HYPEROPIA, OR HYPERMETRO PIA.—From Gr., {,n4), over, and 61,].„ sight, was proposed by Helmholtz. Later Donders introduced the root yeTpor, measure, into the word, changing it to "hypermetropia" to make it correspond with other terms introduced by him as ametropia and emmetropia. The term is often replaced by the abbreviation H.
Definition.—That error of refraction in which the principal focus of the diop tric media lies behind the retina. Rays parallel when they enter the eye tend to focus behind the retina and are inter cepted before they come to a focus: the eyeball is too short from before back ward.
Symptoms.—If the hyperopia is of very high degree the eyeball is usually small in all directions, appears deep set, and noticeably fails to fill the orbit. The pupil is small from contraction of its sphincter, associated with excessive exer tion of the accommodation. Through a similar association there may be con vergent squint, either constant or occur ring when the attempt is made to see near objects clearly.
When the power of accommodation has been lost, as by age or the use of atropine, vision is imperfect at all dis tances. With sufficient accommodation clear vision is possible by excessive exer tion of the ciliary muscle. This causes headache, most frequently frontal, some times occipital. Strain of the accommo dation also causes chronic or recurring conjunctivitis, redness or inflammation of the lid-margins, styes, etc. The use of accommodation to correct the hyper opia leaves less for the focusing of near objects. Hence presbyopia appears early, requiring the use of convex lenses for near work before the age of 45. In high degrees continuous near-seeing may be impossible even from childhood, or jects may be held very close to the eye to make up for imperfect focusing by en larged retinal images. In old age the convex lenses needed are stronger than would be required for failure of accom modation alone. The eye-strain may lead to inflammation of the choroid, optic nerve, or retina. The defect tends to give a distaste for reading and other occupations requiring near vision.
In youth the focusing muscle can so mold the soft crystalline lens that fine print is distinct as near as three or four inches to the eye. As age advances the lens hardens and decreases the active focusing power until fine print, such as foot-notes, can no longer be read at the proper reading distance. This condition is presbyopia. The active focusing power gradually decreases until about the seventieth year, when the focusing muscle loses nearly all power over the hardened lens, and al this age the per fect eye needs for reading a magnifier equal in strength to what the active focusing power was just before presby opia came on. Until the fiftieth year most people with far-sighted, near sighted, and astigmatic eyes can read fine print with glasses which totally correct these imperfections. Many at fifty-five years of age and some at sixty can do this. Many with normal distant vision at fifty to sixty years of age have never used reading glasses, and yet are a trifle far-sighted or astigmatic.
Presbyopia does not come at the fortieth year. Those who need glasses this early have far-sightedness, near-sightedness, or astigmatism. Belladonna is not used in testing the eyes of the aged; so the focusing muscle may mask imperfections after the fortieth year almost as much as earlier. Latent imperfections may exist until the sixty-fifth year. N. B. Jenkins (Medical News, Sept. 2S, 1901).
Etiology.—Hyperopia may be due to flattening of the cornea or crystalline lens, making the focus of the eye unusu ally long: hyperopia of curvature. More commonly it is due to the antero-pos terior axis of the eyeball being shorter than the normal standard: axial hyper opia. It may also be caused by the absence of the crystalline lens, as from injury or extraction for cataract: apha kial hyperopia. Hyperopia is usually congenital. Nearly all eyes are hyper opic at birth, and 70 per cent. continue so throughout life. It tends to increase after the age of forty years through the continued growth of the crystalline lens.
Varieties.—Hyperopia that cannot be corrected by the accommodation the eye possesses is called absolute II. That which is still corrected by accommoda tion in spite of efforts to relax the ciliary muscle is called latent II. That which can be revealed without use of a mydri atic (cycloplegic) is called manifest IL That which can be either corrected or revealed is called facultative II.
Diagnosis.—When distant vision re mains equally good or is rendered clearer by convex lenses hyperopia is present, and the strongest convex lens that allows clear distant vision comes nearest to measuring the hyperopia. The slightest hyperopia is rendered manifest by test ing both eyes together, beginning with convex lenses that are too strong and making them weaker until distant vision is clear. To find its full amount it is often necessary in young persons to em ploy a mydriatic (cycloplegic).
With a convex lens before it the hy peropic eye can see clearly beyond the focal distance of the lens; and by scias copy the point of reversal is found be yond the principal focus of the lens.
Treatment.—Hyperopia requires cor rection by a convex lens. ITsually one strong enough to correct all of it is best. The lenses should be worn constantly if there is convergent squint, headache, or inflammation of the eye or its append ages. If symptoms only arise after use of the eyes for near work, wearing of the correcting lenses at such times may be enough. Diminishing the amount of near work required of the eyes may give relief. Persons who are hyperopic but suffer no inconvenience from the hyper opia require no treatment for it. Hyper opia co-exists with astigmatism in the majority of cases; and the very careful measurement and correction of both errors of refraction may be necessary to render the glasses at all satisfactory.