Comparison of the Schirmer tear test I values after placement in ventral and dorsal conjunctival fornices in healthy cats

Introduction

The precorneal tear film (PTF) is a fluid layer composed of aqueous tear, lipid and mucous in mammals. ¹ It is vital for corneal nutrition and maintenance of corneal clarity. ¹ Quantitative evaluation of PTF is an essential part of each ophthalmic examination in veterinary medicine. ² The Schirmer tear test (STT) is commonly used as a conventional method for estimation of the PTF and helps to diagnose tear-film abnormalities, such as keratoconjunctivitis sicca. ²

To perform the STT, a 35 mm long filter paper strip is placed in the lateral half of the ventral conjunctival fornix and after 1 min, the wetted portion of strips is measured as mm/min. ³

Anatomical and physiological factors of each species, diurnal rhythm, topical and systemic drugs, ocular and systemic diseases, STT strips manufacturer and batch number, and placement of strips are influencing factors in tear-film evaluation. ⁴

The effect of placement of STT strips has been evaluated in dogs and horses.^5,6 In horses, no significant effect of placement of STT strips was found in different parts of ventral conjunctival fornix. ⁶ In dogs, however, a statistically significant difference was reported between placement of STT strips in the ventral and dorsal conjunctival fornices. ⁵

The purpose of this study was to compare the STT values obtained from placement of STT strips in the ventral and dorsal conjunctival fornices and to investigate any correlation between age, body weight and STT values in clinically normal cats.

Materials and methods

The study was approved by the Iran Society for Prevention of Cruelty to Animals in accordance with Iranian ethical codes for studies on laboratory animals and conducted in accordance with the Association for Research in Vision and Ophthalmology statement for the Use of Animals in Ophthalmic and Vision Research.

Twenty-eight intact adult cats (56 eyes) of both sexes (14 males, 14 females) were used for the study. Of the total number of cats, 18 were non-brachycephalic cats (domestic shorthair cats; DSH) and 10 were brachycephalic female cats (Persian cats). Among the DSH cats, 14 were males and four were females. The cats were individually housed in an animal room under standard controlled environmental conditions for 6 days before the beginning of the study. They were exposed to a constant 24 h lighting cycle that included 12 h of light conditions and 12 h of dark conditions. Commercial dry food was offered twice daily throughout the study. Water was given ad libitum for the duration of the study.

The inclusion criteria were: (1) normal physical examination; (2) normal ophthalmic examination (including direct and indirect ophthalmoscopy, fluorescein staining, STT, intraocular pressure measurement and slit-lamp biomicroscopy); (3) normal complete blood count and biochemistry profile; (4) cats older than 12 months; (5) no history of feline herpesvirus infection or any other systemic or ocular diseases; and (6) no history of any systemic or topical medication or vaccination in the past 60 days.

The cats were gently physically restrained in a sternal position, and a Schirmer tear strip (Intervet; Merck Animal Health [lot 15090926]) was placed on each eye in the ventral conjunctival fornix at one-third of the distance from the temporal to nasal canthus for 60 s. Then, the wetted portion of the strips was measured as mm/min (STT of the lower eyelid, or L-STT I). After 48 h, the same procedure was performed on each eye in the dorsal conjunctival fornix (STT of the upper eyelid STT I, or U-STT I).

One examiner (SMR) performed all the ocular tests, examinations and measurements. All the measurements were performed between 9 and 11 AM to minimise the possible variations associated with diurnal changes. Three days after the end of the study, all the cats were examined. No signs of conjunctivitis, keratitis, blepharitis, corneal ulceration or intraocular disease were detected in any of the cats.

Statistical analysis was performed using SPSS statistical software for Mac (SPSS 20.0; IBM). Normality was tested by one-sample Kolmogorov–Smirnov test. The paired-samples t-test was used to compare the STT values obtained from the right and left eyes. Means ± SDs were calculated for all of the eyes combined and for right and left eyes separately. Comparison of the STT values obtained from ventral and dorsal conjunctival fornices was also performed using the paired-samples t-test. A P value of <0.05 was considered statistically significant.

Results

All the data were normally distributed according to one-sample Kolmogorov–Smirnov test (P >0.33).

Twenty-eight cats aged 20.10 ± 7.19 months (range 12–36 months) with a body weight of 2.71 ± 0.63 kg (range 1.8–4 kg) were enrolled in this study. The STT values of the ventral and dorsal conjunctival fornices were 20.80 ± 2.25 mm/min (range 14–25 mm/min; 95% confidence interval [CI] 19.92–21.67) and 18.28 ± 2.27 mm/min (range 15–22 mm/min; 95% CI 17.39–19.16) in all of the study population, respectively. A significant difference was found between the STT values obtained from ventral and dorsal conjunctival fornices (P <0.001).

The results revealed no significant correlation between the age of the animals and the STT I values of both the ventral and dorsal conjunctival fornices (r = −0.045 [P = 0.9] and r = −0.1 [P = 0.7], respectively). Moreover, no significant correlation was found between weight of the cats and the STT I values (r = 0.06 [P = 0.7] and r = 0.08 [P = 0.8], respectively). We did not analyse the effect of sex on the STT I values in our study population. The mean ± SD of the obtained STT values in each breed are shown in Figures 1 and 2.

OPEN IN VIEWER

Figure 1

Mean and SD Schirmer tear test (STT) values (and error bars) of ventral and dorsal conjunctival fornices in domestic shorthair cats. OS = left eye; OD = right eye; OU = both eyes

OPEN IN VIEWER

Figure 2

Mean and SD Schirmer tear test (STT) values of ventral and dorsal conjunctival fornices in Persian cats. OS = left eye; OD = right eye; OU = both eyes

Discussion

STTs are commonly used as a classic and routine method for the assessment of tear production in cats. ⁴ The obtained STT values from both of the strip placements in this study were within the normal reported reference interval. Normal STT I values in adult cats have been reported to range from 14.3 ± 4.7 to 16.92 ± 5.73 mm/min, ⁴ although the range of STT II values in cats is reported to be from 1–33 mm/min. ⁴ Multiple factors influence STT values, including age, sex, breed, diurnal rhythm, lacrimal system diseases, and topical and systemic drugs. ⁴ STT values in cats may be influenced by stress, which can cause a low STT reading. ⁷ Some cats may experience discomfort because of the duration of contact of the STT strips with the cornea. ⁸

The cats tolerated the STT placement well in both the ventral and dorsal conjunctival fornices. As previously described in dogs, ⁵ the placement of the strips in the ventral conjunctival fornix was associated with greater STT values than the dorsal placement of the strips in both the non-brachycephalic and brachycephalic cats in our study. The possible reasons for these findings are the lacrimal meniscus in the ventral conjunctival fornix, ⁵ and difference in corneal sensitivity in different parts of cornea and goblet cell density.^9–11 Corneal sensitivity of the brachycephalic and DSH cats was evaluated. ⁹

Blocker and Van Der Woerdt reported that brachycephalic cats had lower corneal sensitivity than DSH cats. ⁹ They also stated that the central cornea is more sensitive than the peripheral cornea in cats. ⁹ In our study, the brachycephalic cats had the highest lower-eyelid STT values, whereas the DSH cats had the highest upper eyelid STT values.

The highest goblet cell density in feline eyes has been reported in the fornicial region of the ventromedial, dorsolateral, dorsomedial and ventrolateral of eyelids and anterior surface of third eyelid glands. ¹⁰ Also, an increase in the mucus layer of the tear film leads to stabilisation of the aqueous portion of the tear film. ⁵ The effect of age and body weight on the STT results has been reported in cats. The effect of age has been controversial between studies; STT I values, however, have been reported to increase with body weight in cats. ⁴ In our study, no correlation was found between the age and body weight of the animals and tear production.

Future studies are needed to evaluate the effect of weight and reproductive status on U-STT I values on a large population of cats with different breeds and ages.

Conclusions

This study compared the STT I values of dorsal and ventral conjunctival fornices in cats. The difference was significant between the STT I values; however, the L-STT I and U-STT I values were within normal reference intervals. The U-STT I data have diagnostic value, which might be of benefit in cases where there is a problem of the ventral conjunctival fornix.