Myopia

Image:Human_eyesight_two_children_and_ball_normal_vision.jpg Normal vision forachromatopsic colour-blind person. Courtesy NIH National Eye Institute

Image:Human_eyesight_two_children_and_ball_with_myopia_short-sightedness.png The same view when achromatopsicmyopic.

Myopia, also known as nearsightedness or short-sightedness, isrefractive defect whereperson affected usually can see nearby objects clearly but distant objects appear blurred. Myopiacaused byweakening ofciliary muscle which controlseye's lens. The weak muscle can't adjustlens enoughsee far distances, causing far-off thingsbe blurred. About one quarter ofadult population inUnited States has myopia.

Myopiausually caused by doing lots"nearwork", like reading books or usingcomputer screen. Sinceeye rarely hasfocus on far distances,musclerarely used and, asresult, becomes weak. The onsetmyopiausually aroundpre-teen/teenage years,can gradually worsen through adolescence, especially if glassesprescribed. (Corrective lenses (e.g. glasses) dociliary muscle's workit, makingeven weaker.) Eventually during adulthood, myopia levels off.

Thereevidence ofcorrelation between shared genetic materialmyopia, leading somebelieve itinherited (thisan example ofpost hoc ergo propter hoc fallacy). However, other research disputes this. Studies doneNative American tribes found that beforeintroductionwidespread literacy, onlytribal records-keeper had myopia but none ofrest ofpopulation. But after all children were taughtread,percentagechildrenmyopia was approximately equalthat ofrestAmerica. The correlation can be explained by things liketendencypeople whose parents readlotreadlot themselves.

Myopiameasureddiopters, which iscombined power ofcornea andlens neededfocus distant images correctly onretina. Myopia6.00 diopters or greaterconsidered high, or severe, myopia. Peoplehigh myopiaat greater riskmore acute eye problems such as retinal detachment or glaucoma. Theyalso more likelyexperience floaters.

Myopia can be cured throughvarietyeye exercises, designedstretchciliary musclebringbackfull strength. The most importantthese exercisescalled "pumping"involves holding an object, likepencil or finger, nearyour facepractice switching your focus between itan object indistance.

Corrective lenses, - eyepieces, such as those thatvariously termed spectacles or glasses, or contact lensesalso often prescribed but, as noted above, these only makecondition itself worse (althoughrelievesymptoms). It may also be corrected by refractive surgery, such as LASIK. The corrective lenses havenegative dioptric value (i.e.concave) which compensates forexcessive positive diopters ofmyopic eye.

One 2002 study suggested thatmay be caused by over-consumptionbreadchildhood.