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Evaluating tear testing and tear production in cats

Sebbag L, Uhl LK, Schneider B, Hayes B, Olds J, Mochel JP. Investigation of Schirmer tear test-1 for measurement of tear production in cats in various environmental settings and with different test durations. J Am Vet Med Assoc [Internet]. 2020 Mar 15 [cited 2020 Jul 30];256(6):681–6. 

Dry-eye (aqueous tear deficiency) is a common cause of ulcers, conjunctivitis, and even loss of eyes in dogs. However, little information is present on this disease in cats, in part due to the perceived difficulty of measuring tear production accurately in this species. The Schirmer Tear Test 1 (STT1) is a simple test whereby a trip of absorbent paper is placed in the conjunctival fornix for 1 minute, and the distance moisture spreads measured. In cats, high sympathetic tone in clinic and during handling may decrease tear production, leading to erroneously low results and a misdiagnosis of dry-eye. Cats also do not tolerate the presence of the strip in their fornix well, leading to difficulty gathering data. While previous studies in laboratory cats have given meaningful results, there is little information in client-owned pets.

The purpose of this study was to determine a reference range for STT-1 in health, client owned cats; to determine if the degree of “stress” influences the STT; and to determine if times <1 minute give valid results. The study was designed as a prospective observational study.

All cats recruited were free of visible ocular or corneal surface disease. Cats were recruited from three sites:  client owned cats from a primary care practice (n=100); student/staff owned at a veterinary teaching hospital (n=20); and semi-feral barn cats at a TNR program (n=56), for a total of 352 eyes. STT-1 was measured in both eyes, with the first eye chosen randomly and using the same lot number of STT strips. STT was measured every 10 seconds at the TNR clinic and teaching hospital, and every 30 seconds in private practice.

Cats in the teaching hospital had STTs evaluated twice, first in a calm environment, and a second time while listening to a recording of barking dogs and hand cymbals (to induce “stress”).

For statistical purposes, each eye was considered a separate measurement. Two-thirds of the data was used as a “training” set, and the remaining 1/3 for validation. No significant difference was found between Left and Right eyes at 30 or 60 second time points, and so mean data was considered.

There was no significant difference at 30 or 60 second marks between clinic, hospital, or TNR cats. There was a strong correlation between values at 30s and 60s (r= 0.941). Cats under simulated stress conditions had significantly higher heart rates than unstressed cats, however there was no difference in STT at any time point.  No covariate (i.e. age, sex, etc.) had any effect on STT-1 values.

The authors used this data to develop an equation that predicts STT-1 at 1 minute based on STT-1 values at 10 second intervals after strip placement.

The median STT-1 at 60 seconds was 14.3mm (8.2-22.3mm). This is largely in accordance with previously published data.

The authors conclude that the STT-1 is an accurate assessment of tear production in cats. They further conclude that values at time <1 minute may be used to accurately predict values at 1 minute. They further conclude that stress does not play a role in STT values.

This study had a reasonable sample size, though larger data sets are always beneficial. A possible limitation of this study was the inclusion of only healthy cats; it is possible that the conclusions and equations derived may be different in animals with true tear deficiencies. The authors also only evaluated cats with a point source of stress- it is possible that animals with prolonged travel time to the clinic, systemic disease, or other causes of longer term stress may influence tear values. Further work is also needed to determine a cut-off value for tear film deficiency by determining STT-1 values in cats with corneal disease.

Overall, this study is useful for the demonstration of the usefulness of STT-1 testing in cats, and further demonstrates that values read less than 60 seconds have diagnostic utility. (MRK)

See also:

Lim CC, Reilly CM, Thomasy SM, et al. Effects of feline herpesvirus type 1 on tear film break-up time, Schirmer tear test results, and conjunctival goblet cell density in experimentally infected cats. J Am Vet Res 2009;70:394–403.