Refractive Errors Eye72 Refractive Errors



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Refractive Errors Eye72 ()

Refractive Errors

Last updated: September 5, 2017




Types

emmetropia - normal refractive system (light rays focus clearly on retina → sharp image perceived by brain).

ametropia - refractive system defects:

  1. Hyperopia (farsightedness) - images of distant and near objects are focused behind retina:

    1. too short axis of eyeball

    2. too weak refractive power (too flat cornea)

  • most common refractive error.

  • sustained accommodation (even when viewing distant objects!) causes tiring prolonged eye strain (headaches, blurring of vision); prolonged convergence of visual axes (associated with accommodation) may lead eventually to strabismus.

  • convex (plus) corrective lens is used.

Presbyopia - progressive, age-related loss of accommodative amplitude → physiologic hyperopia.

  • primary factors in presbyopia are lens hardening (lens gradually grows less pliable and eventually cannot change shape – Helmholtz theory) and loss of ciliary muscle motility (Schachar theory).

  • begins in adolescence.

  • prevalence 100% among people > 50 yrs.

  • no need for corrective glasses until early to mid-40s.

  • no universally accepted treatments or cures are currently available for presbyopia.



  1. Myopia (nearsightedness) - image of distant object is focused in front of retina:

  1. too long axis of eyeball

  2. too strong refractive power (too steep cornea)

  • concave (minus) corrective lens is used.

  • myopia is genetic in origin; however, eye shape during development is determined in part by refraction presented to it (e.g. in young adults extensive close work accelerates myopia development).

  • 1/4 of world's population is myopic!

  • myopia starts at age ≈ 6 yrs., then progresses (due to increase of eyeball AP diameter) and stabilizes at late teens (in later life increasing myopia may indicate developing cataracts).

Astigmatism - unequal refraction in different meridians of eyeball → distorted retinal image (e.g. point is seen as line):

  1. unequal curvature of cornea.

  2. unequal curvature of lens.

  3. lens dislocation

  • cylindric corrective lens is used (has no refractive power along one axis and is concave or convex along other axis).

  • toric lens - both meridians are curved but not to the same degree.

Anisometropia (vs. isometropia) - significant (usually > 2 diopters) difference between refractive errors of two eyes.

  • if refractive errors are corrected with lenses, differences in image size (aniseikonia) are produced → difficulties in fusion → suppression of one image (amblyopia).

Refraction - act of determining refractive errors and correction by lenses:

static (cycloplegic) refraction – refraction without accommodation.

dynamic (manifest) refraction - refraction during accommodation.

Ametropia can be corrected by 3 different means:

  1. Optical devices (glasses, contact lenses)

  2. Corneal refractive procedures - cannot take care of high refractive errors.

  3. Intraocular procedures (clear lens extraction ± lens implantation; phakic intraocular lens implants)

Contact Lenses

  • provide better visual acuity & peripheral vision than do eyeglasses.

indications

  1. myopia (contact lenses don’t increase or decrease amount of myopia), esp. > -12.0 D

  2. hyperopia, esp. > +10.0 D (incl. aphakia); presbyopia can also be corrected:

    1. nondominant eye is corrected for reading and dominant eye for distant vision (monovision).

    2. bifocal and multifocal contact lenses (fitting procedure is time-consuming - precise alignment is essential!).

  3. astigmatism (toric lenses are used but require expertise for fitting)

  4. anisometropia, aniseikonia

  5. corneal disorders (keratoconus, bullous keratopathy, etc) - soft contact lenses (bandage lenses) ± prophylactic antibiotic eyedrops.

  6. occlusion therapy for amblyopia - soft contact lenses (well-tolerated in children).

Side effects, complications

(rate of complications ≈ 5% per year).

All eyes are altered by contact lens wear!!!

N.B. poor contact lens hygiene may lead to hard-to-reverse inflammatory conditions!



  1. poorly fitting lenses

  2. changes in lens / corneal parameters (swelling of tissues)

  3. harmful (e.g. oxygen-poor, smoky, windy) environment

  4. improper inserting / removing lenses

  5. trapped small foreign particles (e.g. soot, dust)

  6. removing lenses after prolonged use (overwear syndrome); spontaneous healing may occur in day or so if lenses are not worn (in some cases, treatment is required - pupil dilation to prevent posterior iris synechiae, topical antibiotic, sometimes sedation).

No pain should be present at any time; pain is sign of ill-fitting contact lens or corneal irritation!

Cornea

  • corneal epithelium defects can be caused mechanically or chemically (H: preservative-free hydrogen peroxide based disinfection systems).

  1. superficial changes (painless)

  2. abrasions (pain, photophobia, anxiety) - indicate chronic epithelial stress; can allow bacteria to penetrate cornea

  3. ulcers & scars

non-vascularized, dense superficial scar indicates problems with epithelial defects in past contact lens wear:

Source of picture: “Online Journal of Ophthalmology” >>



  1. sterile corneal stroma infiltrates (immune-mediated reaction to bacterial toxins from colonized contact lenses).

  2. bacterial keratitis

  • pathogens - Pseudomonas aeruginosa [can penetrate intact epithelium!], staphylococci.

  • predisposing pathophysiologic factors:

  • corneal epithelium is thinner less sensitive, hypoxic in contact lens wearers;

  • contact lens is barrier between epithelial surface and lid (preventing wiping action of lid and tear exchange);

  • contact lens mechanically damages epithelial barrier.

N.B. primary risk factor is sleeping with lenses!

  1. Acanthamoeba keratitis - Acanthamoeba castellani [found widely in nature; most common source – tap water!].

  • insidious; very painful; after 6 weeks irreversible ring infiltrates form;

  • diagnosis: Acanthamoeba feed on bacteria (corneal scrapings are placed on E coli-layered agar - inspected for characteristic tracks that trophozoites make as they eat their way across dish), Giemsa and calcofluor white stains on corneal scrapings.

Irregular epithelium without defects, localized stromal edema with minor infiltrate, superficial ring-infiltrate:

Source of picture: “Online Journal of Ophthalmology” >>



  • treatment:
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