and three observed cases confirm the exist-
sterilization hyphema in cats at some point during their ca- ence of feline post-sterilization hyphema. When combining
reer. Of those nine positive responses, seven reported using the numbers of retrospective and prospective studies, we esti-
tiletamine/zolazepam, one reported using propofol, and one mate a post-sterilization hyphema incidence of 0.2% (3/1399
reported using a ketamine/diazepam combination as part of cats). Although not all cats received a detailed ophthalmic
their anesthetic protocol. Regarding delivery of maintenance examination, based on the available information, none of the
inhalant anesthetic, 3/9 responses reported use of a facemask, affected animals had any preexisting ophthalmic or systemic
3/9 responses reported intubation, and 3/9 responses reported conditions; there was no ocular trauma. All three of our cases
use of both a mask and intubation. All positive responses re- were juvenile male cats with a body weight <2 kg and anes-
ported cases involving cats <1 year of age and 2/9 also re- thetized with tiletamine/zolazepam, supporting the possible
ported cases involving cats >1 year. Most responses (8/9) association of post-sterilization hyphema with this drug com-
reported the hyphema to be unilateral and transient. bination and/or the age and sex of the animals. The incidence
of post-sterilization hyphema in all our cats receiving tileta-
mine/zolazepam was 0.6% (3/523 cats). Our survey results
3.2 | Review of medical records
During a 14-month time interval between May 2016 and July OD OS
2017, two cases of hyphema were identified out of 1204 cats
(636 males, 82 females) undergoing sterilization surgery 1 hour
(0.2% incidence). Of the 1204 cats, 430 received tiletamine/ post-op
zolazepam and 774 received ketamine/midazolam, and hy-
phema was observed in two juvenile male cats that received
tiletamine/zolazepam.
20 hours
post-op
3.3 | Prospective evaluation of cats
undergoing sterilization surgery
F I G U R E 1 Bilateral hyphema in a 3-mo-old, male domestic
During the 8-week study period in June and July 2016, a total short-haired cat 1-h following surgery (1 h post-op). The hemorrhage
of 195 cats were examined, with one of them developing was more diffuse and extensive in the right (OD) than left eye (OS)
bilateral hyphema during recovery within 1 hour following and was completely resolved by 20 h post-op
4
| SCHENK et al.
TABLE 1 Summary of pre-surgical mean baseline IOPs the still ongoing maturation of anterior segment and aqueous
Number Mean age Mean IOP (±SD) humor outflow pathways.5,6
Age groups of cats (±SD) (Mo) (mm Hg)
ACKNOWLEDGMENTS
Juvenile cats 179 2.5 ± 1.0 11.5 ± 3.8*
The authors thank the staff of the Capital Area Humane
Adult cats 16 16.9 ± 7.0 21.7 ± 4.6*
Society for their technical support and the veterinarians
All cats 195 3.7 ± 4.5 12.3 ± 4.7
who responded to the survey. The study was funded by the
Abbreviations: IOP, intraocular pressure; SD, standard deviation. Michigan State University Graduate School Fellowship
*Significant difference in IOP between juvenile and adult cats (P < .001). Funds and NIH grant R01-EY025752.
indicate that feline post-sterilization hyphema can also occur CONFLICT OF INTEREST
with other anesthetics, such as propofol and ketamine/diaz- There are no conflicts of interest related to this study.
epam combination. To the best of our knowledge, there are
no reports that suggest a link between any of these anesthetic ORCID
induction agents and intraocular hemorrhage. András M. Komáromy https://orcid.
We have no explanation why feline post-sterilization hy- org/0000-0002-8845-0588
phema occurs. Since we found no physical damage in the af-
fected eyes and since the blood disappeared within 20 hours R E F E R E NC E S
with no treatment and no lasting anterior uveitis, we think 1. Association of Shelter Veterinarians' Veterinary Task Force to
that the blood most likely entered the anterior chamber via Advance, Spay-Neuter GB, Bushby PA, et al. The Association
the iridocorneal angle. This is a commonly observed phe- of Shelter Veterinarians' 2016 Veterinary Medical Care
Guidelines for Spay-Neuter Programs. J Am Vet Med Assoc.
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2016;249(2):165-188.
anterior chamber retrogradely through the aqueous humor
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drainage pathways of the iridocorneal angle if the eye is hy- Pathophysiologic considerations. Compend Contin Educ Pract Vet.
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ber in cats during sterilization surgery since we found no tonometer (Tonovet) in clinically normal cat eyes. Vet Ophthalmol.
indications of ocular hypotension when performing tonom- 2010;13(1):31-36.
etry before and after the procedure. We also have no likely 4. McLellan GJ, Kemmerling JP, Kiland JA. Validation of the
TonoVet® rebound tonometer in normal and glaucomatous cats. Vet
explanation how the use of anesthetic face mask could have
Ophthalmol. 2013;16(2):111-118.
resulted in hyphema. Even though we did not measure blood
5. McLellan GJ, Teixeira LB. Feline glaucoma. Vet Clin North Am
pressures, we think that it is unlikely for acute blood pres- Small Anim Pract. 2015;45(6):1307–1333.
sure spikes to have resulted in hyphema, especially since we 6. Adelman S, Shinsako D, Kiland JA, et al. The post-natal develop-
did not observe any vascular damage and the blood disap- ment of intraocular pressure in normal domestic cats (Felis catus)
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During our investigation of post-sterilization hyphema,
we determined mean IOPs in adult cats measured by Icare®
TonoVet as 21.7 ± 4.6 mm Hg, which is comparable to How to cite this article: Schenk AP, McGrath AM,
the previously published normal mean IOPs for adult cats: Beatty CA, et al. Feline post-sterilization hyphema.
20.7 ± 0.5 mm Hg.3-5 Our significantly lower mean IOPs of Vet Ophthalmol. 2020;00:1–4. https://doi.org/10.1111/
11.5 ± 3.8 mm Hg in juvenile compared to adult cats also vop.12760
confirms previous reports of low IOPs in kittens because of
