The physiology of eyestrain

Abstract

Eyestrain, or asthenopia, can be categorized into 2 types: an internal type consisting of sensations of strain and ache felt inside the eye and caused by stress of the accommodative and convergence mechanisms; and an external type consisting of sensations of dryness and irritation on the front of the eye and caused by compromised conditions in the viewing environment. The symptom sensations of the external type mimic those expected due to dry eyes. The likely mechanism is that poor viewing conditions cause contraction of the orbital portion of the orbicularis oculi muscle which causes eye lid squint in an attempt to improve vision. This has been shown to result in secondary reduction in eye lid blink, thereby resulting in dry eye conditions. Electromyography is a sensitive measure of contraction of the orbicularis and has potential to be an objective measure of eyestrain.