Evaluation of intraocular pressure in conscious Hermann's tortoises (Testudo hermanni) by means of rebound tonometry

INTRAOCULAR PRESSURES OF AQUARIUM-HOUSED COWNOSE RAYS (RHINOPTERA BONASUS) WITH NORMAL AND ABNORMAL OPHTHALMIC EXAMS

Cownose rays (Rhinoptera bonasus) are common elasmobranchs in zoos and aquaria; however, there is a lack of published information regarding ocular findings in this species. Intraocular pressure (IOP) was measured in a total of 52 cownose rays (Rhinoptera bonasus) from two unrelated aquaria (n = 22 from A1, n = 30 from A2) using a TonoVet rebound tonometer on two settings (dog = D, and unidentified species = P) as part of a full ophthalmologic examination. Adult (n = 38) and juvenile (n = 14) rays were sampled out of water briefly in sternal recumbency. Intraocular pressure (mean ± SD [range]) in the D setting (9.10 ± 2.57 [4-18] mmHg) was higher than the P setting (5.21 ± 2.32 [0-12] mmHg) (P<0.001). Statistical analysis revealed no difference in IOP between right and left eyes, and no correlation between body weight and IOP. No differences in IOP between sex, age group, and location were identified in either setting. However, a significant difference was observed between levels of severity of corneal disease in IOP D setting (P=0.006) and P setting (P=0.024), and levels of severity of intraocular disease in IOP D setting (P=0.034) only. This study provides baseline IOP values using rebound tonometry in aquarium-housed cownose rays with apparent corneal and intraocular lesions and reveals that the D setting may be more sensitive in identifying IOP changes in eyes with intraocular disease.

COMPARISON OF INTRAOCULAR PRESSURE MEASUREMENT USING REBOUND AND APPLANATION TONOMETRY IN LOGGERHEAD (CARETTA CARETTA) SEA TURTLES

This study aimed to evaluate intraocular pressure (IOP) estimates in healthy eyes of Caretta caretta using rebound tonometry in comparison with applanation tonometry. Twenty-three healthy C. caretta (housed at the Marine Turtle Research Center) without preexisting ophthalmic disease were enrolled in the study. IOP measurements were obtained by the same ophthalmologist, with the turtle in ventral recumbency between 2:30 p.m. and 4:30 p.m., using a rebound tonometer (RT; TonoVet) in dog calibration mode, and after topical anesthesia, an applanation tonometer (AT; Tono-Pen) in both eyes. The average of three readings per instrument was used for analysis. The agreement between the two tonometers was assessed by Bland-Altman analysis and intraclass correlation coefficient (ICC). Moreover, differences in IOP between the two tonometers were analyzed using the Mann-Whitney test. Moderate agreement was found between the two tonometers (ICC, 0.663; 95% confidence interval, 0.206-0.857). The median, Q1, and Q3 IOP obtained with AT (6.2, 4.7, and 9.1 mm Hg) were significantly lower (P = 0.001) than that obtained with RT (9.7, 8.3, and 11.6 mm Hg). It was not possible to obtain an instrument automatically generated mean of four values with AT because of retraction of the globe by the animals, and IOP measurement was unsuccessful in 7 eyes. In conclusion, IOP readings from the RT were statistically higher than those from the AT. RT proved to be more feasible because of the light, short-lasting contact with the cornea.

CLINICAL OPHTHALMIC PARAMETERS OF THE TEXAS TORTOISE (GOPHERUS BERLANDIERI)

Ophthalmic studies of the Texas tortoise (Gopherus berlandieri) established normal ophthalmic parameters for select diagnostic tests in captive tortoises and assessment of differences among individuals of differing size and health status. Sixty-one tortoises of varying weight, shell size, Mycoplasma seroprevalence, and herpesvirus exposure were included. Complete ophthalmic examinations, including neuro-ophthalmic reflexes, phenol red thread test, rebound tonometry, fluorescein staining, palpebral fissure length measurement, slit lamp biomicroscopy, indirect fundoscopy, and ocular ultrasound measurements of axial globe length, anterior chamber depth, lens thickness, and vitreous length, were recorded. All tortoises had negative dazzle and pupillary light reflexes, inconsistent menace responses, and positive palpebral reflexes. Mean ± SD tear production and intraocular pressure (IOP) were 14.2 ± 5.6 mm/15 sec and 13.8 ± 2.4 mm Hg in healthy tortoises, respectively. Mycoplasma-seropositive tortoises (with or without herpesvirus exposure) had significantly increased tear production (20.2 ± 8.1 and 19.9 ± 8.9 mm/15 sec, respectively) compared with healthy seronegative tortoises (14.2 ± 5.6 mm/15 sec; P = 0.02). As body size decreased, so too did palpebral fissure length and ocular ultrasound measurements, while IOP increased. Overall, palpebral fissure length appeared relatively small, and tear production relatively increased compared with other chelonian species, likely on the basis of the relatively arid native habitat. Further work is recommended to establish baseline values in related species, as well as comparison in aquatic versus terrestrial chelonians. The authors further suggest that the finding of relatively increased tear production in tortoises may indicate the need to rule out mycoplasmosis as a cause of upper respiratory tract disease.

Intraocular pressure measurements using the TONOVET® rebound tonometer: Influence of the probe-cornea distance

PURPOSE

To demonstrate the effect of different probe-cornea distances during intraocular pressure (IOP) data acquisition in dogs and rats.

ANIMALS STUDIED

Twenty-four conscious dogs and 15 anesthetized Wistar rats.

METHODS

Three interchangeable three-dimensional printed polylactide plastic spacer collars were used in place of the original Icare TonoVet® collar piece, which provided different distances (4, 6, and 8 mm) between the instrument's probe and the corneal surface. IOP values were obtained in sequence by a single observer, with the tonometer probe at a 4-, 6-, and 8-mm distance from the corneal surface. The dogs were gently restrained, and the rats were anesthetized with isoflurane.

RESULTS

Intraocular pressure values obtained at 4, 6, and 8 mm from the TonoVet® probe to corneal surface distance in both dogs and rats were significantly different (P < .01). There was a small positive correlation between IOP (mm Hg) and probe-cornea distance (mm) (r_s  = 0.39 for dogs and r_s  = 0.51 for rats). In dogs, the mean IOP (± SD mm Hg) obtained at different distances were 16.2 ± 3.0 at 4 mm; 17.6 ± 3.4 at 6 mm; and 19.8 ± 3.8 at 8 mm. In rats, IOP values were 8.2 ± 1.5 at 4-mm; 9.4 ± 1.8 at 6-mm; and 10.5 ± 1.5 mm Hg at 8-mm distance.

CONCLUSIONS

Probe-cornea distance of the Icare TonoVet® significantly affects IOP readings, even within the 4- to 8-mm range recommended by the manufacturer.

OPHTHALMIC EXAMINATION FINDINGS AND INTRAOCULAR PRESSURE MEASUREMENTS IN SIX SPECIES OF ANURA

A complete ophthalmic exam, including intraocular pressure (IOP) measurement, is key to diagnosing ocular diseases such as uveitis and glaucoma in frogs. We performed complete ophthalmic anterior segment examinations and IOP rebound tonometry measurements using two different settings (other "p" and canine "d") for six anuran species. The objectives were to describe common ocular abnormalities found in these species, to compare IOP values between different tonometer settings, and to compare IOPs between species. Examinations revealed abnormalities including cataracts (11/98 total eyes), lenticular sclerosis (10/98) and lipid keratopathy (9/98). IOP was measured with the TonoVet^® and the ranges (oculus uterque, OU mm Hg other "p" setting, canine "d" setting) were giant waxy monkey tree frogs (Phyllomedusa bicolor) (3.5-7.6; 6.5-11.7; n = 5), mission golden-eyed tree frogs (Trachycephalus resinifictrix) (7.0-9.7, 13.2-15.7; n = 6), boreal toads (Bufo boreas boreas) (0.8-5.5, 5.7-10.5; n = 13), Mexican giant tree frogs (Pachymedusa dacnicolor) (3.8-5.0, 8.3-11.8; n = 3), Lake Titicaca frogs (Telmatobius culeus) (8.8-10.5, 14.0-17.2; n = 8), and mossy tree frogs (Theloderma corticale) (9.7-11.0, 15.7-17.0; n = 5). The TonoVet canine "d" setting IOP measurements were statistically higher (P = 0.01) than the other "p" setting measurements for all species except the giant waxy monkey tree frogs. IOP was significantly lower for giant waxy monkey tree frog eyes with cataracts (P < 0.05) with the other "p" setting. IOP did not statistically differ in eyes with lenticular sclerosis. IOP can be measured by rebound tonometer in anurans, but more research is needed for species-specific references using consistent settings.

Ocular findings in a group of healthy captive leopard geckos

OBJECTIVE

Leopard Geckos (Eublepharis macularius) are popular pets and can be affected by a range of ocular disorders. Our objective was to report ocular findings in a group of healthy captive leopard geckos and to establish reference ranges for commonly performed ocular diagnostic tests.

ANIMALS STUDIED

Twenty-six healthy male geckos aged 1 year old (n = 4) and >2 years old (n = 22).

PROCEDURES

All animals underwent ophthalmic examination, corneal esthesiometry, modified Schirmer tear test (mSTT), rebound tonometry, conjunctival bacterial aerobic and fungal culture, and measurement of ocular dimensions. Student's t test was used to compare values of corneal esthesiometry, tonometry and mSTT between groups. Multiple correlations were assessed by Pearson correlation coefficient.

RESULTS

All animals had a normal ocular examination. Tear production as measured with a mSTT (mean ± SD) technique was 3.1 ± 1.3 mm/min and tonometry values (mean ± SD) were 8.2 ± 1.7 mm Hg. Corneal touch threshold (median, range) was 4.4 cm, 2.5-5.0. Younger animals had a significantly increased corneal sensitivity compared to older animals (P = .0383). Results of culture showed no growth for fungal organism in any animals. Conjunctival bacterial isolation rates were low, with only 7/26 samples positive for nine bacterial species.

CONCLUSIONS

Leopard geckos are amenable to ophthalmic examination and ocular diagnostic database testing with minimal manual restraint.

The influence of the tonometer position on canine intraocular pressure measurements using the Tonovet® rebound tonometer

The objective of this study was to assess the variability of readings made using the Tonovet^® rebound tonometer for measurement of intraocular pressure (IOP) in the peripheral cornea and in angulated positions on the canine corneal surface. Forty-six client-owned dogs admitted for ophthalmic evaluation at the Queen's Veterinary School Hospital, University of Cambridge were included in the study. IOP readings were taken at a variety of locations and using the tonometer at a number of different angles to the cornea: 1) Perpendicularly at center of the cornea (CC); 2) At the center of the cornea but with the tonometer positioned at four angles, and 3) At four different points on the peripheral cornea. All values were compared with the values recorded at the recommended CC position. IOP values were significantly underestimated in seven positions, with median and interquartile range from 12.1 ± 4 mmHg (nasal on periphery) to 15 ± 5 mmHg (laterally angled at center), varying between 0 mmHg to 2.9 mmHg from the CC value. While dorsally angled in the central cornea were not significantly different from those at CC (p = 0.09). Median values were lower for measurements in peripheral positions when compared to angled central positions. These results demonstrate that angling the tonometer or measuring in peripheral regions can result in small but statistically significant underestimation of IOP values.

MEASURING INTRAOCULAR PRESSURE IN WHITE'S TREE FROGS (LITORIA CAERULEA) BY REBOUND TONOMETRY: COMPARING DEVICE, TIME OF DAY, AND MANUAL VERSUS CHEMICAL RESTRAINT METHODS

Ocular diseases reported in frogs include uveitis and glaucoma, which are associated with changes in intraocular pressure (IOP). The objectives of this study were to characterize the normal IOP for White's tree frogs ( Litoria caerulea ) using two types of rebound tonometers, and to assess whether time of day or method of restraint affected IOP. Eighteen conscious, unrestrained, ophthalmologically normal frogs were used to measure IOP using TonoVet® and TonoLab® tonometers, at three time points during the day. In a subset of 12 frogs, IOP was measured while under manual restraint using the TonoVet. Anesthesia was induced in 9 frogs using two different concentrations of MS-222 (0.5 g/L and 2 g/L) in order to evaluate for changes in IOP with the TonoVet. Mean (± SD) IOP values for the TonoLab (16.8 ± 3.9 mm Hg) were significantly higher than TonoVet values (14.7 ± 1.6 mm Hg; P < 0.01). TonoVet IOP values did not significantly change with time of day. TonoLab values were significantly lower in the evening (1600-1800; 14.5 ± 3.1 mm Hg), compared with morning and midday measurements (0800-1000 and 1200-1400; 18.0 ± 3.8 mm Hg; P < 0.01). Manually restrained frogs had significantly lower IOP (13.4 ± 1.5 mm Hg) compared with unrestrained frogs (15.3 ± 1.2 mm Hg; P < 0.01). Chemical restraint did not cause significant changes in IOP. Intraocular pressure can be measured with both types of rebound tonometers in White's tree frogs, but time of day and manual restraint can affect IOP values.

Tonometer validation and intraocular pressure reference values in the normal chinchilla (Chinchilla lanigera)

OBJECTIVES

To determine accuracy and precision of three commonly used tonometers (TonoVet^® and TonoLab^® (ICare Oy, Finland)-rebound tonometers, and Tono-Pen VET™ (Reichert, NY)-applanation tonometer) in normal chinchillas, and to establish a normal intraocular pressure (IOP) reference range in this species.

METHODS

The anterior chambers of three chinchilla eyes were cannulated ex vivo and readings obtained at manometric IOPs from 5 to 80 mmHg, using each of the three tonometers in random order. Data were analyzed by linear regression, ANOVA, and Bland-Altman plots. Tonometry was performed in both eyes of 60 chinchillas (age 8 weeks-16.2 years) using the TonoVet^® and relationship between age and IOP analyzed using linear regression. For all statistical tests, P < 0.05 was significant.

RESULTS

Intraocular pressure values obtained using the Tono-Pen VET™ and TonoVet^® (in dog calibration mode;'d') showed strong linear correlation with manometry within the physiologic and clinically relevant range of IOP (0-50 mmHg). The TonoVet^® 'd' setting displayed significantly greater precision over the full range of IOP evaluated than the Tono-Pen VET™, and both TonoVet and Tono-Pen VET™ were significantly more accurate than the TonoLab^® tonometer. Mean ± SD IOP (TonoVet^® 'd') in chinchillas was 9.7 ± 2.5 mmHg, and the 95% reference interval was 4.7-14.7 mmHg.

CONCLUSIONS

Both the Tono-Pen VET™ and TonoVet^® provided clinically acceptable estimates of IOP in chinchillas. The TonoVet^® provides accurate and precise IOP values, while Tono-Pen VET™ derived measurements showed greater variability. Values obtained either with the TonoLab^® or TonoVet^® used in the 'unspecified' calibration setting were inaccurate in this species.

Allometry and Scaling of the Intraocular Pressure and Aqueous Humour Flow Rate in Vertebrate Eyes

In vertebrates, intraocular pressure (IOP) is required to maintain the eye into a shape allowing it to function as an optical instrument. It is sustained by the balance between the production of aqueous humour by the ciliary body and the resistance to its outflow from the eye. Dysregulation of the IOP is often pathological to vision. High IOP may lead to glaucoma, which is in man the second most prevalent cause of blindness. Here, we examine the importance of the IOP and rate of formation of aqueous humour in the development of vertebrate eyes by performing allometric and scaling analyses of the forces acting on the eye during head movement and the energy demands of the cornea, and testing the predictions of the models against a list of measurements in vertebrates collated through a systematic review. We show that the IOP has a weak dependence on body mass, and that in order to maintain the focal length of the eye, it needs to be an order of magnitude greater than the pressure drop across the eye resulting from gravity or head movement. This constitutes an evolutionary constraint that is common to all vertebrates. In animals with cornea-based optics, this constraint also represents a condition to maintain visual acuity. Estimated IOPs were found to increase with the evolution of terrestrial animals. The rate of formation of aqueous humour was found to be adjusted to the metabolic requirements of the cornea, scaling as Vac(0.67), where Vac is the volume of the anterior chamber. The present work highlights an interdependence between IOP and aqueous flow rate crucial to ocular function that must be considered to understand the evolution of the dioptric apparatus. It should also be taken into consideration in the prevention and treatment of glaucoma.

MEASUREMENT OF INTRAOCULAR PRESSURE USING TONOVET® IN EUROPEAN POND TURTLE (EMYS ORBICULARIS)

Twenty-two captive adult European pond turtles (12 males and 10 females) were unrestrained without sedation while intraocular pressure (IOP) was measured by means of a Tonovet®. Mean±SD IOP values between 8 and 10 am for all turtles were 5.42±0.96 mm Hg (range, 3-9 mm Hg). IOP between the right and left eye and between males and females was not significantly different. There was no correlation between IOP and body weight or body length of animals.

Survey of ophthalmic anterior segment findings and intraocular pressure in 95 North American box turtles (Terrapene spp.)

OBJECTIVE

To describe the ophthalmic biomicroscopy findings and intraocular pressures (IOP) in a captive population of box turtles and to determine whether a relationship exists between body morphometrics or health status and IOP.

PROCEDURES

Hundred and three box turtles (69 Gulf coast, 24 three-toed, one ornate, one eastern, and eight unidentified) were triaged into three different color-coded groups: green (healthy), yellow (abnormal physical examination with no need for immediate care), and red (immediate care required). Both eyes were evaluated by rebound tonometry and slit-lamp biomicroscopy. Body weight and morphometric data were recorded.

RESULTS

Intraocular pressures measurements were available for 190 eyes, slit-lamp biomicroscopy was available for 170 eyes, and morphometric data were available for 81 turtles. IOP in Gulf coast turtles (138 eyes) was 6.7 ± 1.4 mmHg OU. IOP in three-toed turtles (48 eyes) was 8.3 ± 1.5 mmHg OU, which was significantly higher than in Gulf coast turtles (P < 0.0001). No significant IOP differences were noted between genders in both subspecies (P = 0.768). There was a correlation between IOP and health status in three-toed turtles only. There was a mild negative correlation between morphometrics and IOP in Gulf coast and three-toed turtles. Fifteen of 87 turtles had unilateral corneal or lenticular opacities; 3/87 had bilateral corneal or lenticular disease; and 3/87 had adnexal abnormalities.

CONCLUSIONS

Different subspecies of box turtles have different normal intraocular pressures as measured by rebound tonometry, which was influenced by the animals' health status in one subspecies. Some morphometric parameters were found to be associated with IOP. Box turtles are often affected with ophthalmic abnormalities of unknown clinical significance.

Intraocular pressure in free-ranging anuran species in Oklahoma

Ocular disease appears to be a common issue in anurans. Intraocular pressures were measured for six species of free-ranging anurans in central Oklahoma. No significant differences were identified between left or right eyes. There was a significant negative relationship between the weight of the anuran and intraocular pressure. The intraocular pressure range for the six species was 3-10 mm Hg. Tonometry values in anurans are, to the authors' knowledge, previously unreported and this study provides initial information on intraocular pressure measurement in anurans.

Evaluation of rebound tonometry in red-eared slider turtles (Trachemys scripta elegans)

OBJECTIVE

To evaluate feasibility and accuracy of intraocular pressure (IOP) measurement by rebound tonometry in adult red-eared slider turtles and determine the effects of manual and chemical restraint on IOP.

ANIMAL STUDIED

Seventeen adult red-eared slider turtles.

PROCEDURES

Intraocular pressure was measured with TonoLab® and TonoVet® tonometers in conscious, unrestrained turtles. To evaluate the effects of manual restraint, turtles were restrained by digital pressure on the rostral head or proximal neck. The effect of two chemical restraint protocols (dexmedetomidine, ketamine, midazolam [DKM] and dexmedetomidine, ketamine [DK] subcutaneously) on IOP was evaluated. Triplicate TonoLab® and TonoVet® readings were compared with direct manometry in three ex vivo turtle eyes.

RESULTS

TonoLab® correlated better with manometry at IOPs < 45 mmHg than TonoVet® (linear regression slopes of 0.89 and 0.30, respectively). Mean (±SD) IOP in unrestrained conscious turtles was significantly lower (P < 0.01) with TonoLab® (10.02 ± 0.66 mmHg) than with TonoVet® (11.32 ± 1.57 mmHg). Manual neck restraint caused a significant increase in IOP (+6.31 ± 5.59 mmHg), while manual rostral head restraint did not. Both chemical restraint protocols significantly reduced IOP (DKM: −1.0 ± 0.76 mmHg; DK: −1.79 ± 1.17) compared with measurements in conscious unrestrained turtles.

CONCLUSIONS

Chemical and manual neck restraint affected IOP. Rostral head restraint had no significant effect on IOP and is, therefore, recommended as the appropriate restraint technique in red-eared slider turtles. TonoLab® measurements estimated actual IOP more accurately, within physiologic range, than measurements obtained using the TonoVet®.