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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. 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. 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 …