Topical Ocular Anesthesia Has no Effect on Photophobia Induced by Red and Blue Light Stimulus

Abstract

Purpose : It is an intriguing question whether the newly discovered melanopsin-containing intrinsically photosensitive ophthalmic trigeminal ganglion cells (ipTGCs) mediate photophobia. We designed a psychophysical paradigm to test the hypothesis that if ipTGC photoactivity makes a significant contribution to photophobia, then topical anesthesia will significantly increase the photophobic threshold to melanopsin-activating blue light, but not to melanopsin-silent red light. If ipTGCs have no significant contribution to photophobia, topical ocular anesthesia will have no effect on the photophobic threshold for either blue or red light.

Methods : 10 visually normal participants completed the experiment consisting of an Anesthesia condition in which light stimuli were presented binocularly following administration of 0.5% proparacaine eye drops, and a Control condition where placebo drops were used. The two conditions were tested on two separate visits in a double-blinded manner. In each condition, a randomized series of 280 blue and red light flashes were presented over 7 intensity steps with 20 repetitions of each color/intensity combination. Participants reported whether they perceived the stimulus as “uncomfortably bright” or not by button press. The proportion of positive responses was used to generate psychometric functions, and the psychophysical photophobic thresholds were defined as the light intensity at which each individual perceived the stimulus to be uncomfortably bright 50% of the time. The two conditions were compared using paired sample t-tests with Bonferroni correction.

Results : Under blue light stimulation, there was no significant difference in the photophobic thresholds between the two conditions [Anesthesia ( = 32.07, σ = 9.20 cd/m²), Control ( = 32.65, σ =12.06 cd/m²)]; t(9) = 0.17, p = 0.44, d = 0.05. Similarly, during red light stimulation, no significant difference in threshold values was found between conditions [Anesthesia ( = 106.67, σ = 67.95 cd/m²), Control ( = 109.48, σ = 72.83 cd/m²)]; t(9) = 0.28, p = 0.39, d = 0.09.

Conclusions : Topical ocular anesthesia does not alter the psychophysical photophobia thresholds for either blue or red light, suggesting that ipTGC photoactivity makes little contribution to photophobia under physiological conditions. However, these data do not exclude the possibility that ipTGC photoactivity mediates pathological photophobia.

This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.