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James E. Sheedy, Sowjanya Gowrisankaran; Viewing compromised visual stimuli causes dry eye symptoms: Role of the orbicularis muscle. Journal of Vision 2006;6(6):73. doi: 10.1167/6.6.73.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Two types of asthenopia (internal and external) have been previously identified: internal asthenopia is a pain or ache inside the eye caused by accommodative and convergence stress; external asthenopia is dry eye-like symptoms (irritation and burning) caused by adverse visual stimuli such as glare or reading compromised images. This study investigates the role of the orbicularis muscle in external symptoms. Methods: Twenty subjects read seven passages under the following asthenopia-inducing conditions: glare, reduced contrast, small font, mixed astigmatic refractive error, upward gaze, accommodative stress and convergence stress. Orbicularis activity was recorded using surface electromyography (EMG). Video images were analyzed for the amount of squint. Results: Refractive error (p=0.0001), glare (p=0.0001), reduced contrast (p=0.007), small font (p=0.034) and up gaze (0.001) resulted in significant increase in EMG power and amplitude. Refractive error (p=0.0001) and glare (p=0.0001) also caused significant squint. Conclusion: An increase in orbicularis EMG occurred in all conditions that are associated with external asthenopia. However only those conditions that benefit from squint (refractive error and glare) caused measurable squint. Accommodative and convergence stress, which cause internal asthenopic symptoms, resulted in neither a significant EMG nor squint response. The results indicate that muscular tension in the orbicularis is caused by conditions that compromise the visual stimulus, but the ocular aperture is decreased only for those that benefit from it. These results suggest that orbicularis tension is a reflex response to adverse visual conditions and is related to dry eye-like symptoms.
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