This article does not cite any sources. (January 2010) (Learn how and when to remove this template message)
Eye strain, also known as asthenopia (from Greek asthen-opia, Ancient Greek: ἀσθεν-ωπία, transl. weak-eye-condition), is an eye condition that manifests through nonspecific symptoms such as fatigue, pain in or around the eyes, blurred vision, headache, and occasional double vision. Symptoms often occur after reading, computer work, or other close activities that involve tedious visual tasks.
|Other names||asthenopia, aesthenopia|
When concentrating on a visually intense task, such as continuously focusing on a book or computer monitor, the ciliary muscle tightens. This can cause the eyes to get irritated and uncomfortable. Giving the eyes a chance to focus on a distant object at least once an hour usually alleviates the problem.
A CRT computer monitor with a low refresh rate (<70Hz) or a CRT television can cause similar problems because the image has a visible flicker. Aging CRTs also often go slightly out of focus, and this can cause eye strain. LCDs do not go out of focus but are also susceptible to flicker if the backlight for the LCD uses PWM for dimming. This causes the backlight to turn on and off for shorter intervals as the display becomes dimmer, creating noticeable flickering which causes eye fatigue.
A page or photograph with the same image twice slightly displaced (from a printing mishap, or a camera moving during the shot) can cause eye strain by the brain misinterpreting the image fault as diplopia and trying in vain to adjust the sideways movements of the two eyeballs to fuse the two images into one.
Eye strain can also happen when viewing a blurred image (including images deliberately partly blurred for censorship), due to the ciliary muscle tightening trying in vain to focus the blurring out.
Sometimes asthenopia can be due to specific visual problems—for example, uncorrected refraction errors or binocular vision problems such as accommodative insufficiency or heterophoria. It is often caused by forcing the eye to focus and interpret visual data on a small region for a prolonged period of time. This can be caused while reading a book, driving a vehicle, using a digital screen or any similar task which fatigue the eye muscles. The condition worsens due to the buildup of fatigue over an extended period of time.
Eye strain can also be a result of the distortion caused by the refractive properties of certain types of spectacle lenses. The subtle blurriness caused by this distortion in peripheral vision, requires eye muscles to strain in order to retain clear vision. Such prolonged distortion can lead to an increase in strain which is eventually felt by muscles surrounding the eye (in severe cases, even muscles of the upper cheek and forehead). Plastic lenses cause greater distortion than glass lenses and this can easily be verified by focusing both eyes on a screen directly in front and turning the head left or right while continuing to look at the same spot on the screen while wearing spectacles.