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Susan Anderson, Jan Full, Pieter Poolman, John Pienta, Andrew Russo, Randy Kardon; Photosensitive Post -TBI Patients Show a Disproportionate Discomfort Index Compared to Brightness Sense Compared to Normal Subjects and Migraineurs. Invest. Ophthalmol. Vis. Sci. 2013;54(15):2358.
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© ARVO (1962-2015); The Authors (2016-present)
The prevalence of photosensitivity after post-traumatic brain injury (TBI) is high, reported to be 59%. Photic-EMG responses from the squinting muscles in photo- sensitive humans showed an abnormal increase to light (Anderson and Kardon et al, ARVO 2012). Here we compare the brightness and discomfort sense in normal subjects, migraine patients and TBI patients as a function of light intensity.
Normal subjects (n=15), Migraine patients (n=8; in between headaches) and TBI patients (n=4) were tested as a function of increasing log light intensity with photopically matched 640nm red light and 485nm blue light. One-second duration, full-field light stimuli were delivered with a Ganzfeld ERG apparatus, over a 6 log unit range of intensity (0.5 log unit steps) up to 2.6 log cd/m2 brightness. Each subject was asked to rate the brightness and their discomfort following every light stimulus on their own internal scale (magnitude of estimation technique). After recording the ratings, the brightness and discomfort ratings were normalized to 100% for a person’s brightness rating at the 2.6 log cd/m2 blue light. In this way the discomfort rating could be internally compared to the brightness rating as a function of log intensity using a Naka-Rushton sigmoid curve fit.
All subjects showed a brightness and discomfort rating that highly correlated with log stimulus light intensity for both red and blue lights (R>0.97). There was no difference in brightness sense between red and blue lights, or between test groups indicating photopic matching of red and blue intensity and no effect of migraine or TBI on brightness sensation per se. The median discomfort index relative to brightness sense was significantly greater for the blue light (median=28.5% for red light vs 42% for blue light, p<0.001) indicating that blue light is perceived as more aversive than photopically matched red light. Group ANOVA analysis showed that the TBI group had a significantly greater discomfort index compared to the control or migraine group for either red or blue light (p<0.04).
Blue light causes more discomfort relative to brightness sensation across all subjects, which may be mediated by melanopsin. TBI patients reported significantly greater discomfort relative to brightness sense compared to normal subjects and migraine patients in between headaches.
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