The effects of phenylephrine on pupil diameter and accommodation in rhesus monkeys

Abstract

Purpose: Phenylephrine is used to dilate the iris through alpha-adrenergic stimulation of the iris dilator muscle. Sympathetic stimulation of the ciliary muscle is believed to be inhibitory, decreasing accommodative amplitude. Investigations in humans have suggested some loss of functional accommodation after phenylephrine. It is unclear whether this loss is due to direct action of phenylephrine on the ciliary muscle or to secondary optical factors associated with mydriasis. The purpose of this study was to determine whether phenylephrine affects Edinger-Westphal (EW)-stimulated accommodation in rhesus monkeys.

Methods: The time course for maximum mydriasis was determined by videographic pupillography after phenylephrine instillation in 10 normal rhesus monkeys. Static and dynamic EW-stimulated accommodative responses were studied in five iridectomized rhesus monkeys before and after phenylephrine instillation. Accommodative amplitude was measured with a Hartinger coincidence refractometer. Dynamic accommodative responses were measured with infrared photorefraction, and functions were fitted to the data to determine peak velocity versus accommodative response relationships.

Results: The maximum dilated pupil diameter of 8.39 +/- 0.23 mm occurred 15 minutes after administration of phenylephrine. In iridectomized monkeys, postphenylephrine accommodative amplitudes were similar to prephenylephrine amplitudes. Dynamic analysis of the accommodative responses showed linear peak velocity versus accommodative amplitude relationships that were not statistically different before and after phenylephrine.

Conclusions: alpha-Adrenergic stimulation causes a strong pupil dilation in noniridectomized monkey eyes but does not affect EW-stimulated accommodative amplitude or dynamics in anesthetized, iridectomized rhesus monkeys.

Figure 1

Four different phenylephrine dosage protocols were tested on one noniridectomized monkey eye on different occasions to determine the appropriate dose needed to elicit maximum pupil dilation. Maximum pupil dilation was achieved by 20 minutes with two doses of 10% phenylephrine.

Figure 2

Pupil diameters were measured in one eye each of 10 noniridectomized monkeys after two 0.1-mL doses of 10% phenylephrine. Mean maximum dilation was achieved after 15 minutes.

Figure 3

The linear relationship (from linear regressions) between peak velocity and accommodative amplitude (main sequence relationship) was not significantly different before and 25 minutes after phenylephrine instillation for (A) accommodation (P = 0.52) or (B) disaccommodation (P = 0.24).

Figure 4

Baseline refraction measurements were similar before and 30 minutes after instillation of 10% phenylephrine (t-test: t = −1.4931, df = 4, P = 0.2097).

Figure 5

Stimulus response functions measured with a Hartinger coincidence refractometer were similar before and 30 minutes after 10% phenylephrine (A–E). (F) Saline control.

Figure 6

Hartinger-measured accommodative responses to the same stimulus amplitudes before and after phenylephrine administration were not significantly different (orthogonal regression: slope = 1.017, intercept = −0.476, r = 0.9898).