Topical and intravenous pilocarpine stimulated accommodation in anesthetized rhesus monkeys

Abstract

Many studies have used pilocarpine to stimulate accommodation in both humans and monkeys. However, the concentrations of pilocarpine used and the methods of administration vary. In this study, three different methods of pilocarpine administration are evaluated for their effectiveness in stimulating accommodation in rhesus monkeys. Experiments were performed in 17 iridectomized, anesthetized rhesus monkeys aged 4-16 years. Maximum accommodation was stimulated in all these monkeys with a 2% pilocarpine solution maintained on the cornea for at least 30 min in a specially designed perfusion lens. In subsequent topical pilocarpine experiments, baseline refraction was measured with a Hartinger coincidence refractometer and then while the monkeys were upright and facing forward, commercially available pilocarpine (2, 4, or 6%) was applied topically to the cornea as 2 or 4 drops in two applications or 6 drops in three applications over a five minute period with the eyelids closed between applications. Alternatively, while supine, 10-12 drops of pilocarpine were maintained on the cornea in a scleral cup for 5 min. Refraction measurements were begun 5 min after the second application of pilocarpine and continued for at least 30 min after initial administration until no further change in refraction occurred. In intravenous experiments, pilocarpine was given either as boluses ranging from 0.1mg/kg to 2mg/kg or boluses followed by a constant infusion at rates between 3.06 mg/kg/h and 11.6 mg/kg/h. Constant 2% pilocarpine solution on the eye in the perfusion lens produced 10.88+/-2.73 D (mean+/-SD) of accommodation. Topically applied pilocarpine produced 3.81 D+/-2.41, 5.49 D+/-4.08, and 5.55 D+/-3.27 using 2%, 4%, and 6% solutions respectively. When expressed as a percentage of the accommodative response amplitude obtained in the same monkey with constant 2% pilocarpine solution on the eye, the responses were 34.7% for 2% pilocarpine, 48.4% for 4% pilocarpine, and 44.6% for 6% pilocarpine. Topical 4% and 6% pilocarpine achieved similar, variable accommodative responses, but neither achieved maximum accommodation. IV boluses of pilocarpine achieved near maximal levels of accommodation at least ten times faster than topical methods. Doses effective for producing maximum accommodation ranged from 0.25mg/kg to 1.0mg/kg. IV pilocarpine boluses caused an anterior movement of the anterior lens surface, a posterior movement of the posterior lens surface, and a slight net anterior movement of the entire lens. Considerable variability in response amplitude occurred and maximum accommodative amplitude was rarely achieved with topical application of a variety of concentrations of commercially available pilocarpine. Intravenous infusion of pilocarpine was a rapid and reliable method of producing a nearly maximal accommodative response and maintaining accommodation when desired.

Fig. 1

Maximum accommodative responses were obtained from the 14 monkeys using constant perfusion of 2% pilocarpine. Subsequently, monkeys received topical applications of 2%, 4%, or 6% pilocarpine. Empty spaces in the graph indicate where monkeys were not treated with a particular concentration of pilocarpine.

Fig. 2

The results from each topical treatment of each monkey were expressed as a percentage of the maximum accommodation achieved in that monkey using constant pilocarpine perfusion in the perfusion lens. The mean responses were 34.7% for 2% pilocarpine, 48.4% for 4% pilocarpine, and 44.6% for 6% pilocarpine.

Fig. 3

Inter-individual variability in accommodative responses to identical treatments with pilocarpine. Responses are expressed as a percentage of the maximum accommodative response obtained in each individual monkey with the perfusion lens. (A) Four monkeys which were treated with a total of 4 drops of 4% pilocarpine applied over 5 min had responses from 13.0% to 95.8% of the maximum accommodation achieved with the perfusion lens. (B) Three monkeys were treated with 4 drops of 6% pilocarpine over 5 min. One of these monkeys (#111) received this treatment on two different occasions. The responses varied from 18.0% to 46.2% of maximum accommodation.

Fig. 4

The results from Fig. 2 are separated by method of application. Responses are expressed as a percentage of the maximum accommodative response obtained in each individual monkey with the perfusion lens. The individual monkeys are identified above each bar. The number of drops applied is shown at the base of each bar. (A) Eye drops applied topically to the cornea followed by closing the eyelids resulted in highly variable responses for all concentrations with 6% producing relatively low responses. (B) Experiments where 10–12 drops of pilocarpine were held on the eye with an eyecup for 5 min show a higher accommodative response with 6% pilocarpine, but still considerable variation. In both methods, in only one monkey was the full 100% accommodative response achieved.

Fig. 5

Two sample time-course graphs of accommodative responses to pilocarpine are shown for three different methods: topical drops, constant perfusion, and intravenous bolus infusion of 1 mg/kg. The results in each graph are from three different experiments performed on the same monkey on three different days.

Fig. 6

Accommodative responses to increasing boluses of intravenous pilocarpine are shown as a function of time. The bolus doses used were 0.1, 0.25, 0.5, 1.0, and 2.0 mg/kg pilocarpine. The accommodative fluctuations are due to slow eye movements that sometimes occur under propofol anesthesia.

Fig. 7

Accommodative responses to pilocarpine boluses of increasing dosage were obtained in five monkeys and repeated in one monkey. (A) Total accommodation in diopters is shown plotted as function of pilocarpine dosage. (B) Accommodative levels are normalized to the maximum accommodation achieved during the experimental session and plotted against pilocarpine dosage.

Fig. 8

CUB measurements were made on the contra-lateral eye during the same experiment as shown in Fig. 6. Measurements are shown here over time for (A) lens thickness, (B) anterior chamber depth, and (C) anterior segment depth (lens thickness + anterior chamber depth).

Fig. 9

Movements of the anterior lens surface (solid black line), lens center (solid gray line), and the posterior lens surface (dashed black line) are plotted against time for pilocarpine bolus experiments in monkeys (A) #50, (B) #38, and (C) #4. Negative values indicate the anterior direction and positive values indicate the posterior direction. Movements are calculated as changes from the average baseline value during the first 2 min. In A, five increasing bolus doses of 0.1, 0.25, 0.5, 1.0, 2.0 mg/kg were given. In B, 4 bolus doses of 0.25, 0.5, 1.0, 2.0 mg/kg were given. In C bolus doses of 0.1, 0.25, 0.5 mg/kg were in most cases, boluses were given before accommodation had returned to baseline from the prior doses.

Fig. 10

In three experiments, CUB measurements were made on one eye at the same time that photorefraction was measured on the other. Combined accommodative data from multiple periods of rapid accommodation are plotted here against concurrent changes in lens surface position for monkeys (A) #50, (B) #38, and (C) #4. The data show an anterior movement of the anterior lens surface and a posterior movement of the posterior lens surface during accommodation.

Fig. 11

Photorefraction accommodative responses measured simultaneously in both eyes (left eye: black; right eye: gray) in Monkey #50 in response to boluses of 0.1, 0.25, 0.5, and 1.0 mg/kg pilocarpine followed by 0.25 mg/kg atropine.

Fig. 12

Photorefraction measured accommodative refractive responses to bolus doses followed immediately by constant i.v. infusion of pilocarpine recorded in three monkeys. Experiments were repeated once in monkey #70 and twice in monkey #50. Constant doses of pilocarpine are shown in black below each accommodative response and bolus amounts of pilocarpine (black) and atropine (gray) are specifically labeled.

Fig. 13

Biometric measurements on the contra-lateral eye in each of the experiments shown in Fig. 12. Constant doses of pilocarpine are shown in black below each accommodative response and bolus amounts of pilocarpine (black) and atropine (gray) are specifically labeled.