In this study, an average reduction in resting pupil size of 1.78 mm was observed in brimonidine-treated eyes, which is consistent with the results observed in the other studies.
32,36,37
The relationship between log stimulus intensity and pupil contraction amplitude will vary depending on the area of retina being illuminated, in addition to stimulus brightness. The Ganzfeld bowl light stimulus used in this study will undoubtedly produce a greater pupil response than a smaller-sized light stimulus, although scatter of bright light across the retina even with a smaller-sized light tends to recruit larger areas of retina at brighter lights. Nonetheless, the mechanical constraints on pupil movement are likely to come into play at the same pupil size in a given eye, no matter what area of retina is stimulated, as long as the summated area of retina at a given brightness causes the pupil size to drop below its mechanical limit.
Limitations on pupil contraction were observed in all brimonidine-treated eyes. The first sign of a limitation of pupil contraction in the miotic eye was at the point where pupil contraction (in millimeters) in the treated eye started to fall below that in the untreated eye. This point is presumed to be where iris mechanics starts to limit pupil contraction, which we term the “mechanical threshold” (
Fig. 4). The “mechanical threshold” found in our study (range from 2.38 to 4.44 mm; mean 3.25 ± 0.61 mm) is strikingly similar to what was observed by Loewenfeld and Newsome
28 (the lower limit of linear range of pupil contraction ranged from 2.75 to 3.80 mm, mean ± SD 3.41 ± 0.37 mm), when anisocoria was produced in six healthy subjects using cocaine, an indirect sympathomimetic, to produce mydriasis instead of miosis, as was done in the present study. It was not surprising to see that plotting pupil response in percent contraction amplitude reduced the confounding effect of iris mechanics on the assessment of afferent light transduction (
Fig. 5, right), because percent contraction represents pupil contraction
relative to baseline pupil size. However, even the percent contraction appears to plateau at high stimulus intensities, as repeatedly observed in the study of Park et al.,
35 using the same stimulus protocol. A question was raised as to whether the plateau of the pupil response was contributed solely by a limitation imposed by iris mechanics or also by a plateau signifying neuronal nonlinearity of the PLR at high stimulus intensities.
Our results from brimonidine testing suggest that most of the nonlinearity of the pupil response likely comes from a limitation on iris movement and not from nonlinearity of neuronal activity. The relative contribution of the sphincter and dilator muscles to iris mechanics are best demonstrated by the plot of the pupil size at peak contraction in the untreated and treated eyes (
Fig. 3, bottom of each plot). When pupil size in the treated, more miotic eye (black opened circles) plateaus, the limitation on pupil contraction should be mainly due to the maximum contraction that can be attained by the iris sphincter muscles; the sympathetic tone is blocked by brimonidine. The pupil of the untreated eye was never able to reach as small a pupil size even at the peak contraction as the treated eye (see
Fig. 3, comparing lower red and black curves with open circles), indicating difference in iris muscle tone between untreated versus treated eye at the peak contraction. We modeled what would be the full excursion of the pupil in the untreated eye, should the untreated eye achieve the minimal pupil size at peak contraction that was reached by the treated eye. The pupil contraction amplitude of the untreated eye (in both millimeters and percentage) was recalculated by subtracting the minimum pupil size achieved in the brimonidine-treated eye from the baseline pupil size in the untreated eye for each contraction (predicted pupil contractions, green open circles in
Fig. 6). The resulting pupil response curve appears to be more linear. This linear pupil response curve more likely represents parasympathetic mediated pupil contraction from the sphincter muscles when sympathetic tone is blocked by brimonidine.
These findings seem to suggest that the limitation of peak pupil contraction in the untreated eye compared with the brimonidine-treated eye might be at least partly contributed by the presence of residual dilator muscle tone through sympathetic activity that is still present in the untreated eye at peak of contraction. This finding is somewhat different from the previous hypothesis that the behavior of PLR was attributed to the length-tension characteristics of the sphincter muscle alone.
28,29 One has to be aware that this is a simplified model with respect to the receptor mechanism of the iris smooth muscles. The iris is found to be heavily innervated by sympathetic, parasympathetic, and sensory nerve terminals through both postsynaptic and prejunctional receptors in various species.
38–40 The innervation of iris sphincter muscle in humans is found to be mainly contributed by cholinergic nerve fibers and substance P,
33,41 although other adrenergic receptors have been found on sphincter muscles
33,42,43 and may modulate its tone. Brimonidine exerts most of its pharmacologic effect through α2-adrenoreceptor–mediated downregulation of norepinephrine release.
44–51 At present, there is no evidence that the α2 agonist activity of brimonidine exerts a modulatory effect on the iris sphincter and its α1 biological activity has been found to be insignificant.
47