Ocular findings and intraocular lens power in captive chimpanzees (Pan troglodytes)

Abstract

Purpose. To perform a descriptive investigation of the chimpanzee (Pan troglodytes) eye, conduct selected ophthalmic diagnostic tests, document intraocular measurements and determine the intraocular lens (IOL) power for this species. Methods. Thirty-three anaesthetised captive chimpanzees (20 females with a mean age of 21.8 ± 7.06 years and a median age of 15 years, 13 males with a mean age of 16.4 ± 17.48 years and a median age of 15 years) underwent ophthalmic examination as part of routine health examinations. Selected diagnostic tests performed: intraocular pressure (IOP) by rebound tonometry (TonoVet®), central corneal thickness (CCT) by ultrasonic pachymetry, keratometry using a handheld automatic keratometer, A- and B-mode ultrasonography. IOL power was calculated using the Retzlaff and Binkhorst theoretical formulas. Results. Most common ophthalmic findings (eyes): iridal melanosis (11), dyscoria (3) and mature cataracts (2). IOP: 22.59 ± 5.34 mmHg (“d” setting), 14.34 ± 4.03 mmHg (“P” setting). Mean horizontal corneal radius: 7.35 ± 0.47, mean vertical corneal radius: 6.55 ± 0.55 mm. Mean horizontal and vertical corneal curvatures were 46.19 ± 2.80 D and 51.74 ± 4.39 D respectively. CCT: 0.45 ± 0.04 mm. No significant differences were found between right and left eyes, age or sex for any parameter evaluated except for a sex difference in IOP where males had lower IOP than females. Ocular biometry: axial globe length: 21.41 ± 0.76 mm, anterior chamber depth (ACD): 3.63 ± 0.47 mm, crystalline lens thickness (LT): 3.81 ± 0.68 mm, posterior segment depth: 13.98 ± 1.00 mm. The estimated postoperative ACD (PACD) was calculated as ACD + 0.5 x LT. To address the effect of IOL position on lens power, additional calculations using PACD +2mm and PACD -2mm were performed. Calculated IOL powers (using the Retzlaff and Binkhorst formulas respectively): PACD: 21.86 ± 5.73 D and 22.81 ± 5.79 D; PACD +2mm: 27.67 ± 7.36 D and 28.83 ± 7.43 D; PACD -2mm: 17.71 ± 4.59 D and 18.51 ± 4.65 D. Conclusions. Normal parameters described in this study will aid in the identification of ocular pathology in chimpanzees. Determination of the IOL power will facilitate correct IOL selection for the phacoemulsification candidate, although further studies evaluating pseudophakic refraction and determination of actual PACD are required.