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72 AJVR • Vol 77 • No. 1 • January 2016
The tear film is vital to the physiologic function
of eyes and is essential for the maintenance of
corneal clarity. It serves as the cranial refracting sur-
face of the eye and provides nutrition for the corneal
surface.1,2
The importance of tear film evaluation during as-
sessment of ocular health has long been recognized.
Tear film tests are categorized as quantitative tests
that are used to evaluate the volume of tear film or
qualitative tests that are used to assess quality of tear
film. Quantitative tests for the evaluation of tear film
include the STT, PRTT, and EAPPTT.
In clinical veterinary practice, quantitative clini-
cal evaluation of the precorneal tear film is most fre-
quently limited to use of the STT because published
standard values for the STT in domestic species are
accepted and clinically useful for the identification of
Results of selected ophthalmic diagnostic tests
for clinically normal Syrian hamsters
(Mesocricetus auratus)
Seyed Mehdi Rajaei dvm, dvsc
Maneli Ansari Mood dvm, dvsc
Reza Sadjadi dvm
David L. Williams ma, vetmd, phd
Received February 25, 2015.
Accepted May 6, 2015.
From the Department of Clinical Sciences, Faculty of
Specialized Veterinary Sciences, Science and Research
Branch, Islamic Azad University, Tehran, Iran (Rajaei,
Ansari Mood, Sadjadi); and the Department of Veterinary
Medicine, University of Cambridge, Cambridge, CB3
0ES, England (Williams). Dr. Rajaei’s present address is
Department of Clinical Sciences, College of Veterinary
Medicine, Karaj Branch, Islamic Azad University, Alborz,
Iran.
Address correspondence to Dr. Rajaei (mehdi_13r@
hotmail.com).
OBJECTIVE
To determine values for tear production, horizontal palpebral fissure length
(HPFL), eye blink frequency, and intraocular pressure (IOP) in healthy Syrian
hamsters (Mesocricetus auratus).
ANIMALS
40 healthy adult Syrian hamsters (80 eyes).
PROCEDURES
Tear production was measured with the phenol red thread test (PRTT), modi-
fied Schirmer tear test (mSTT), and endodontic absorbent paper points tear
test (EAPPTT). The IOP was measured by use of rebound tonometry. Cor-
relations between test results and body weight were evaluated.
RESULTS
Mean ± SD values for the IOP, PRTT, EAPPTT, mSTT, HPFL, and blink fre-
quency for all 80 eyes were 4.55 ± 1.33 mm Hg, 5.57 ± 1.51 mm/15 s, 4.52 ±
1.55 mm/min, 2.07 ± 0.97 mm/min, 5.84 ± 0.45 mm, and 1.68 ± 0.43 blinks/
min, respectively. For all variables, values did not differ significantly between
the right and left eyes or between males and females. There was no correla-
tion between measured variables and body weight.
CONCLUSIONS AND CLINICAL RELEVANCE
Results for this study provided information on values for the IOP, PRTT,
mSTT, EAPPTT, HPFL, and eye blink frequency in healthy Syrian hamsters.
It was important to determine reference intervals for this species because
they commonly are kept as pets or used as research animals. (Am J Vet Res
2016;77:72–76)
quantitative tear film deficiencies.2 Small domestic,
wild, and exotic animals have a small palpebral fissure
length; thus, narrow (2.5 and 4 mm wide) mSTT strips
have been recommended for measurement of tear
production in these animals.3
The PRTT was developed for use because of vari-
able results, poor repeatability, and low sensitivity of
the STT for detecting inadequate tear production in
humans.4 It is performed by placing a 75-mm-long cot-
ton thread impregnated with pH-sensitive phenol dye
(which changes from yellow to red when it absorbs
tears that are slightly alkaline) in the ventral fornix of
an eye for 15 seconds.2
The EAPPTT was proposed in 2012 as a new
method for tear film assessment.5 Standardized end-
odontic absorbent paper points are commonly used
in dentistry because their highly absorptive proper-
ties promote drying after irrigation, allow carriage
of medicants (eg, antiseptics and disinfectants), and
assist in collection of samples for microbiological
culture.5,6 They also can be used as an alternative
method for tear film measurement. For those mea-
surements, 1 standardized absorbent paper point is
inserted in the ventral conjunctival fornix of an eye
ABBREVIATIONS
EAPPTT Endodontic absorbent paper points tear test
HPFL Horizontal palpebral fissure length
IOP Intraocular pressure
mSTT Modified Schirmer tear test
PRTT Phenol red thread test
STT Schirmer tear test
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AJVR • Vol 77 • No. 1 • January 2016 73
and allowed to remain there for 1 minute; the pa-
per point is then removed, and the wet portion is
measured by use of a digital calipers graduated in
millimeters.
Intraocular pressure is controlled and regulated
by the CNS, which maintains a balance between aque-
ous humor production and outflow.7,8 Assessment of
IOP is a critical component of a complete ophthalmic
examination because an abnormally high or low IOP
is evidence of ocular disease, such as glaucoma or
uveitis.9
The purpose of the study reported here was to de-
termine tear secretion by use of the mSTT, PRTT, and
EAPPTT and to measure IOP by means of rebound to-
nometry in the eyes of healthy adult Syrian hamsters
(Mesocricetus auratus). Additionally, HPFL and eye
blink frequency were evaluated because these 2 vari-
ables could directly affect measurement of tear pro-
duction and spread of the tear film.10
Materials and Methods
Animals
The study population consisted of 40 healthy
adult Syrian hamsters (21 males and 19 females).
Animals were housed indoors beginning 7 days be-
fore the first day of testing; Syrian hamsters were
housed separately in labeled cages in an air-condi-
tioned room with a constant temperature (20° to
22°C) and relative humidity (50% to 55%). The light-
ing cycle consisted of 12 hours of light and 12 hours
of darkness. Animals were fed a commercial diet
formulated for hamsters, and water was available ad
libitum. The study was approved by the Iran Society
for Prevention of Cruelty to Animals in accordance
with the Iranian Ethical Code for Studies on Labora-
tory Animals.
Procedures
A complete physical examination and ophthalmo-
scopic examination that included direct and indirect
ophthalmoscopy,a fluorescein staining,b and slit lamp
biomicroscopyc were performed. All the animals were
included in the study on the basis that no abnormali-
ties were detected during the physical and ophthal-
mic examinations.
A PRTT,d EAPPTT,e and STTf were performed.
Each test was produced by a single manufacturer and
was from the same batch with a single lot number. A
sequence of procedures was performed on each Syr-
ian hamster. Eye blink frequency and IOP were as-
sessed on day 1, the PRTT was performed on day 3, the
EAPPTT was performed on day 5, and the mSTT and
HPFL were assessed on day 7. On day 14, complete
physical and ophthalmoscopic examinations were
performed on all Syrian hamsters.
One investigator (SMR) conducted all ocular tests,
examinations, and measurements. All tests were con-
ducted between 4 pm and 6 pm to minimize possible
variations associated with diurnal changes.
Eye blink frequency was counted. Each animal
was placed in a cage that was made of clear plastic,
which was intended to provide familiar surroundings.
Syrian hamsters were not restrained or handled dur-
ing counting. Two investigators (SMR, MAM), 1 located
on each side of the cage, counted the number of eye
blinks during a 5-minute period. The mean value for
the 2 investigators was calculated and used for statisti-
cal analysis.
For IOP measurement, animals were physically
restrained without any pressure applied to the eye-
lids or neck. One of the investigators grasped a Syrian
hamster by the nape of the neck between a thumb
and forefinger and simultaneously maintained a grip
on the tail and supported the animal’s body against
the palm of the other hand; a second investigator then
obtained IOP values. Protrusion of the eyeballs was
not observed during tonometry. A tonometerg with a
disposable probe was held horizontally perpendicular
at a distance of 4 to 5 mm from the central corneal sur-
face. The device was calibrated by use of the p setting.
Six consecutive measurements were obtained. The se-
ries of measurements was repeated until the tonom-
eter indicated that there was an acceptable SD for the
6 measurements. The procedure then was repeated for
the contralateral eye.
To measure the aqueous portion of the tear film,
the ventral eyelid of each Syrian hamster was everted.
A 3-mm folded head of a phenol red cotton thread was
placed into the ventral conjunctival fornix and allowed
to remain there for 15 seconds. The thread was then re-
moved, and the portion of the thread that had changed
from yellow to red was immediately measured.
To measure the aqueous tear volume with the
EAPPTT, 1 absorbent paper point was inserted in the
ventral conjunctival fornix of each eye and allowed to
remain there for 1 minute. Each paper point was then
removed, and the wet portion was immediately mea-
sured by use of a digital calipers.
The mSTT strips were obtained by longitudinal-
ly dividing standard (35 mm in length and 5 mm in
width) commercial STT strips aseptically with a scal-
pel blade and stainless steel ruler to yield 2 strips that
were 35 mm in length and 2.5 mm in width. Forceps
were used to insert an mSTT strip in the ventral con-
junctival fornix. Strips were allowed to remain in the
fornix for 1 minute. Strips then were removed, and
the wet portion was measured. Because of the small
amount of tears in most of the eyes, the notch of the
mSTT strip often was not reached; thus, the distance
from the end of a strip to the point at which the strip
was wet was measured, rather than measuring the
length of the wet strip beginning at the notch as is
conventional for other species.11
For measuring HPFL, the distance between the
inner end of the ocular caruncle and the temporal
canthus (termed the palpebral fissure length) was
measured. Measurements were obtained by use of a
waterproof digital caliper with a liquid-crystal display
screen.h
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74 AJVR • Vol 77 • No. 1 • January 2016
Statistical analysis
Statistical analysis was performed by use of a sta-
tistical software program.i A 1-sample Kolmogorov–
Smirnov test was used to assess data normality. Paired
sample t tests were used to compare IOP, PRTT,
EAPPTT, mSTT, and HPFL values obtained for the right
and left eyes. Mean and SD were calculated for all the
eyes and for right and left eyes separately. An inde-
pendent sample t test was used to compare mean
IOP, PRTT, EAPPTT, and mSTT values for sex and body
weight. A Pearson correlation analysis was used to
evaluate the relationship between body weight and
mean IOP, PRTT, EAPPTT, mSTT, and HPFL. Values were
considered significant at P < 0.05.
Results
Ocular discomfort was not observed in any of the
Syrian hamsters for up to 7 hours after measurements
were obtained during the study. No signs of conjuncti-
vitis, keratitis, blepharitis, corneal ulcers, or intraocular
disease were detected in any of the Syrian hamsters.
All the continuous numeric data obtained for the
study were normally distributed as determined by use
of the 1-sample Kolmogorov-Smirnov test (P > 0.2).
Mean ± SD body weight for all Syrian hamsters was
83.40 ± 18.20 g (range, 42.2 to 122.0 g). Mean body
weight of females and males was 83.13 ± 15.61 g and
83.81 ± 20.76 g, respectively (Table 1). Mean values
for IOP, PRTT, EAPPTT, and mSTT for all 80 eyes were
4.55 ± 1.33 mm Hg, 5.57 ± 1.51 mm/15 seconds, 4.52
± 1.55 mm/min, and 2.07 ± 0.97 mm/min, respectively.
We did not detect significant differences in values be-
tween the right and left eyes or between males and
females.
A correlation (r = 0.541; P = 0.014) was detected
between PRTT and EAPPTT values but not between
PRTT and mSTT values or between EAPPTT and
mSTT values. Moreover, there was no correlation be-
tween the measured variables and body weight of the
Syrian hamsters.
Discussion
A paucity of information exists on ocular variables
for wild and exotic animals because of the large num-
ber of species involved.10 Some wild and exotic spe-
cies are maintained as pets, whereas others are used as
research animals. Regardless of their use, good veteri-
nary care must be provided for each species, and refer-
ence intervals need to be determined for physiologic
variables before diagnosis of abnormal conditions
is possible. This is particularly true for ophthalmic
examination.
Syrian hamsters are desert-dwelling species. Low
tear production may be a fluid conservation mecha-
nism for animals living in arid areas.12 Nevertheless,
low tear production by Syrian hamsters is adequate to
protect the ocular surface.13 In contrast to results of
a previous study13 on tear production by Syrian ham-
sters in which investigators found a significantly in-
creased PRTT value for male animals, compared with
results for female animals, no significant difference
was found between PRTT values of the males and fe-
males in the present study.
The mean ± SD EAPPTT for Wistar rats (Rattus
norvegicus) and Swiss Webster mice (Mus musculus)
is 6.18 ± 2.06 mm/min and 4.39 ± 1.45 mm/min, re-
spectively.10 The mean EAPPTT for Syrian hamsters in
the present study was slightly higher, compared with
values for the mice of that other study.10
Another alternative quantitative tear test is the
mSTT, which has been used in dogs,14,15 birds,16 rhe-
sus monkeys (Macaca mulatta),17 black-tufted mar-
mosets (Callithrix penicillata),5 and red-ear sliders
(Trachemys scripta elegans).18 In the present study,
STT values for Syrian hamsters were exceptionally
low, which made it difficult to evaluate tear produc-
tion by use of this method. A more precise measure-
ment was possible with the mSTT. The mSTT has been
used in birds16; however, the strips used in that study16
were only 2 mm wide.
During STT measurement, the filter paper strip ab-
sorbs all the tears produced as well as those compris-
ing the tear film. Once the tear film has been absorbed,
tear uptake by the test strip equals tear production by
the lacrimal and Harderian glands.19
In the present study, results for the PRTT and
EAPPTT were positively correlated, and volume of
fluid measured by use of the PRTT and EAPPTT was
small. We postulate that the PRTT and EAPPTT were
measuring tear volume in the conjunctival sac rather
Mean ± SD Range
Male Female Male Female
Variable All (n = 40) (n = 21) (n = 19) All (n = 40) (n = 21) (n = 19)
IOP (mm Hg) 4.55 ± 1.33 4.90 ± 1.41 4.20 ± 1.22 2–8 2–8 2–7
mSTT (mm/min) 2.07 ± 0.97 2.09 ± 1.15 2.05 ± 0.76 0–5 0–5 0–4
PRTT (mm/15 s) 5.57 ± 1.51 5.15 ± 1.56 6.00 ± 1.41 3.0–10.0 3.0–7.5 4.5–8.5
EAPPTT (mm/min) 4.52 ± 1.55 4.40 ± 1.48 4.65 ± 1.68 2–7 3–7 2–7
HPFL (mm) 5.84 ± 0.10 5.87 ± 0.49 5.81 ± 0.50 5.16–6.72 5.32–6.70 5.16–6.72
EBF (blinks/min) 1.68 ± 0.43 1.66 ± 0.53 1.70 ± 0.35 1.2–2.4 1.2–2.4 1.4–2.4
Body weight (g) 83.40 ± 18.20 83.81 ± 20.76 83.13 ± 15.61 42.2–122.0 42.2–122.0 51.0–105.0
EBF = Eye blink frequency.
Table 1—Mean ± SD and range for ophthalmic variables measured in both eyes of each of 40 Syrian hamsters (Mesocricetus
auratus).
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AJVR • Vol 77 • No. 1 • January 2016 75
than assessing de novo tear production by the lac-
rimal glands.
The HPFL for Wistar rats and Swiss Webster mice is
6.45 ± 0.09 mm and 3.59 ± 0.27 mm, respectively.10 Adult
Syrian hamsters of the present study had a larger HPFL
than did mice of similar body weight. It is worth men-
tioning that the ease with which globes prolapse with
handling of hamsters is related to the longer lid aperture.
Measurement of IOP is important for evalu-
ation of ocular health. Reference IOP values for
mice and rats have been obtained with a rebound
tonometer.j Mean ± SD IOP of conscious rats is 18.4 ±
0.1 mm Hg.20 Mean IOP differs among strains of
mice (10.6 ± 0.6 mm Hg for Balb/c mice, 13.3 ±
0.3 mm Hg for C57-BL/6 mice, and 16.4 ± 0.3 mm Hg for
CBA mice).20 Mean IOP determined by use of a rebound
tonometerg in New Zealand White rabbits is 9.51 ±
2.62 mm Hg.21 The mean IOP of 4.55 ± 1.33 mm Hg for
Syrian hamsters of the present study was significantly
lower than values measured in mice, rats, and rabbits.
Differences in handling and restraint of animals, time of
day, and position of the body or head could have been
responsible for the difference between IOP of Syrian
hamsters and IOP of other rodents; however, in the au-
thors’ opinion, such factors are unlikely to result in such
a marked difference. The low IOP in Syrian hamsters
requires further evaluation.
A comparison of 2 types of rebound tonometersg,j
has been performed for chinchillas22 and red-ear slid-
ers.23 The rebound tonometerj for laboratory animals
may be more accurate than the veterinary rebound
tonometerb used for red-ear sliders.23 However, no sig-
nificant differences were observed in IOP of chinchil-
las for the various models of rebound tonometer.22 In
the present study, IOP was obtained by use of a veteri-
nary rebound tonometerg with the device calibrated
by use of the p setting. Use of a veterinary rebound
tonometerg would appear to be most appropriate
owing to its widespread availability as a diagnostic de-
vice in veterinary clinics.
Contact between the cornea and probe rarely
causes a corneal reflex in dogs.24 Similar to results for
Hermann’s tortoises (Testudo hermanni),25 impact of
the probe invariably induced a blink reflex in the Syr-
ian hamsters during the study reported here. Although
this discrepancy may be attributed to the size of the
ocular globe of Syrian hamsters, interspecies differ-
ences in corneal innervation cannot be excluded.25
Eye blink frequency of Syrian hamsters in the
present study ranged from 1.2 to 2.4 blinks/min. Eye
blink frequency of guinea pigs is 2 to 5 blinks/20 min-
utes.11 Eye blink frequency of dogs, cats, horses, cat-
tle, black-tufted marmosets, and pigs is 3 to 5 blinks/
min,26 1 to 5 blinks/5 minutes,26 5 to 25 blinks/min,26
5 blinks/min,26 3 to 5 blinks/min,5 and approximately
10 blinks/min,26 respectively. Blinking maintains the
physiologic thickness of the preocular surface by
spreading tears over the corneal surface.27
The study reported here provided reference val-
ues for ophthalmic examinations of adult Syrian ham-
sters. The IOP was measured by means of rebound to-
nometry, and tear production was assessed by use of a
number of tests.
Acknowledgments
The study was performed at the Faculty of Specialized Veteri-
nary Sciences, Science and Research Branch, Islamic Azad Univer-
sity, Tehran, Iran.
This research received no specific grant from any funding
agency in the public, commercial, or not-for-profit sectors. The
authors declare that they have no conflicts of interest.
Footnotes
a. Binocular indirect ophthalmoscope, Welch Allyn Inc,
Skaneateles Falls, NY.
b. Fluorescein Glostrips, Nomax Inc, St Louis, Mo.
c. PSL portable slit lamp, Reichert Inc, Depew, NY.
d. Zone-Quick, Menicon America Inc, San Mateo, Calif.
e. Roeko color, number 30, Coltene/Whaledent GmbH & Co KG,
Langenau, Germany.
f. Opstrip, Ophtechnics Inc, Haryana, India.
g. TonoVet, Icare, Tiolat, Helsinki, Finland.
h. IP54, 0-150 mm, resolution, 0.01 mm, Guanglu, Guilin, China.
i. IBM, SPSS version 17.0 for Windows, SPSS Inc, IBM Co, Chicago,
Ill.
j. TonoLab, Icare, Tiolat, Helsinki, Finland.
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