Veterinarians should be aware of specific aspects regarding ophthalmic examination of reptiles because the ophthalmic anatomy and physiology of most reptiles differ from those of mammals. Many reptilian species have small globes and other anatomic idiosyncrasies such as striated iris musculature or the presence of scleral ossicles, which can cause altered rigidity and form stability that may result in pressure-related ophthalmic disorders characterized by a nonlinear relationship with IOP1 and that represent challenges for ophthalmic examination.1,2
Additionally, the lack of reference intervals for ophthalmic variables such as tear production and IOP for most reptiles makes detection of ophthalmic abnormalities in those species difficult.3,4 Consequently, reports of ocular diseases in reptiles are typically limited to case reports of neoplasia,1 blepharitis,5 conjunctivitis,6 lid edema,7 keratoconjunctivitis sicca,3 hypopyon,8 and husbandry-related problems such as retained spectacles or periorbital swelling9 in individual animals.
Measurement of IOP is an essential part of a complete ophthalmic examination for the detection of glaucoma and uveitis.10,11 To our knowledge, glaucoma has not been diagnosed in any reptilian species, possibly because of the lack of established reference intervals for IOP in reptiles3,4 and the difficulty associated with the measurement of IOP in species with small globes.4,12–14 The use of various tonometers for the measurement of IOP has been described in several reptilian species (including tortoises,12 sea turtles,13 and alligators15) but has not, to our knowledge, been described for bearded dragons (Pogona spp). The objective of the study reported here was to evaluate the feasibility of the use of rebound and applanation tonometry for the measurement of IOP in healthy inland bearded dragons (Pogona vitticeps) and to assess the effects of diurnal variation and topical anesthesia on the IOP in that species.
Materials and Methods
Animals
The study was approved by the Lower Saxony State Office for Consumer Protection and Food Safety, the regional office responsible for oversight of animal research. Fifty-six (27 female and 29 male) healthy inland bearded dragons that were housed indoors at animal shelters or education centers or that were privately owned were used for the study. Consent was obtained from the owner or appropriate entity for all animals prior to study enrollment. The age of the bearded dragons ranged between 4 months and 11 years; however, the age for 8 animals was unknown, and in those instances, the subjects were categorized as juveniles (n = 2) or adults (6). In all, 8 juvenile and 48 adult bearded dragons were enrolled in the study. The body weight of the study subjects ranged from 62 to 482 g.
All bearded dragons were enrolled in the study from March through August of the same year. Following enrollment, each bearded dragon was individually housed for 3 days in an enclosure that was equipped with a standardized UVB light and a heat spot lamp to maintain adequate temperatures; hiding spots and fresh water were also provided. The mean temperature inside the enclosure as determined by an environmental thermometera ranged from 26°C in the morning to 35°C in the evening, with higher temperatures achieved at designated basking areas. All husbandry activities and evaluations took place in the same room with a temperature range of 26° to 28°C.
Ophthalmic examination
Only healthy bearded dragons without detectable ophthalmic abnormalities were included in the study. Each animal was determined to be healthy on the basis of results of a physical examination, serum biochemical analysis, and radiographic examination of the body and skull. For each animal, a complete ophthalmic examination that included slit-lamp biomicroscopyb and fluorescein staining was performed. One of 2 investigators (EJS or JS) evaluated the palpebral reflex by touching the skin on the medial edge of the eyelid. One investigator (EJS) assessed the direct pupillary light reflex by use of a beam of bright light. Clinical examination of the posterior ocular segment was not possible because of the small size of the pupil and the apparent resistance of this species to conventional mydriatics.2,16,17
To better assess the corneal dimensions of the study subjects, 1 investigator (EJS) measured the horizontal corneal diameter in the enucleated eyes of 21 bearded dragons that were euthanized for medical reasons and were not enrolled in the study. Further dimensions of the cornea such as the central corneal thickness, curvature, and rigidity could not be assessed because of the lack of access to the equipment necessary to perform those measurements.
Tonometry
For each bearded dragon, the IOP of both eyes was measured by reboundc and applanationd tonometry in accordance with the manufacturers’ guidelines at each of 3 times (morning [between 9 AM and 10 AM], midday [between 1 PM and 2 PM], and evening [between 5 PM and 7 PM]) throughout the day to determine whether diurnal variation was present. The order (right vs left) in which the eyes were evaluated at each time was random. For each tonometric procedure, 3 sequential valid IOP measurements with < 5% variation were obtained for each eye. Valid measurements were indicated by a bar on the rebound tonometer or in the form of an SD < 5% on the applanation tonometer. At each time, IOP was measured by rebound tonometry first followed by applanation tonometry 10 minutes later. In the evening, 1 drop of a topical anesthetic solutione was applied to each eye and an additional rebound tonometric measurement was obtained after 2 to 3 minutes to determine whether topical anesthesia had an effect on the IOP and the tolerance of the study subjects to IOP measurement. All tonometry measurements were performed by the same investigator (EJS).
All rebound tonometry measurements were obtained with the subjects unrestrained in sternal recumbency. The calibration setting for dogs was used on the basis of the manufacturer's recommendationf and the fact that IOP measurements in red-eared sliders did not vary significantly among calibration settings when obtained with the canine, equine, and other species settings.16
Prior to applanation tonometry, 1 drop of topical anesthetic solutione was applied to each eye in accordance with the manufacturer's recommendation. After 2 to 3 minutes, the body and head of each dragon was restrained by an investigator, with care taken to avoid the application of excessive pressure to the subject's neck, and measurement of IOP by applanation tonometry was attempted. Measurement of IOP by applanation tonometry was restricted to a maximum of 3 attempts per time and 2 attempts in general to minimize stress on the dragons. After all tonometric measurements were obtained, fluorescein staining was performed on all eyes to identify corneal microlesions.
Statistical analysis
For each eye and each tonometric method, the mean of the 3 IOP measurements obtained during each time was used for analysis. For each eye, descriptive data (mean, SD, median, range, quartiles, and coefficient of variation) were generated for each time (morning, midday, and evening) and for the entire day as well as after application of topical anesthesia. The Kolmogorov-Smirnov test was used to evaluate the distribution of the IOP measurements for normality. Because the data were not normally distributed, nonparametric methods were used for analysis, and the results were reported as the median, 95% confidence interval, and range. The Wilcoxon test for paired differences was used to compare the mean IOP between the right and left eyes, and because the mean IOP did not differ significantly between the right and left eyes, the data for both eyes were pooled for further analyses. To evaluate the effect of topical anesthesia on the IOP, the measurements obtained during the evening before and after topical anesthesia were compared. Two-sided tests were used for all analyses, and values of P < 0.05 were considered significant. All analyses were performed with commercial statistical software.g
Results
Response to tonometry
The mean ± SD horizontal corneal diameter of the bearded dragons included in the study was 4.14 ± 0.74 mm (range, 3.0 to 5.2 mm). Most of the study subjects responded with extensive blinking and aggressive avoidance behavior during applanation tonometry, and it was impossible to collect sufficient data for statistical analysis with that method. We suspect that the adverse reactions to the applanation tonometer were caused by the proportionately large size of the probe tip in relation to the corneal diameter (Figure 1).
Photograph depicting the proportionately large size of the applanation tonometer probe tip in relation to the cornea of an inland bearded dragon (Pogona vitticeps).
Citation: American Journal of Veterinary Research 76, 6; 10.2460/ajvr.76.6.494
The tip of the rebound tonometer in relation to the corneal diameter was proportionately smaller than that of the applanation tonometer (Figure 2), and all dragons tolerated rebound tonometry well, although a few (n = 10) responded to the tonometer by blinking or turning their heads. The relatively intense bounce of the probe tip on the corneal surface appeared to cause some discomfort as evidenced by a noticeable rebound of the head in the majority (45/56 [80%]) of the animals; however, shortly after this rebound response, the subjects returned their heads to the initial position and did not attempt to escape. The rebound of the head in response to the tonometer was noticeably reduced following application of the topical anesthetic to the eyes. No severe adverse effects associated with the application of the topical anesthetic were observed, although a few (n = 10) of the subjects shook their heads immediately after application of the topical anesthetic solution as if it caused some slight discomfort. The results of the fluorescein staining after completion of all tonometry procedures were negative for all eyes.
Photograph depicting the proportionate size of the rebound tonometer probe tip in relation to the cornea of an inland bearded dragon.
Citation: American Journal of Veterinary Research 76, 6; 10.2460/ajvr.76.6.494
IOP
The IOP did not vary significantly between the right and left eyes, so the data for both eyes were pooled for all analyses and summarized (Table 1). There appeared to be diurnal variation in the IOP. The median IOP in the morning was significantly (P < 0.01) higher than that at midday; however, the median IOP did not differ significantly between the morning and evening or between midday and evening. The IOP was not significantly affected by topical anesthesia (P = 0.799) or sex (P = 0.197).
Summary statistics for the IOP of 56 healthy inland bearded dragons (Pogona vitticeps) for the entire day, at each of 3 times (morning [between 9 AM and 10 AM], midday [between 1 PM and 2 PM], and evening [between 5 PM and 7 PM]) throughout the day, and after the evening application of a topical anesthetic solution to both eyes.
| IOP (mm Hg) | ||
|---|---|---|
| Time | Median (range) | 95% confidence interval |
| Entire day | 6.16 (4.89–7.72) | 5.61–6.44 |
| Morning | 6.50 (5.00–8.17) | 6.17–6.83 |
| Midday | 6.08 (4.50–7.67)* | 5.83–6.33 |
| Evening | 6.17 (3.50–7.83) | 6.00–6.50 |
| After topical anesthesia | 6.33 (4.50–8.17) | 6.00–6.50 |
The values for the entire day represent the median, range, and 95% confidence interval for all measurements obtained during the morning, midday, and evening prior to the application of topical anesthesia.
Value is significantly (P < 0.05) lower than the corresponding value for morning.
All IOP measurements were obtained with the subjects unrestrained in sternal recumbency by use of a commercially available rebound tonometer with the canine calibration setting as recommended by the manufacturer in a room with an ambient temperature that ranged from 26° to 28°C. To determine the effect of topical anesthesia on IOP, 1 drop of a topical anesthetic solution was applied to each eye after the evening measurements were obtained, and rebound tonometry was repeated 2 to 3 minutes later. For each eye at each time (morning, midday, evening, and after topical anesthesia), 3 sequential IOP measurements with < 5% variation were obtained; the mean was calculated for those 3 measurements and used for analysis. The IOP did not differ significantly between the right and left eyes, so the data for both eyes were pooled for analyses.
Discussion
To our knowledge, the present study was the first to investigate the use of tonometry to measure IOP in captive bearded dragons. Although the IOP has been measured in various reptilian species,4,12–16 extrapolation of IOP results from 1 species to another should be avoided because of anatomic variations among species.18,19
Examination of the fundus in reptiles is challenging because the striated muscles of the iris allow voluntary control of the pupil size3 and are unaffected by the mydriatics commonly used in mammals.2,3,16,17 Intracameral injection of mydriatics such as that described for birds20 is not recommended for reptiles because of the risk of damage to intraocular structures. Topical neuromuscular blocking agents such as rocuronium bromide have been successfully used in birds.21 However, to our knowledge, the use of those agents has not been described in reptiles, and they were not used in the present study because of the lack of information about their dose, efficacy, and adverse effects in reptiles. Consequently, mydriasis was not induced in the bearded dragons of the present study because we wanted to avoid the need for anesthesia and its effects on IOP.4 All of the bearded dragons included in the present study were apparently healthy with no evidence of visual impairment and had normal behavior.
Applanation tonometry has been used to successfully measure the IOP of red-footed tortoises,12 loggerhead sea turtles,13 and alligators,15 but it was not tolerated well by the bearded dragons of the present study. Most of the study subjects did not tolerate > 1 corneal contact with the applanation tonometer and responded with extensive blinking or defensive movements. We believe the proportionately large applanation tonometer probe tip in relation to the corneal diameter of the bearded dragons prevented IOP measurements from being obtained without substantial eye distortion or indentation of the globe. A similar issue was observed in a study16 that involved red-eared sliders, which have a corneal diameter similar to that of the bearded dragons of the present study. We believe that the small corneal diameter of the bearded dragons prevented the proper placement of the tonometer tip. Additionally, investigators of other studies22,h have suggested that the steep curvature of corneas with diameters < 9 mm prevents accurate placement of the applanation tonometer tip and causes inaccurate IOP measurements. On the basis of the results of the present study, use of applanation tonometry for the measurement of IOP in bearded dragons is not recommended.
Rebound tonometry allows quick, easy, and hygienic measurement of IOP23 and can be performed without animal restraint, which avoids potential measurement artifacts caused by the stress of handling or the inadvertent application of excessive pressure to the head or neck.4,24 Although most of the bearded dragons of the present study had signs of slight discomfort associated with the intense bounce of the tonometer tip on the cornea, fluorescein staining after tonometry was completed did not reveal any evidence of corneal damage. The tip of a rebound tonometer specifically designed for use in rodents has a slightly smaller diameter (1 to 2 mm), compared with that of the rebound tonometer used in the present study, and may have elicited less reaction from the bearded dragons. Unfortunately, that tonometer was not available for use in the present study; however, it was used successfully in a study4 that involved red-eared sliders, which suggests that it would be appropriate for measurement of IOP in species with small corneas.
In the present study, the mean IOP as determined by rebound tonometry in the evening (between 5 PM and 7 PM) before and 2 to 3 minutes after the application of a topical anesthetic solution to both eyes did not differ significantly, which suggested that IOP was not affected by topical anesthesia. Following topical anesthesia, the rebound response of the heads of the bearded dragons to the application of the tonometer to the cornea was noticeably less than that prior to the topical anesthesia. This finding suggested that topical anesthesia improved the tolerance of the study subjects to rebound tonometry in a manner similar to that observed in red-eared sliders.16 Information regarding the pharmacokinetics and effects of topical anesthetic solutions on the corneal properties of reptiles is lacking. Generally, caution should be used when topical ophthalmic drugs are administered to reptiles at doses recommended for humans or other mammals because of the possibility of overdose or altered drug absorption in the relatively small reptilian eyes.25 For the bearded dragons of the present study, the only adverse effect associated with topical anesthesia was the transient shaking of the head immediately after application of the anesthetic solution in 10 of 56 (18%) subjects. Consequently, it is recommended that only 1 drop of topical anesthetic solution be applied to each eye of bearded dragons prior the use of rebound tonometry for measurement of IOP. It is unknown whether topical anesthesia has any long-term effects on the measurement of IOP because rebound tonometry was performed at only 2 to 3 minutes after application of the anesthetic solution.
Although the results of the present study suggested there was significant diurnal variation in the IOP of bearded dragons, the numeric difference between the median morning (between 9 AM and 10 AM) and midday (between 1 PM and 2 PM) IOPs was small and not considered clinically relevant. Therefore, for practical reasons, we summarized the IOP measurements for the entire day. The overall median IOP for the bearded dragons of the present study (6.16 mm Hg; range, 4.89 to 7.72 mm Hg) was similar to the IOP of red-eared sliders16 (6.1 mm Hg; range, 3 to 9 mm Hg) as determined by rebound tonometry and the IOP of juvenile loggerhead sea turtles13 (5 mm Hg; range, 4 to 9 mm Hg) as determined by applanation tonometry, and markedly less than the IOP of alligators15 (range, 11.6 to 23.7 mm Hg) as determined by applanation tonometry. The differences in the IOP among reptilian species are most likely the result of interspecies variations in corneal properties such as rigidity, curvature, and central thickness.26–28
Diurnal variations in IOP have been described in chickens,29 rhesus monkeys,30 mice,31 and rabbits32; however, little is known about the diurnal pattern of IOP or other ocular variables such as tear production in reptiles. For the bearded dragons of the present study, the IOP was significantly higher in the morning, compared with the IOP at midday. A similar finding was observed in rabbits, which suggests that the IOP of rabbits has a circadian rhythm that is associated with dark-to-light alterations.32 Contrary to rabbits, the activity33 and ecology34 of reptiles is dependent on temperature; therefore, we theorize that diurnal variation in the IOP of bearded dragons is primarily affected by environmental temperature rather than dark-to-light alterations. In the present study, the bearded dragons appeared quite sleepy and were inactive during the morning IOP measurement, and the environmental temperature in their enclosures in the morning was slightly cooler than that during the midday and evening IOP measurements. A positive association between inactivity and IOP in bearded dragons seems physiologically feasible because, in leopard geckos, the IOP during brumation was significantly higher than the IOP during more active periods.i
In the present study, the IOP of bearded dragons was not significantly associated with sex. This finding was similar to the results of other studies that involved loggerhead sea turtles13 and birds of prey.j
The IOP measurements obtained in the present study should be interpreted with caution because the manufacturer of the rebound tonometer used has not calibrated it for use in bearded dragons, and the ocular anatomy of reptiles has certain aspects (eg, scleral ossicles and a thin-walled cornea) that substantially differ from the ocular anatomy of mammals,35 which might have affected the IOP measurements. Therefore, we cannot conclude that the IOP measurements obtained in the present study are externally valid for all other methods of IOP measurement. The results of multiple studies4,11,36 indicate a substantial difference between the IOP measurements obtained by rebound tonometry and those obtained by manometry, which is considered the gold standard. Hence, the results of the present study should only be used as initial guidelines for the IOP of healthy bearded dragons as determined by the rebound tonometer used in the study. The accuracy of measurement of IOP by use of rebound tonometry in bearded dragons should be investigated in conjunction with manometric calibration.
Results of the present study indicated that the measurement of IOP by rebound tonometry was feasible, rapid, and well tolerated by bearded dragons, whereas measurement of IOP by applanation tonometry was not tolerated by this species. Because the bearded dragons appeared to tolerate the rebound tonometry procedure better after the administration of topical anesthetic solution to the eyes and the IOP did not vary significantly before and after application of the topical anesthetic solution, we recommend that topical anesthesia be used to measure IOP in bearded dragons that are uncooperative or prone to stress. Bearded dragons are ectotherms, and diurnal variations or temperature-dependent effects should be considered during the interpretation of IOP measurements in this species. Practitioners should be aware of the possible effects of species-specific anatomic and physiologic ophthalmic alterations on the measurement of IOP, especially when the tonometric device used to obtain that measurement has not been calibrated for the species on which it is being used.
Acknowledgments
Presented in abstract form at the meeting of the German Study Group of Amphibian and Reptile Veterinarians, part of the German Society of Herpetology and Herpetoculture, Düsseldorf, Germany, May 2014.
ABBREVIATION
| IOP | Intraocular pressure |
Footnotes
Thermometer 83–2 (–40° to 150°C), Hugo Sachs Electronik Harvard Apparatus, March-Hugstetten, Germany.
SL-15 Portable Slit Lamp, Kowa, Aichi, Japan.
TonoVet, Icare Finland Oy, Espoo, Finland.
TonoPenVET, Reichert, Germany.
0.5% Proparacain-POS, Ursapharm, Saarbrücken, Germany.
Haensel C, Icare Finland Oy, Espoo, Finland: Personal communication, 2014.
SPSS Statistics, version 21, IBM Corp, Chicago, Ill.
Tandler H. Tonometrie am Vogelauge mittels TonoVet und TonoLab im Vergleich zum TonoPenXL. PhD dissertation, University of Munich, Munich, Germany, 2013.
Di Girolamo N, Andreani V, Guandalini A, et al. Evaluation of rebound tonometry in conscious leopard geckos (abstr), in Proceedings. 20th Annu Conf Assoc Reptilian Amphib Vet 2013;114–115.
Reuter A. Evaluation des Rebound-Tonometers TonoVet für die Bestimmung des Augeninnendrucks bei Greifvögeln und Eulen. PhD dissertation, Free University of Berlin, Berlin, Germany, 2010.
References
1. Darrow BG, Johnstone McLean NS, Russman SE, et al. Periorbital adenocarcinoma in a bearded dragon (Pogona vitticeps). Vet Ophthalmol 2013; 16: 177–182.
2. Williams DL. The reptile eye. In: Williams DL, ed. Ophthalmology of exotic pets. New York: John Wiley and Sons, 2012;159–196.
3. Lawton MPC. Reptilian ophthalmology. In: Mader DR, ed. Reptile medicine and surgery. 2nd ed. St Louis: Elsevier Inc, 2006;323–342.
4. Delgado C, Mans C, McLellan GJ, et al. Evaluation of rebound tonometry in red-eared slider turtles (Trachemys scripta elegans). Vet Ophthalmol 2014; 17: 261–267.
5. Thomas CL, Artwohl JE, Pearl RK, et al. Swollen eyelid associated with Foleyella sp infection in a chameleon. J Am Vet Med Assoc 1996; 209: 972–973.
6. Cooper JE, McClelland MH, Needham JR. An eye infection in laboratory lizards associated with an Aeromonas sp. Lab Anim 1980; 14: 149–151.
7. Elkan E, Zwart P. The ocular disease of young terrapins caused by vitamin A deficiency. Pathol Vet 1967; 4: 201–222.
8. Tomson FN, McDonald SE, Wolf ED. Hypopyon in a tortoise. J Am Vet Med Assoc 1976; 169: 942.
9. Reavill D, Schmidt RE. Pathology of the reptile eye and ocular adnexa, in Proceedings. 19th Annu Conf Assoc Reptilian Amphib Vet 2012;87–97.
10. Kniestedt C, Punjabi O, Lin S, et al. Tonometry through the ages. Surv Ophthalmol 2008; 53: 568–591.
11. Reuter A, Müller K, Arndt G, et al. Accuracy and reproducibility of the TonoVet rebound tonometer in birds of prey. Vet Ophthalmol 2010; 13: 80–85.
12. Selmi AL, Mendes GM, McManus C, et al. Intraocular pressure determination in clinically normal red-footed tortoise (Geochelone carbonaria). J Zoo Wildl Med 2002; 33: 58–61.
13. Chittick B, Harms C. Intraocular pressure of juvenile loggerhead sea turtles (Caretta caretta) held in different positions. Vet Rec 2001; 149: 587–589.
14. Selleri P, Di Girolamo N, Andreani V, et al. Evaluation of intraocular pressure in conscious Hermann's tortoises (Testudo hermanni) by means of rebound tonometry. Am J Vet Res 2012; 73: 1807–1812.
15. Whittaker CJ, Heaton-Jones TG, Kubilis PS, et al. Intraocular pressure variation associated with body length in young American alligators (Alligator mississipiensis). Am J Vet Res 1995; 56: 1380–1383.
16. Labelle AL, Steele KA, Breaux CB, et al. Tonometry and corneal aesthesiometry in the red-eared slider turtle (Trachemys scripta elegans). J Herpetol Med Surg 2012; 22: 30–35.
17. Holmberg BJ. Ophthalmology of exotic pets. In: Maggs DJ, Miller PE, Ofri R, eds. Slatter's fundamentals of veterinary ophthalmology. 4th ed. St Louis: Elsevier Inc, 2008;427–441.
18. Nuhsbaum MT, Gionfriddo JR, Powell CC, et al. Intraocular pressure in normal llamas (Lama glama) and alpacas (Lama pacos). Vet Ophthalmol 2000; 3: 31–34.
19. Knollinger AM, La Croix NC, Barrett PM, et al. Evaluation of a rebound tonometer for measuring intraocular pressure in dogs and horses. J Am Vet Med Assoc 2005; 227: 244–248.
20. Murphy CJ. Raptor ophthalmology. Compend Contin Educ Pract Vet 1987; 9: 241–260.
21. Barsotti G, Briganti A, Spratte JR, et al. Safety and efficacy of bilateral topical application of rocuronium bromide for mydriasis in European kestrels (Falco tinnunculus). J Avian Med Surg 2012; 26: 1–5.
22. Korbel R. Tonometrie beim Vogel mit dem TonoPen XL. Tierarztl Prax 1999; 27: 208–217.
23. TonoVet product manual. Espoo, Finland: Icare Finland Oy, 2013.
24. Pauli AM, Bentley E, Diehl KA, et al. Effects of the application of neck pressure by a collar or harness on intraocular pressure in dogs. J Am Anim Hosp Assoc 2006; 42: 207–211.
25. Millichamp NJ. Management of ocular disease in exotic species. Semin Avian Exotic Pet Med 1997; 6: 152–159.
26. Kotecha A. What biomechanical properties of the cornea are relevant for the clinician? Surv Ophthalmol 2007; 52: S109–S114.
27. Bhan A, Browning AC, Shah S, et al. Effect of corneal thickness on intraocular pressure measurements with the pneumotonometer, Goldmann applanation tonometer, and Tono-Pen. Invest Ophthalmol Vis Sci 2002; 43: 1389–1392.
28. De Moraes CGV, Prata TS, Liebmann J, et al. Modalities of tonometry and their accuracy with respect to corneal thickness and irregularities. J Opt 2008; 1: 43–49.
29. Prashar A, Guggenheim JA, Erichsen JT, et al. Measurement of intraocular pressure (IOP) in chickens using a rebound tonometer: quantitative evaluation of variance due to position inaccuracies. Exp Eye Res 2007; 85: 563–571.
30. Bito LZ, Merritt SQ, DeRousseau CJ. Intraocular pressure of rhesus monkey (Macaca mulatta). I. An initial survey of two free-breeding colonies. Invest Ophthalmol Vis Sci 1979; 18: 785–793.
31. Saeki T, Aihara M, Ohashi M, et al. The efficacy of TonoLab in detecting physiological and pharmacological changes of mouse intraocular pressure—comparison with TonoPen and microneedle manometery. Curr Eye Res 2008; 33: 247–252.
32. Pereira FQ, Bercht BS, Soares MG, et al. Comparison of a rebound and an applanation tonometer for measuring intraocular pressure in normal rabbits. Vet Ophthalmol 2011; 14: 321–326.
33. Huey RB, Stevenson RD. Integrating thermal physiology and ecology of ectotherms: a discussion of approaches. Am Zool 1979; 19: 357–366.
34. Dawson WR. On the physiological significance of the preferred body temperatures of reptiles. In: Gates DM, Schmerl RB, eds. Perspectives of biophysical ecology. New York: Springer, 1975;443–473.
35. Wyneken J. Reptilian eyes and orbital structures, in Proceedings. 19th Annu Conf Assoc Reptilian Amphib Vet 2012;75–82.
36. Löbler M, Rehmer A, Guthoff R, et al. Suitability and calibration of a rebound tonometer to measure IOP in rabbit and pig eyes. Vet Ophthalmol 2011; 14: 66–68.
