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

TonoVet Plus®: Higher reliability and repeatability compared with Tono-Pen XL™ and TonoVet® in rabbits

OBJECTIVE

To evaluate the accuracy and repeatability of the Tono-Pen XL™, TonoVet® and TonoVet Plus® tonometers by manometric evaluation, and to establish adjustment equations for intraocular pressure (IOP) estimates in rabbits.

ANIMAL STUDIED

Rabbits.

PROCEDURES

A postmortem study was conducted on seven rabbit eyes to verify the correlation between manometry and tonometry with an artificial incremental increase in IOP from 5 and 60 mmHg. A clinical study was conducted to evaluate accuracy and to establish reference values for the species, with measurement of IOP in 17 animals, for 2 consecutive days, with the same tonometers and calibrations used in the postmortem evaluations.

RESULTS

There were strong linear trends for all evaluated tonometers. In the in-vivo evaluation, the mean IOP values were: 14.23 ± 1.75 (Tono-Pen XL™); 13.89 ± 2.07 (TonoVet® calibration mode 'd'); 8.88 ± 1.24 (TonoVet calibration mode 'p'); 18.59 ± 1.94 (Tonovet Plus®). There was a significant difference in the two evaluation times for the two TonoVet® calibration modes. The adjustment equations generated from the manometry for the evaluated tonometers were: Y = 0.2570X + 2.219 (Tono-Pen XL™), Y = 0.2289X + 2.389 (TonoVet® 'd'), Y = 0.4043X + 4.062 (TonoVet® 'p'), Y = 0.1233X + 0.3644 (TonoVet Plus®) (X is device-estimated IOP).

CONCLUSIONS

All evaluated tonometers were well correlated with the manometry, with an underestimation of IOP by all devices. Applying adjustment formulas may compensate for systematic errors. TonoVet Plus® was well tolerated, and showed better repeatability and reliability in successive evaluations.

Repeatability, reproducibility, and agreement of three tonometers for measuring intraocular pressure in rabbits

The aim of this study was to evaluate repeatability, reproducibility, and agreement of three commonly used tonometers in animal research (TonoLab, TonoVet, and TonoPEN AVIA) in a cohort of 24 rabbits. Additionally, the impact of sedation on IOP was investigated in 21 New Zealand White rabbits with the TonoVet tonometer. Repeatability was determined using the coefficient of variation (CoV) for two observers. For the TonoLab (6.55%) and TonoVet (6.38%) the CoV was lower than for the TonoPEN AVIA (10.88%). The reproducibility was highest for the TonoVet (0.2 ± 3.3 mmHg), followed by the TonoLab (0 ± 12.89 mmHg) and lowest for the TonoPEN AVIA (− 1.48 ± 10.3 mmHg). The TonoLab and TonoVet showed the highest agreement (r = 0.85, R² = 0.73). After sedation, a significant IOP reduction (often > 25%) was observed. Our results show that among the three tonometers tested, the TonoVet tonometer is best for use in rabbits while the TonoLab should be avoided. The impact of sedation on IOP was substantial and should be taken into account during experimentation.

Evaluation of a dexmedetomidine-midazolam-ketamine combination administered intramuscularly in captive ornate box turtles (Terrapene ornata ornata)

OBJECTIVE

To characterize the effects of a combination protocol of dexmedetomidine-midazolam-ketamine (DMK) administered intramuscularly (IM) in ornate box turtles (Terrapene ornata ornata).

STUDY DESIGN

Prospective experimental trial.

ANIMALS

A total of 16 apparently clinically healthy adult ornate box turtles (eight male, eight female).

METHODS

Each turtle was treated with dexmedetomidine (0.1 mg kg^-1), midazolam (1 mg kg^-1) and ketamine (10 mg kg^-1) administered IM. Time to first response, time to maximal effect, the plateau phase and time to recovery from reversal administration were recorded. Physiologic variables, muscle tone, reflexes and the ability to perform endotracheal intubation were recorded at 5 minute intervals. Movement in response to an IM injection of 0.1 mL sterile 0.9% NaCl administered in the left pelvic limb, using a 25 gauge needle to a depth of just past the bevel of the needle, was assessed every 15 minutes. Atipamezole (0.5 mg kg^-1) IM and flumazenil (0.05 mg kg^-1) SC were administered 60 minutes after the initial DMK injections.

RESULTS

The mean time to first response, time to maximal effect, the plateau phase and time to recovery were 2.1, 14.9, 38.7 and 7.8 minutes, respectively. A respiratory rate was not observed in most turtles. The body temperature significantly increased over time. The palpebral reflex was persistent in 43% of turtles and the tail pinch reflex remained intact in 13% of turtles. All turtles recovered with no observed adverse effects.

CONCLUSIONS AND CLINICAL RELEVANCE

In this study, this DMK protocol administered to ornate box turtles resulted in a rapid-onset, light anesthesia lasting approximately 40 minutes and a smooth recovery with no adverse effects noted.

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.

Intraocular Pressure Measurement by Rebound Tonometry (TonoVet) in Normal Pigeons (Columba livia)

We evaluated the applicability of a rebound tonometer (TonoVet) in pigeon eyes and established normal reference intraocular pressure (IOP) values in healthy pigeons; 20 eyes of euthanized pigeons were used for calibration of the TonoVet and 48 eyes of 24 adult pigeons were used for measurement of reference IOP. First, IOP of pigeon eyes ex vivo were measured using the 'd' and the 'p' modes of the TonoVet and compared to manometric IOP values from 5 to 80 mm Hg. Then, to establish normal reference values, IOP was measured from clinically normal pigeons in vivo. The 'd' and the 'p' modes of the TonoVet showed a strong linear correlation with the manometric IOP (R² = .996 and .991, respectively). The obtained regression formulas were: y₁ = 0.439x + 2.059 and y₂ = 0.330x - 0.673, respectively (y₁, 'd' mode of TonoVet; y₂, 'p' mode of TonoVet; x, manometric IOP). The 'd' and the 'p' modes consistently measured one-half and one-third of the actual IOP, respectively. Therefore, the formula obtained through the 'd' mode was applied to obtain reference values. The calibrated IOP of normal pigeon eyes was 19.5 ± 4.4 mm Hg. The actual IOP could be calculated using the presented formula. Considering the limitations of the 'p' mode, use of the 'd' mode is more appropriate. Therefore, the TonoVet rebound tonometry under the 'd' mode is a reliable method for measuring IOP in pigeons.

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.

Calibration of the TonoVet and Tono-Pen Vet tonometers in the porcine eye

OBJECTIVE

The pig has an increasingly important role in ocular drug delivery models, but the most accurate tonometer in this species is unknown. The purpose of this study was to evaluate the accuracy of TonoVet and Tono-Pen Vet tonometers in the ex vivo porcine eye.

PROCEDURE

Four freshly enucleated normal porcine eyes were cannulated with two 25-gauge needles; one connected via tubing to a mercury manometer calibrated continuous physiologic recorder and the other connected to a reservoir of lactated Ringer's solution on an adjustable stand. Triplicate IOP readings were taken with the TonoVet and then the Tono-Pen Vet at 5, 10, 15, 20, 25, 30, 35, 40, 50, 60, 70, 80 mmHg.

RESULTS

Linear regression showed strong linear trends for both the TonoVet (r² = 0.969) and Tono-Pen Vet (r² = 0.983). The TonoVet slightly underestimated IOP at lower pressures and slightly overestimated IOP at higher pressures (y = 1.092x - 4.0, where y = tonometer reading, x = manometer reading, and 4.0 = intercept). The Tono-Pen Vet consistently underestimated IOP (y = 0.773x - 2.1). These differences were statistically significant (P = <0.001, one-way repeated-measures ANOVA).

CONCLUSION

As in other species, both the TonoVet and Tono-Pen Vet tonometers do not measure true IOP in the porcine eye; however, the TonoVet more closely approximated true IOP in the normal porcine eye than the Tono-Pen Vet and may be the tonometer of choice for 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.