• View in gallery

    Photograph showing B-mode ocular ultrasonography being performed for 1 of 17 rescue facility–housed tigers (Panthera tigris) in a study to calculate the pseudophakic intraocular lens power needed to approximate emmetropia in adult tigers. The image was obtained after induction of anesthesia and immediately prior to intubation.

  • View in gallery

    Illustration depicting ocular globe dimensions as obtained by use of ocular ultrasonography. Reprinted from McMullen RJ and Gilger BC.6 Reproduced with permission of John Wiley and Sons. Copyright 2006 American College of Veterinary Ophthalmologists. ACD = Anterior chamber depth. CLT = Crystalline lens thickness.

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    Illustration depicting keratometry measurements. Reprinted from McMullen RJ and Gilger BC.6 Reproduced with permission of John Wiley and Sons. Copyright 2006 American College ofVeterinary Ophthalmologists.

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Keratometry, biometry, and prediction of intraocular lens power in adult tigers (Panthera tigris)

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  • 1 Blue Pearl Veterinary Partners, Tampa, FL
  • | 2 Mars Veterinary Health, Vancouver, WA
  • | 3 Big Cat Rescue, Tampa, FL
  • | 4 Department of Veterinary Ophthalmology, JT Vaughan Large Animal Teaching Hospital, College of Veterinary Medicine, Auburn University, Auburn, AL

Abstract

OBJECTIVE

To calculate the necessary pseudophakic intraocular lens (IOL) power to approximate emmetropia in adult tigers.

ANIMALS

17 clinically normal adult tigers.

PROCEDURES

33 eyes of 17 clinically normal adult tigers underwent routine ophthalmic examination and B-scan ultrasonography while anesthetized for unrelated procedures. Specific ultrasound data (globe measurements and corneal curvature) and estimated postoperative IOL positions were utilized to calculate predicted IOL power by use of Retzlaff and Binkhorst theoretical formulas. Applanation tonometry and refraction were also performed.

RESULTS

Mean ± SD axial globe length was 29.36 ± 0.82 mm, preoperative anterior chamber depth was 7.00 ± 0.74 mm, and crystalline lens thickness was 8.72 ± 0.56 mm. Mean net refractive error (n = 33 eyes) was +0.27 ± 0.30 diopters (D). By use of the Retzlaff formula, mean predicted IOL power for the postoperative anterior chamber depth (PACD), PACD – 2 mm, and PACD + 2 mm was 43.72 ± 4.84 D, 37.62 ± 4.19 D, and 51.57 ± 5.72 D, respectively. By use of the Binkhorst equation, these values were 45.11 ± 4.91 D, 38.84 ± 4.25 D, and 53.18 ± 5.81 D, respectively. Mean intraocular pressure for all eyes was 14.7 ± 2.69 mm Hg.

CLINICAL RELEVANCE

The calculated tiger IOL was lower than reported values for adult domestic felids. Further studies evaluating actual PACD and pseudophakic refraction would help determine the appropriate IOL power to achieve emmetropia in this species.

Abstract

OBJECTIVE

To calculate the necessary pseudophakic intraocular lens (IOL) power to approximate emmetropia in adult tigers.

ANIMALS

17 clinically normal adult tigers.

PROCEDURES

33 eyes of 17 clinically normal adult tigers underwent routine ophthalmic examination and B-scan ultrasonography while anesthetized for unrelated procedures. Specific ultrasound data (globe measurements and corneal curvature) and estimated postoperative IOL positions were utilized to calculate predicted IOL power by use of Retzlaff and Binkhorst theoretical formulas. Applanation tonometry and refraction were also performed.

RESULTS

Mean ± SD axial globe length was 29.36 ± 0.82 mm, preoperative anterior chamber depth was 7.00 ± 0.74 mm, and crystalline lens thickness was 8.72 ± 0.56 mm. Mean net refractive error (n = 33 eyes) was +0.27 ± 0.30 diopters (D). By use of the Retzlaff formula, mean predicted IOL power for the postoperative anterior chamber depth (PACD), PACD – 2 mm, and PACD + 2 mm was 43.72 ± 4.84 D, 37.62 ± 4.19 D, and 51.57 ± 5.72 D, respectively. By use of the Binkhorst equation, these values were 45.11 ± 4.91 D, 38.84 ± 4.25 D, and 53.18 ± 5.81 D, respectively. Mean intraocular pressure for all eyes was 14.7 ± 2.69 mm Hg.

CLINICAL RELEVANCE

The calculated tiger IOL was lower than reported values for adult domestic felids. Further studies evaluating actual PACD and pseudophakic refraction would help determine the appropriate IOL power to achieve emmetropia in this species.

Introduction

The lens, despite its small size, plays a crucial role in refraction of light onto the retina to achieve vision. Any change to the specific biochemical composition of the lens can cause a change in clarity and thus a change in an animal’s vision. Cataract formation, or the increased opacification of the lens, is 1 such abnormality that can cause progressive vision loss. Cataracts can be classified by etiology as inherited (primary) and acquired (secondary). Causes of acquired cataracts include nutritional deficiencies, certain toxins, trauma, inflammation, endocrine disorders (eg, diabetes mellitus), and senility.1 Cataracts are a well-documented cause for vision loss in both domestic and nondomestic species, and tigers are not an exception to this rule.1,2,3,4,5,6,7,8,9,10 Milk replacer formulas, as opposed to natural milk, have been implicated as possible causes of cataracts, dietary intolerance, and reduced weight gain due to differences in protein, fatty acid, and carbohydrate composition in both domestic and nondomestic felids.4,11,12 More specifically, deficiencies in taurine, arginine, phenylalanine, tryptophan, and histidine are possible causes of the development of skin problems, cataracts, and strabismus in tiger cubs fed particular artificial milk replacers.4 In addition, the tiger is listed as endangered on the International Union for the Conservation of Nature Red List of Threatened Species,13 and there are now more tigers estimated to be in captivity than in the wild.

The gold standard of treatment for cataract-induced vision impairment is surgical removal of the affected lens contents. Phacoemulsification, or the use of ultrasonic power to break up lens contents that are then aspirated from the eye, is 1 type of surgical technique. This operation has been documented in tigers.1 However, removal of the lens without placement of an artificial lens (aphakia) leaves the eye severely hyperopic (farsighted) due to the important role of the lens in optical refraction.2,14 As such, veterinary ophthalmologists have attempted to approximate emmetropia in several species postphacoemulsification by implanting an intraocular lens (IOL).5,8,10,15,16,17 Additional research has been performed to determine correct IOL refractive power prior to surgical implantation by use of mathematical formulas, of which there are many that have been created and reviewed in human ophthalmology.18,19,20 Formulas can be separated loosely into theoretical formulas and regression formulas. Theoretical formulas utilize variables including refractive indices of aqueous and vitreous humors, axial globe length (AGL), corneal curvature, and estimated postoperative (or pseudophakic) anterior chamber depth (PACD).21,22 Regression formulas are more recent and were created by use of regression analysis and constants calculated by use of large population studies and postoperative evaluations. As PACD cannot be measured prior to surgery, this variable must be estimated or extrapolation must be made from previous postoperative measurements of ACD in the respective species.5,14,17

Previous studies have described the theoretical calculation of diopter (D) power to achieve emmetropia in canine,23 feline,5,24 equine,6,25 avian (eagle7 and owl8), and lagomorph (rabbits9) species utilizing various theoretical formulas. To our knowledge, there is only 1 abstract describing IOL implantation by use of a +30 D foldable acrylic lens in a Bengal tiger (Panthera tigris), with no description of IOL refractive power calculation prior to implantation.1 In addition, we are unaware of any previously published studies that describe the mean ocular biometric data of an ophthalmologically healthy tiger eye required for calculations of IOL power. The AGL in P tigris has previously been reported to be 30 mm, as measured from prepared slides of histologic cross sections of eyes.26 Although tonometry and refractive error are not required for theoretical calculation, both are important in complete ophthalmic examination and monitoring preoperatively and postoperatively in phacoemulsification patients.

The objectives of this study were to determine the mean intraocular pressure (IOP) obtained via applanation tonometry, mean phakic refraction, and mean ocular biometric data of the tiger eye (AGL, preoperative ACD, crystalline lens thickness [CLT], and corneal curvature) to calculate an estimated pseudophakic IOL power to achieve emmetropia in the tiger. Our hypothesis was that the IOL power required to achieve emmetropia in the tiger eye would be similar to that of domestic cats.

Materials and Methods

The study protocol was reviewed and approved by the Blue Pearl Science Internal Review Board and was performed in compliance with guidelines of the US Animal Welfare Act.

Study sample

Adult tigers (P tigris) housed at a nondomestic felid rescue facility (accredited by the Global Federation of Animal Sanctuaries) were evaluated in this study. These tigers were all rescued at variable points in their lifetime; their complete histories were unknown. The exact subspecies of each tiger was also unknown and could have been mixed. These tigers were placed under general anesthesia for procedures unrelated to the eye (eg, routine physical examination, blood sample collection for various CBC and biochemical analyses, radiographic and ultrasonographic examination, dental examination and procedures, etc).

Sedation was achieved with a combination of medetomidine (0.02 mg/kg, IM), midazolam (0.2 mg/kg, IM), and ketamine (2 mg/kg, IM) administered via dart gun in the tigers’ habitats. If additional premedication was needed for induction, handling, or movement, an additional dose of ketamine (1 mg/kg, IM) was given. Patients were then transported to the rescue facility hospital, intubated, and maintained on isoflurane inhaled anesthetic. Monitoring during procedures included electrocardiography, pulse oximetry, and indirect (Doppler) blood pressure. An IV catheter was placed and fluids (Normosol-R) administered at roughly 5 to 10 mL/kg/h (due to weight of patients, fluid was given at the maximum rate of the IV equipment used).

When under general anesthesia for any reason, all nondomestic felids at the rescue had a complete ocular examination by a veterinary ophthalmologist routinely performed as part of the physical examination. This included staining with 1.0-mg fluorescein sodium ophthalmic strips (Ful-Glo; Akorn Operating Co LLC), slit-lamp biomicroscopy (SL-15; Kowa Co Ltd), applanation tonometry (TonoPen XL; Mentor), direct ophthalmoscopy with a 3.5-V coaxial ophthalmoscope (Welch Allyn), and indirect ophthalmoscopy (Vantage Plus Convertible Slimline Wireless; Keeler) with a 28-D lens (Volk Optical Inc). The IOPs were measured following topical administration of 0.5% proparacaine hydrochloride ophthalmic solution (Alcain) with the tigers in sternal or lateral recumbency. Positioning of the tigers varied and was dependent on the other procedures for which the tiger was under anesthesia. For this study, 5 successive readings were obtained for each eye and only those with ≤ 5% variance were documented. Results obtained were combined, and a mean ± SD value was calculated for each eye. One author performed all measurements (TMM), and the applanation tonometer used was within current calibration specifications of the manufacturer.

For this study, refractive error of each eye of each tiger was evaluated by use of streak retinoscopy. Streak retinoscopy was performed by use of a streak retinoscope (Heine Optotechnik) and a set of retinoscopy bars with 16-mm-diameter lenses with a range of ± 0.5 D to ± 15 D (Luneau Technology Operations) at working distance of 67 cm by 1 author (TMM). For each eye, retinoscopy was repeated 5 times in both the horizontal and vertical meridians and results were recorded individually. Results obtained were combined, and a mean value ± SD was calculated to determine the net refractive error of each eye. These results were then averaged for all left eyes, right eyes, and net refraction of all eyes in this study.

Tigers were excluded from this study if ophthalmic abnormalities were detected on examination (eg, cataract, glaucoma, lens instability, or hypertensive retinopathy). Adnexal diseases or other systemic diseases did not lead to exclusion from this study.

Ocular dimensions

For this study, a B-mode portable ultrasound machine (LOGIQ-E; GE Healthcare) with an 8-MHz probe was utilized to measure ocular dimensions (Figure 1). Sterile coupling gel was applied to the probe, and the tip was touched to the central corneal surface, ensuring optimal contact and minimal pressure to the globe as to not distort corneal curvature and dimensions. The same author (TMM) performed all measurements. Preoperative ACD, CLT, and AGL were then calculated from the captured ultrasound images by use of the ultrasound program’s internal calipers (Figure 2). Each measurement was performed once per image on 6 captured images and recorded individually. Results were then averaged per eye, and total ocular dimensions were then averaged for the entire sample. Vitreous chamber depth (VCD) was calculated from the subtraction of preoperative ACD and CLT from AGL.

Figure 1
Figure 1

Photograph showing B-mode ocular ultrasonography being performed for 1 of 17 rescue facility–housed tigers (Panthera tigris) in a study to calculate the pseudophakic intraocular lens power needed to approximate emmetropia in adult tigers. The image was obtained after induction of anesthesia and immediately prior to intubation.

Citation: American Journal of Veterinary Research 83, 2; 10.2460/ajvr.21.04.0060

Figure 2
Figure 2

Illustration depicting ocular globe dimensions as obtained by use of ocular ultrasonography. Reprinted from McMullen RJ and Gilger BC.6 Reproduced with permission of John Wiley and Sons. Copyright 2006 American College of Veterinary Ophthalmologists. ACD = Anterior chamber depth. CLT = Crystalline lens thickness.

Citation: American Journal of Veterinary Research 83, 2; 10.2460/ajvr.21.04.0060

Corneal curvature/keratometry

Calculation of corneal curvature was performed as previously published with minor modifications.6 The digital ultrasound images were uploaded to an imaging software program (Keystone version 1.8.220; Asteris). Corneal curvature was calculated by determining the radius of the cornea by 1 observer and veterinary ophthalmologist (TMM). Six measurements were taken from 6 separate images of each plane (horizontal and vertical), and the averages for each were calculated. Corneal curvature was measured by digitally plotting 3 independently placed points along the surface of the corneal epithelium as indicated by the hyperechoic corneal ultrasound reflection (Figure 3). Straight lines were then drawn to connect these 3 individual points. Perpendicular lines were drawn bisecting the 2 lines by use of the imaging software to ensure a perfect 90° angle. The point of their intersection represented the corneal radius. Five measurements were obtained for both horizontal corneal curvature and vertical corneal curvature (corneal curvature in D), and the mean corneal radius was calculated.6 Mean corneal curvature for each patient was used in the Binkhorst21 and Retzlaff22 theoretical formulas. Mean ± SD corneal curvature for all eyes was calculated and recorded.

Figure 3
Figure 3

Illustration depicting keratometry measurements. Reprinted from McMullen RJ and Gilger BC.6 Reproduced with permission of John Wiley and Sons. Copyright 2006 American College ofVeterinary Ophthalmologists.

Citation: American Journal of Veterinary Research 83, 2; 10.2460/ajvr.21.04.0060

Estimated PACD

The PACD was calculated as the distance between the corneal epithelium and the center of the lens (equal to the preoperative ACD plus 50% of the anterior to posterior CLT) by use of the B-mode ultrasound measurements. Since the actual postoperative position of an intracapsular IOL could not be predicted, we also calculated the PACD at 2 mm anterior (ie, PACD – 2 mm) and 2 mm posterior (ie, PACD ± 2 mm) to this central position to determine the effect of lens position on the refractive power of an IOL to achieve emmetropia, as has been previously described.6

Prediction of IOL power

The mean AGL, mean corneal curvature, and mean estimated PACD were used to calculate the IOL power necessary to achieve emmetropia by use of Retzlaff21 and Binkhorst22 theoretical formulas. The Retzlaff21 formula was as follows: P = (N/[L – C]) – ([N X K]/N – [K X C]), where P = emmetropic IOL power (in D), n = refractive index of aqueous and vitreous humors (1.336), L = AGL (m), K = corneal curvature (D), and C = PACD (m). The Binkhorst22 formula was as follows: P = (1.366 X [4r – L]))/([L – C] X [4r – C]), where P = emmetropic IOL power (D), r = corneal radius (mm), L = axial length (mm), C = PACD (mm), and 1.366 = a constant for refractive index of aqueous humor. Additional IOL calculations using PACD – 2 mm and +2 mm were made to evaluate the effect of IOL position on refraction.

Data analysis

Descriptive statistics of measurements are reported. Normality was determined by use of the D’Agostino-Pearson method. Results for the 2 IOL formulas in calculated IOL power and certain tiger subgroups were compared by use of the paired t test. Comparisons between male and female tigers were made by use of the Mann-Whitney U test. Relationships between age, weight, average AGL, average preoperative ACD, and average CLT, IOP, and refraction were made by use of linear regression analysis. Because both eyes from a tiger are correlated and not considered independent, the results were combined from each eye. Means were considered significantly different at P < 0.05.

Results

Seventeen tigers (9 males and 8 females) and a total of 33 eyes were included in this study. One tiger had an eye enucleated due to corneal perforation with concurrent cataract, anterior lens luxation, and glaucoma; however, the remaining eye was considered ophthalmologically normal at the time of examination. The results that follow exclude the previously enucleated eye.

Tigers ranged in age from 11 to 21 years, with a mean ± SD age of 16.7 ± 3.49 years. Weight ranged from 208 to 438 kg, with a mean weight of 319.1 ± 64.4 kg. All tigers were sexually intact. When variables between sexes were compared, the only significant correlation was between sex and weight (mean female weight, 263 ± 41 kg; mean male weight, 362 ± 47 kg).

Tonometry

The IOP results ranged from 5 to 22 mm Hg. Mean IOP for all 33 eyes was 14.7 ± 2.69 mm Hg. There was no significant (P = 0.06) difference between the left and right eyes. There was no statistical correlation between tonometry results and age, sex, or weight. Difference in IOP between tigers in lateral or sternal recumbency was not evaluated.

Streak retinoscopy

Net spherical refraction results when all eyes were averaged (n = 33 eyes) were +0.27 ± 0.30 D for the right eyes and +0.26 ± 0.31 D for the left eyes. Individual variations were noted to be between 0 and +1 D. Net refractive error (n = 33 eyes) was +0.27 D ± 0.30. There was neither a significant (vertical, P > 0.99; horizontal, P = 0.38) difference between the left and right eyes nor a significant correlation between spherical refraction and age, sex, or weight.

Ultrasound biometry

Overall mean biometry results of the 17 tigers were as follows: AGL, 29.36 ± 0.82 mm; preoperative ACD, 7.00 ± 0.74 mm; and CLT, 8.72 ± 0.56 mm. Mean VCD was 13.7 mm. Weight was not associated with axial globe length based on regressive analysis (P = 0.77). There was no significant correlation between any ocular dimensions (AGL, preoperative ACD, or CLT) and age, weight, or sex.

Keratometry

Mean horizontal corneal curvature was 24.04 D, and mean vertical corneal curvature was 24.50 D (overall mean, 24.27 ± 2.25 D). There was no significant correlation between corneal curvature and age, weight, or sex.

IOL calculations

By use of the Retzlaff21 formula, the overall mean IOL power was 43.72 ± 4.84 D. By use of the Binkhorst22 formula, the overall mean IOL power was 45.11 ± 4.91 D. Mean dioptric power was then recalculated6 by use of PACD – 2 mm and PACD + 2 mm for each theoretical formula to determine changes to the dioptric power if the IOL placed was not in the exact center of the lens capsule. If the IOL was placed 2 mm more anteriorly (PACD – 2 mm), the mean dioptric power was 37.62 ± 4.19 D and 38.84 ± 4.25 D by use of Retzlaff and Binkhorst formulas, respectively. If the IOL was placed 2 mm posteriorly (PACD + 2 mm), the mean dioptric power was 51.57 ± 5.72 D and 53.18 ± 5.81 D by use of Retzlaff and Binkhorst formulas, respectively. There was a significant difference in the mean dioptric power produced by the 2 formulas (P < 0.001) as well as for the results for dioptric power when 2 mm was added and subtracted from the estimated PACD. The Retzlaff formula produced a consistently lower IOL power estimate than did the Binkhorst formula. There were no significant correlations between IOL calculations and age, weight, or sex.

Discussion

The mean AGL (29.36 ± 0.82 mm) was measured by use of B-mode ultrasound in this study. This AGL was similar to a previous study26 that showed a mean AGL of 30 mm by use of histopathologic sections of tiger eyes. Tigers had an apparently greater AGL than domestic cats (20.98 mm)5 and dogs (20.43 mm)23 but a shorter AGL than horses (39.23 mm).6 Mean ACD was greater in tigers than in cats (4.97 mm),5 dogs (4.95 mm),23 and horses (5.63 mm).6 Mean CLT was thicker in tigers than in cats (7.89 mm)5 and dogs (7.14 mm)23 but was thinner than in horses (11.75 mm).6 Removing values (ACD and CLT) for the anterior chamber from the AGL allowed us to make a general statement about the VCD in these species. Cats and dogs have a similar VCD (8.12 and 8.34 mm respectively),5,23 but the other 2 species are considerably different. Horses6 have a VCD of approximately 21.85 mm, while tigers in the present study had a mean VCD of 13.7 mm.

Mean corneal curvature for the tigers of the present study was 24.27 D. This was less than in domestic cats (38.93 D)5 and dogs (39.94 D)6 but greater than in horses (16.46 D).23 A lower D value indicates a less steep (ie, flatter) corneal curvature, requiring light rays to travel a longer distance through the globe before they are focused on the retina. Thus, corneal curvature has an important and direct effect on the strength of an IOL necessary to refract light onto the retina.

It was unknown whether there was any variation in biometric measurements, keratometric measurements, or refractive error due to age, weight, subspecies, or habitat as was previously noted in other species.23,27,28,29,30,31,32 Our sample consisted of 17 tigers (9 males and 8 females) that were all of a mature age. Based on our results, there was no significant difference in ocular dimensions (AGL, preoperative ACD, and CLT), keratometry results, or refractive error based on weight, age, and sex.

Mean estimated dioptric power to approximate emmetropia in the tigers of the present study was estimated to be 43.72 ± 4.84 D (Retzlaff21) and 45.11 ± 4.91 D (Binkhorst22), based on estimated PACD without the described 2-mm adjustments for the potential variations in IOL position, and these values were higher than a previously noted IOL power (+30 D) utilized in a tiger case report.1 Calculations in the present study suggested that tigers may require a higher IOL power than has been reported for other species, including dogs23 (41.00 D) and horses6 (29.56 D), but a lower IOL power requirement than reported for domestic cats5 (52.80 D) for achieving emmetropia. Thus, our hypothesis that theoretical IOL power for a tiger would be similar to domestic felids was not supported. This hypothesis was based on the expectation that the overall ratios of the variables (AGL, corneal curvature, and PACD) included in the IOL mathematical equations would be comparable between tigers and domestic felids; however, this was not the case. Tigers and cats do have similar lens thicknesses (tigers having a mean CLT of 8.72 mm and domestic felids5 having a mean CLT of 7.89 mm), but the other variables, specifically the differences between PACD and AGL and corneal curvatures, were considerably different. Anterior chamber depth differed between domestic cats and tigers by only roughly 2 mm, but, relative to the AGL (a difference of 10 mm), the tiger’s lens rests more anteriorly than in domestic cats. When compared with horses,6 tigers have a crystalline lens that is positioned further posteriorly in relation to AGL, resulting in a deeper ACD. There is an inverse relationship between PACD and the IOL strength required to refract incoming light rays on the retina. Many variables are responsible for the refraction of light onto the retina, but corneal curvature (in D) is a general indication of an animal’s AGL. The flatter the corneal curvature, the farther light must travel before it reaches the retina.33 The ACD and CLT, and subsequently the PACD, all play an important role in fine-tuning the refraction once light passes through the cornea.33

Retzlaff and Binkhorst theoretical formulas were utilized in the present study to determine an estimated IOL power to achieve emmetropia after phacoemulsification. Both formulas use preoperative measurements of corneal curvature and AGL, mathematical constants for refractive indices within the eye, and PACD. These formulas were chosen for their simplicity and historical use in veterinary medicine, which allowed comparisons. As with any study requiring measurements, there was concern about intraobserver reproducibility of ocular dimensions. As noted in previous studies,25,34,35 ultrasound biomicroscopy was considered to have acceptable intraobserver and interobserver repeatability and reproducibility of ocular biometric measurements, at least for larger measurements such as AGL and ACD. In addition, these formulas were produced for evaluation of human eyes. It is yet to be determined whether these formulas are appropriate for estimation of IOL power in tigers. When the 2 formulas were compared with each other, there was a significant difference in IOL dioptric power noted between them. The Retzlaff formula consistently produced an IOL power lower than did the Binkhorst formula. This was also noted in studies of horses.6 This difference is considered to be related to the use of different constants in each study (Retzlaff21 uses 1.336, while Binkhorst22 uses 1.366). At the time our study was conducted, it was unknown whether the Retzlaff or Binkhorst equation was considered more accurate; additional research would be necessary to test both IOL dioptric powers and compare results.25

Based on previous similarly designed studies in other species, there was concern that overcorrection resulting in myopia can occur with these formulas.10,17,24 There can be considerable differences between in vitro and in vivo studies for IOL power, most notably due to differences in PACD. Estimated PACD utilizing preoperative measurements is not considered ideal; however, no tigers that had undergone phacoemulsification with IOL implantation were available for measurements in the present study. In addition, the implanted IOL often rests more anteriorly rather than aligning with the equator within the lens capsule as noted in other species with PACD calculations after surgery.17 For comparison of the effects of final IOL resting placement, 2 mm was added and subtracted to the estimated PACD.6 As has been previously noted in other studies, even minute changes in the location of the IOL implant and PACD can cause considerable change in the required refractive power of the IOL.6,10,17 The estimated power by use of the Retzlaff formula changed by approximately –6 D for an IOL placed 2 mm more anteriorly and +8 D for an IOL placed 2 mm more posteriorly. The Binkhorst formula yielded somewhat similar results: –6.2 D for an IOL placed 2 mm more anteriorly and +3.5 D for an IOL placed 2 mm more posteriorly. These changes were to be expected; if all other ocular dimensions and refractive surfaces or indices were kept the same, moving the lens more anteriorly (ie, farther away from the retina) would lead to less refractive power necessary and vice versa for a lens placed closer to the retina to achieve a focal point resting on the retina itself. To our knowledge, there have been no published calculations of PACD or postoperative refractive power performed in tigers. Serial postoperative measurements would be necessary to evaluate this conclusion. Additionally, a previous study17 in horses yielded a constant to help minimize overcorrection by comparing a mean preoperative ACD-to-PACD ratio to predict a more accurate PACD. This same calculation could be performed in other species including tigers to better estimate a more accurate IOL power.

In the tigers of the present study, there was little overall variation in spherical refraction because refractive error only fluctuated between 0 and +1 D for all eyes. This indicated that, at least in our sample of phakic tigers, spherical refraction was close to emmetropic or slightly hyperopic. There was no significant relationship between spherical refraction and age, sex, or weight.

All tonometry results were in patients that had been premedicated (medetomidine, xylazine, and ketamine), placed under general anesthesia (isoflurane), and placed either in sternal or lateral recumbency. Intraocular pressure results can be directly affected by certain medications used for sedation and anesthesia.36,37,38,39,40,41,42,43,44 Dissociative anesthetics such as ketamine have been consistently shown to cause a spurious increase in IOP in other species.3638 Previous research in adult lions did not show a significant difference in IOP measurements among anesthetic protocols, but both anesthetic protocols in that study included ketamine.39 α-2 Receptor agonists such as medetomidine and dexmedetomidine have been shown to cause decreases in IOP or have no effect when given IV depending on when IOP is measured in other species.40,41,42 Isoflurane anesthesia has been shown to decrease IOP in other species, but this testing has not been performed in tigers.43,44 Although tonometry results without the confounding effects of anesthetics would be preferred, this testing is not considered safe to perform in tigers. Tonometry readings taken clinically in adult tigers would most likely be performed with some combination of the above drugs; as such, the findings may be clinically relevant.

The effects of body position and signalment of the animal on IOP values have been evaluated previously in other species. One study45 found that there was no significant change in IOP when measured in lateral recumbency compared to sternal recumbency in domestic cats. Other factors that were shown to affect IOP in large felids include age, sex, and reproductive status (luteal phase or progesterone concentrations in female lions).39,46,47 Juvenile male lions were shown to have elevated IOP in comparison to juvenile female lions.39 The IOPs varied within female lions depending on progesterone concentration: higher progesterone concentrations were associated with higher IOP measurements.47 Further research revealed a significant difference in IOP in lions < 1 year of age versus older lions.46 Intraocular pressures in lions appear to increase linearly with age for the first 20 months of life until 40 months of age, at which point they decrease gradually.46 These variables were not examined in the present study because no tigers < 1 year of age were included. Ofri et al46 indicated potential differences between subspecies, which was not evaluated in the present study.

Limitations of the present study included the possibility of user error in both measurements and calculations, a small sample size, no standard body positioning, an inability to measure variables including IOPs in awake animals without effects of sedation and anesthesia, and an incomplete range of tigers in terms of age (the youngest tiger was 11 years of age). It is unknown whether our results are generalizable to a larger population of tigers. Additional research is warranted to determine whether tigers are a species in which ocular dimensions, corneal curvature, and ametropia can vary with age, sex, weight, and subspecies, and whether there is a difference in ocular variables between tigers bred in captivity versus in the wild. Further study is also needed on the effects of anesthesia, positioning, and signalment on IOP in tigers. Research into prototype IOLs specifically designed for tigers should be performed to determine appropriate IOL power to achieve emmetropia via serial streak retinoscopy and PACD measurements.

Acknowledgments

Supported by Blue Pearl Science.

The authors declare that there were no conflicts of interest for this study.

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Contributor Notes

Corresponding author: Dr. Michau (tammy.miller.michau@effem.com)