Trbolova and Ghaffari Acta Vet Scand (2017) 59:35
DOI 10.1186/s13028-017-0303-2
Acta Veterinaria Scandinavica
Open Access
RESEARCH
Results of the Schirmer tear test
performed with open and closed eyes
in clinically normal horses
Alexandra Trbolova1 and Masoud Selk Ghaffari2*
Abstract
Background: The Schirmer tear test (STT) is widely used in both human and veterinary ophthalmology. Two types of
STTs have been developed: STT I and SST II. The STT I measures the basal and reflex tear production and is the most
widely used STT. However, several factors influence the STT results such as the person performing the test and the
location of the strip placement within the conjunctival sac. The aim of this study was to measure the basal and reflex
tear production (STT I) in clinically normal horses with open versus closed eyes.
Results: Forty clinically healthy horses without any ocular diseases were included. On day 1, the STT I was first performed on all the horses with the eyes open followed by an STT I with closed eyes performed 30 min later. On day 2,
all horses had their eyes closed during the first STT and the eyes open during the second test performed 30 min later.
The mean value of the STTs performed on open eye was significantly less than the STT performed on closed eye on
both days of examination.
Conclusion: This study showed a small but statistically significant difference between STT values obtained with open
versus closed eyes in clinically normal horses.
Keywords: Horse, Equine, Schirmer tear test, STT, Open and closed eyes STT
Findings
Tears play an important role in maintaining the health
and normal functions of the conjunctiva and cornea and
deficiency in tear production results in keratoconjunctivitis sicca (KCS) [1]. The measurement of tear production is an important diagnostic test when deficiency of
the lachrymal system is suspected. The Schirmer tear test
(STT) devised by Otto Schirmer a century ago is widely
used in both human and veterinary ophthalmology as
a basic assessment of tear production [2]. Two types of
STTs have been developed: STT I and STT II. The SST I
is the most commonly used STT and measures the basal
and reflex tear production. The STT II on the other hand
evaluates basal tear production after topical application
*Correspondence: selkghaffari@gmail.com
2
Department of Clinical Sciences, College of Veterinary Medicine, Karaj
Branch, Islamic Azad University, Alborz, Iran
Full list of author information is available at the end of the article
of an anesthetic and is of predictive values in animals
with corneal ulceration, which do not tolerate the STT I.
Several factors influence the STT results such as inconsistencies in the absorptive capacities of STT strips due
to differences in filter papers, the person performing the
test, and the location of strip placement within the conjunctival sac [3].
The results of the STT in clinically normal horses have
been reviewed in several studies, but no studies have
focused specifically on the results of open eye STT versus
closed eye STT in horses. The aim of the present study
was therefore to compare the STT values obtained with
open versus closed eyes in clinically normal horses.
Forty clinically healthy horses (20 males and 20
females) without any ocular diseases were included. The
horses were selected based on normal physical and ocular examinations as determined by STT I, biomicroscopy,
indirect ophthalmoscopy, and fluorescein staining.
© The Author(s) 2017. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License
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and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/
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Trbolova and Ghaffari Acta Vet Scand (2017) 59:35
Page 2 of 3
STT I values were evaluated in both eyes of all horses
using a commercial STT strip of a single lot number
(Schirmer-Tränentest, Vet Eickemeyer, Tuttlingen, Germany). On day 1, the STT was first performed on all
the horses with the eyes open followed by an STT with
closed eyes 30 min later. On day 2, all horses had the eyes
closed during the first STT and the eyes open during the
second test performed 30 min later.
The STT was performed by inserting a STT strip inside
the lower eyelid approximately one-third of the distance
from the temporal to the nasal canthi for one minute.
The eyelids were kept open/closed by the use of the
examiner’s fingers. All examinations were carried out in
the housing area of the horses at the same time of day
(between 10:00 a.m. and 12:00 noon) and all the measurements were performed by the same investigator (AT).
A chemical restraint was not used.
The statistical analysis was performed by using the
software package SPSS 12.0 for Microsoft windows
(SPSS Inc., Chicago, IL, USA). The data were reported as
mean ± standard deviation (SD).
Student’s t test was used to compare the STT values
obtained on each of the days for the right and left eyes
and to evaluate the STT values for open eyes versus
closed eyes. A P value of <0.05 was considered as statistically significant.
The obtained STT values and corresponding P values
are presented in Table 1.
The mean value of the STTs of the left and right open
eyes versus the left and right closed eyes was significantly
less for both days (P < 0.001). Left and right open eye
measurements were not significantly different at any of
the days (P = 1.0 and 0.3 for days 1 and 2, respectively).
However, when analyzing the results for closed eyes
measurements, the left–right values for day 1 were significantly different (P = 0.02) while they did not differ at
day 2 (P = 1.0).
Table 1 Values of the Schirmer tear test (mean and standard deviation in mm/min) in 40 clinically normal horses
measured twice daily on two consecutive days
Test day
Day 1
Day 2
Eye status and no.
of measurements
Eye
Left
P value
Right
Open T0 (n = 40)
21.5 ± 3.1 21.5 ± 3.3 1.0
Close T30 (n = 40)
22.3 ± 3.0 21.9 ± 3.1 0.02
P value
<0.001
Close T0 (n = 40)
23.7 ± 2.8 23.7 ± 2.8 1.0
0.027
Open T30 (n = 40)
23.0 ± 3.1 22.8 ± 2.9 0.3
P value
0.004
Number of measurements per eye per day = 80
<0.001
The overall mean and SD for all measurements on open
eyes was 22.2 ± 2.9 mm/min, while the value for closed
eyes was 22.9 ± 2.8 mm/min.
The study shows that the results of a STT I depend on
whether the test is done on open or closed eyes as the
STT I values were significantly higher for closed eyes
than for open eyes.
The values of the STT I observed in the horses were
similar to those reported in other studies of normal
horses. Previous studies have reported the normal STT
I values in horses as 20.6 ± 6.5 [4] and 24.8 ± 4.8 mm/
min [5, 6].
Our results differ from those found in humans, in
which the STT I values with open eyes showed significantly higher values when compared to closed eyes. In
humans, possible mechanisms for higher STT values
with open eyes are influence by external factors such as
temperature, evaporation, and humidity [7–9]. It is also
possible that the human cornea is more sensitive than
the equine cornea thus producing more tears. Also, in
humans, the duration of a STT measurement is generally
5 min compared to 1 min in horses. The longer time with
the eyes open with the strip inserted may increase tearing
[10, 11].
The specific details of lacrimal gland innervation are
poorly understood in horses. Fibers from the ophthalmic division of the trigeminal nerve, facial nerve, pterygopalatine ganglion, and sympathetic fibers from the
carotid plexus have been traced to the lacrimal gland. The
nictitans gland surrounds the base of the third eyelid cartilage and is innervated by parasympathetic fibers from
the glossopharyngeal cranial nerve [12, 13].
It is generally believed that manipulative procedures,
such as corneal or conjunctival scrapings, flushing of the
lacrimal apparatus, and potentially even application of
bright light to an inflamed eye may result in artificially
elevated STT values. For these reasons, if the STT is to
be performed, it should be done very early in an ophthalmic examination [13].
The exact reason for the mild increased tear production
in horses of our study with closed eye STT I is uncertain
but a possible explanation could be a reflex tearing by
stimulating the trigeminal nerve via the eyelid sensation.
As the STT strip itself irritates the eye and stimulates the
production of reflex tears, topical anesthetic eliminates
this irritation and thereby eliminates the reflex tearing.
Thus, in many species, STT II values are lower than STT
I values [14, 15].
Several studies have evaluated the effects of age, season,
environment, sex, and time of day on tear production in
horses [5, 16]. The STT values can also vary depending
on inconsistencies in the absorptive capacities of the STT
Trbolova and Ghaffari Acta Vet Scand (2017) 59:35
strips due to differences in filter papers or the individual
performing the test [3].
Keratoconjunctivitis sicca in horses is considered relatively rare compared to other species and is mostly associated with facial nerve paralysis resulting from head
trauma, fractures of the mandible, petrous temporal or
stylohyoid bones, or equine protozoal encephalomyelitis.
Other reported causes include eosinophilic granulomatous dacryoadenitis and hypothyroidism. Determining
the normal levels of tear secretion in healthy horses could
be of help for the accurate diagnosis of KCS in this species [13, 17–19].
In conclusion, this study shows a small, but statistically significant difference between STT I measurements
in normal horses with eyes either being open or closed.
Although the difference may be clinically insignificant,
this information is useful in two ways. First, it implies
that STT I can be measured with open or closed eyes in
the horse with clinically relevant accuracy and second
that the difference in STT values should be taken into
consideration when evaluating horses with ocular surface
disease where the method of performing the test may be
of more significance.
Authors’ contributions
MSG analyzed the data and drafted the manuscript. AT initiated and carried
out the study. Both authors participated in writing the manuscript. Both
authors read and approved the final manuscript.
Author details
1
Clinic of Small Animals, University of Veterinary Medicine and Pharmacy,
Kosice, Slovakia. 2 Department of Clinical Sciences, College of Veterinary Medicine, Karaj Branch, Islamic Azad University, Alborz, Iran.
Competing interests
The authors declare that they have no competing interests.
Availability of data and materials
The datasets during and/or analysed during the current study available from
the corresponding author on reasonable request.
Ethics approval
This study did not require ethical approval.
Publisher’s Note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Received: 18 April 2016 Accepted: 21 May 2017
Page 3 of 3
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