Studying the effect of iris mechanics on the pupillary light reflex using brimonidine-induced anisocoria.

Publications // Yanjun Chen // Apr 26 2013

PubMed ID: 23513058

Author(s): Chen Y, Kardon RH. Studying the effect of iris mechanics on the pupillary light reflex using brimonidine-induced anisocoria. Invest Ophthalmol Vis Sci. 2013 Apr 26;54(4):2951-8. doi: 10.1167/iovs.12-10916. PMID 23513058

Journal: Investigative Ophthalmology & Visual Science, Volume 54, Issue 4, Apr 2013

PURPOSE To study and correct for the limiting effect of iris mechanics on the amplitude of light-evoked pupil contractions in order to derive a more clinically accurate assessment of afferent input to the visual system.

METHODS Transient pupil responses were recorded to a series of 1-second red Ganzfeld light stimuli with a stepwise increase in stimulus intensity using a binocular infrared computerized pupillometer. One eye of eight healthy subjects was treated with 0.2% brimonidine tartrate ophthalmic solution to induce pupil size reduction. The amount of pupil contraction as a function of stimulus intensity was compared between the brimonidine-treated, miotic eye and the untreated eye.

RESULTS BRIMONIDINE TREATMENT PRODUCED SIGNIFICANT REDUCTION IN PUPIL SIZE IN HEALTHY SUBJECTS (MEAN REDUCTION IN PUPIL SIZE: 1.78 ± 0.35 mm, P < 0.05). For increasing light intensity, the treated pupil started to show reduced pupil contractions compared with the contralateral untreated pupil when the peak of pupil contraction reached an average pupil size of 3.25 ± 0.61 mm (range, 2.38-4.44 mm). When measured by percent pupil contraction (contraction amplitude/baseline pupil diameter), the pupil response as a function of stimulus intensity in the treated, miotic eye did not differ from that in the untreated eye.

CONCLUSIONS Iris mechanics limits the amount of pupil contraction and can act to reduce the assessed neuronal integration of the pupil light reflex. Pupil response assessed by using percent contraction amplitude is least affected by mechanical effects and provides a more accurate approximation of afferent input.