Intraocular pressure (IOP) is a fundamental physiological parameter widely used in both human and veterinary medicine for the diagnosis, monitoring, and treatment of ophthalmological diseases such as glaucoma, uveitis, and conjunctivitis ^3,4. IOP reflects the dynamic balance between aqueous humor production and drainage. Disruption of this balance may result in optic nerve damage, permanent vision loss, and serious clinical conditions such as glaucoma ^5,6. Therefore, accurate and reliable measurement of IOP is of great importance, particularly in post-traumatic evaluations and cases of systemic diseases ⁷.

Although reference values for IOP have been defined in veterinary literature for cats, dogs, horses, rats, and some exotic species, there are no available reference values for gazelles ^8-10. This gap creates uncertainty, particularly during diagnostic and therapeutic processes, and the use of reference values extrapolated from different species, reduces the measurement accuracy and reliability of clinical decisions. Moreover, considering physiological variables, such as the effects of anesthetic agents on IOP and the influence of body position on measurements, the lack of species-specific reference values for Gazella subgutturosa represents a significant clinical limitation ^11-13.

This study aimed to perform systematic IOP measurements in healthy male and female Gazella subgutturosa individuals using the rebound tonometry method without sedation and in a natural posture and to define scientifically based reference values for this species. The original dataset intended to be obtained is expected to provide clinical reference values applicable to the diagnosis, monitoring, and treatment of glaucoma, conjunctivitis, uveitis, and traumatic ocular diseases. In addition, these data are anticipated to contribute to clinical decision-making processes on an objective basis and to provide a scientific foundation for the long-term health management of this endangered species.

Animal Material and Study Design: The study was conducted on 20 gazelles (Gazella subgutturosa) aged between 0 and 6 months, who were brought to the Harran University Faculty of Veterinary Medicine Animal Hospital for routine health examinations and were considered clinically healthy. Ten gazelles were female, and ten were male. Both eyes of each individual were evaluated separately, for a total of 40 eyes.

Prior to IOP measurement, all animals underwent a detailed ophthalmological examination. This evaluation included assessment of blink reflex, pupillary light reflex, and direct ophthalmoscopy (Gowllands® Ophthalmoscope, United Kingdom). Individuals with no signs of ophthalmic or systemic disease were included in the study.

Tonometry Procedure: IOP measurements were performed using the Icare® TONOVET Plus device (Finland), which operates based on the rebound tonometry principle (Figure 1). Because no species-specific calibration mode is available for gazelles, the "d" mode was selected based on its widespread use in medium-sized mammals with comparable corneal biomechanics. Manufacturer-specific recommendations for Gazella subgutturosa are currently unavailable. Measurements were conducted in the device's "d" mode, in accordance with the manufacturer's instructions, without the use of topical or systemic anesthetics, and without sedation ³. All measurements were carried out by the same investigator within the same time interval to minimize circadian variability. All measurements were performed between 09:00 and 12:00 to minimize the effects of diurnal variation in intraocular pressure. During the procedure, gazelles were manually restrained in their natural standing posture. Measurements were first obtained from the right eye and subsequently from the left eye. The tonometer probe was positioned perpendicular to the center of the cornea at an approximate distance of 4–8 mm, and the mean of six consecutive measurements was used for evaluation. A new probe tip was used for each animal ¹⁴. Care was taken to avoid pressure on the neck region and eyelid manipulation was minimized. All the procedures were performed in a quiet environment to prevent stress. The values obtained from each eye were recorded separately and the arithmetic mean of the right and left eye measurements was calculated as the individual IOP value.

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Figure 1: Measurement of intraocular pressure in a gazelle using the Icare® TONOVET Plus rebound tonometer without sedation

Statistical Analysis: Statistical analyses were performed using the IBM SPSS Statistics 22.1 software (IBM Corp., Armonk, NY, USA). The distribution characteristics of the quantitative data were assessed using the Shapiro–Wilk test. Results are reported as the mean ± standard deviation (SD) and median (min–max). To determine the differences between IOP values obtained from the right and left eyes, a paired-samples t-test was used when the data were normally distributed, whereas the Wilcoxon signed-rank test was applied when normality was not met. To avoid pseudoreplication, the mean of the right and left eye measurements was calculated for each individual and used as the unit of analysis in all subsequent comparisons. Comparisons of IOP between sex groups were performed using an independent-samples t-test under parametric conditions and the Mann–Whitney U test. Homogeneity of variances was assessed using Levene's test prior to one-way ANOVA. One-way analysis of variance (ANOVA) was used to assess differences in mean IOP values among age groups, and the Kruskal–Wallis test was preferred when parametric assumptions were not satisfied. In all statistical analyses, the level of significance was set at p<0.05.

In comparisons based on sex, IOP values in female gazelles (24.3±9.1 mmHg) were significantly higher than those in males (17.6±6.8 mmHg). The independent-sample t-test demonstrated that this difference was statistically significant (p=0.026). This finding suggests that sex may have a potential influence on IOP.

Evaluation according to age groups showed no statistically significant difference in IOP values based on the results of one-way analysis of variance (ANOVA) (p=0.222). Among the age groups, the most commonly represented group, 30-day-old individuals, exhibited a mean IOP value of 21.3±9.3 mmHg, which was comparable to those of the other age groups (Table 2).

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Table 1: Descriptive statistics of intraocular pressure (IOP) values obtained by rebound tonometry in healthy Gazella subgutturosa individuals

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Table 2: Descriptive statistics of intraocular pressure (IOP) values measured in Gazella subgutturosa individuals according to age groups

Despite the lack of interocular statistical differences, each eye should be evaluated individually, as certain ophthalmic disorders may develop unilaterally. However, for greater reliability and to reduce measurement error, evaluation of both eyes together is considered more appropriate ^5,8,15,16. The significantly higher IOP observed in females compared with males in the present study (24.3±9.1 mmHg vs. 17.6±6.8 mmHg; p=0.026) suggests the presence of species-specific and potentially sex-related physiological differences. Although similar sex-related effects have been reported in some avian species in association with head position (e.g., higher IOP in male adult cranes) ^17, sex-related differences are not consistent between members of the Cervidae and Bovidae families. Therefore, confirmatory studies on gazelles that concurrently investigate variables such as ocular biometry, hormonal status, and corneal thickness are warranted. However, given the relatively small overall sample size, this finding should be interpreted cautiously and warrants confirmation in larger population-based studies.

The mean IOP obtained in gazelles in the present study (both eyes: 21.3±9.0 mmHg) was close to, but slightly lower than, the values reported in conscious red deer measured by rebound tonometry (26.8±4.1 mmHg) ⁸. When compared with the mean values reported using applanation tonometry in eland and fallow deer (14.6±4.0 and 11.9±3.3 mmHg, respectively) ^4, it is evident that differences in device type and measurement method can significantly influence absolute IOP values. Indeed, in white-tailed deer, measurements obtained with the TonoPen® were higher than those obtained with the TonoVet® in the same individuals (15.6±2.9 vs. 12.9±2.6 mmHg), demonstrating a systematic measurement bias between the two devices ⁹. Similar rebound–applanation comparisons in dogs and horses have also indicated small but clinically significant interdevice offsets ¹⁸. Previous studies in cervids and other ungulate species have reported variability in intraocular pressure measurements depending on the selected TonoVet calibration mode and device type. Therefore, further manometric validation studies are required to determine the most appropriate calibration mode for gazelles. In addition, deviations from manometric measurements may be significant among the species modes of TonoVet Plus®; for example, in the porcine eye, the "equine" mode yielded the highest accuracy, whereas the "canine" and "leporine" modes systematically overestimated IOP ¹⁹. In the present protocol, the "d" mode was used; however, because no species-specific calibration is available for gazelles, further studies involving manometric validation or determination of the most appropriate mode are considered necessary.

In this study, all measurements were performed with the animals in a standing position, without applying pressure to the head or neck region, and with minimal manipulation of the eyelids to avoid position- and manipulation-related increases or decreases in IOP. This approach is consistent with strong evidence in horses showing that IOP increases markedly when the head is lowered below the heart level and decreases when it is elevated ²⁰. A wide range of IOP changes associated with head position during hoisting or inversion have been reported ¹¹. In dogs, measurable short-term changes in IOP related to body position have been described ²¹. In cranes, the position of the head relative to the heart level has been shown to significantly affect IOP ¹⁷. This body of literature reinforces the necessity of recording body position during both field and clinical IOP measurements as well as ensuring identical conditions during repeated measurements.

No significant differences were detected among age groups in the present study, which is consistent with findings in white-tailed deer showing no age-related differences in IOP ⁹. However, in some species, IOP has been reported to increase with age and body weight during development (e.g., in crane chicks) ¹⁷. Sensitivity to environmental conditions may vary among species. For instance, significant location-related variability in IOP has been reported in Pygoscelis penguins ^7, highlighting the potential influence of environmental stressors and climate in field studies. In addition, a slight but significant age-related decrease in IOP has been reported in black-footed penguins ^22, which may be associated with changes in corneal or ocular biomechanics or autonomic tone. Because central corneal thickness (CCT) was not measured in the present study, the potential effect of CCT cannot be excluded. Although reports in some species indicate that the influence of CCT on IOP measurements is limited or negligible ^22, this relationship requires confirmation in gazelles.

Veterinary studies have demonstrated that anesthetic and sedative agents can significantly reduce IOP and, more rarely, increase it; moreover, sensitivity may vary depending on the measurement device and mode used ^13,15. Pronounced device- and mode-related differences have been reported in coatis ²³. TonoVet® has been shown to be more accurate and sensitive than TonoLab® in chinchillas ^24, and TONOVET Plus has demonstrated high accuracy within the clinical range in the rabbit eye ²⁵. In the present study, the use of a sedation-free measurement protocol was considered particularly important for reference value generation because it eliminates pharmacological influences. In addition, the distance between the probe and cornea and the perpendicular contact of the probe with the corneal surface are critical for measurement accuracy ¹⁸. During application in gazelles, strict adherence to these standardization criteria was ensured.

The findings of this study provide the first reference IOP values that allow objective interpretation of IOP in gazelles in cases of trauma, suspected uveitis, or glaucoma, and postoperative follow-up. Despite the absence of interocular differences, bilateral assessment remains essential in clinical settings. The higher IOP values observed in females should be interpreted with caution until validated by larger studies. It should be emphasized that IOP values obtained without standardization of body position, head and neck pressure, and measurement techniques may lead to false-positive or false-negative results ^11,17,20,21. Device selection and calibration mode should also be considered when defining clinical thresholds ^18,19,24.

This study had several limitations. First, the sample sizes within the age subgroups were small, limiting the statistical power of the subgroup comparisons. The absence of statistically significant age-related differences should be interpreted cautiously, as the statistical power of subgroup analyses was limited by small and highly unbalanced sample sizes across age categories. Second, the single-center design and performance of measurements within a single time period of the day did not allow the assessment of known diurnal fluctuations in IOP. Third, because CCT and ocular biometry were not measured in gazelles, the potential effects of these covariates on IOP cannot be excluded. Furthermore, multicenter studies encompassing different habitats and climatic regions would provide a more comprehensive understanding of the effects of environmental factors on IOP ⁷.

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