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Refractive Errors
Robert M. Kershner, MD, MS, FACS
image BASICS
DESCRIPTION
  • Refraction is the bending of a wave of light as it passes through the optical system of the eye. The degree of bending is dependent on the index of refraction through the air, cornea, and lens and is described quantitatively by Snell law. A refractive error refers to the inability of the eye to produce a focused image on the fovea or central part of the retina.
  • Emmetropia: When light rays are in perfect focus, the image being viewed is seen clearly.
  • Ametropia: any refractive error of the eye that prevents normal focusing of the image
  • Hyperopia: When the cornea of the eye is too flat or the eye is too short, light rays fall in focus behind the retina, and the individual is “farsighted.”
  • Myopia: When the cornea is too steep or the length of the eyeball is too long, the focal point for light rays lies short of the retina, and the individual is “nearsighted.”
  • Presbyopia: The natural tendency of the crystalline lens to harden/become sclerotic with age, limiting the focusing of the eye on near objects (accommodation). By the age of 40 years, most people do not have enough room within the eye to allow normal excursion of the lens and accommodation; viewing of near objects is blurred, and reading glasses are required.
  • Astigmatism: When the cornea is steeper in one meridian more than the other or the globe is not round (i.e., is oval or almond shaped), visual blurriness occurs due to the absence of a single point of focus.
  • Anisometropia: an unequal refractive error between the two eyes
  • System(s) affected: nervous
Geriatric Considerations
Presbyopia occurs after the age of 40 years.
Pediatric Considerations
Refractive errors can be detected early in life.
EPIDEMIOLOGY
  • Predominant age: Refractive errors may be present at birth and can increase in magnitude with age until the eye is fully developed. Often they are not detected until puberty.
  • Predominant gender: male = female
  • Individuals >40 years of age are more likely to experience presbyopia/the normal loss of accommodation that occurs with age, necessitating the use of reading glasses for close work.
Prevalence
Of the general U.S. population, 70% have some form of ametropia.
ETIOLOGY AND PATHOPHYSIOLOGY
  • Developmental (most common)
  • Ocular trauma
  • Iatrogenic (e.g., post refractive surgery or cataract removal)
Genetics
Inherited
GENERAL PREVENTION
No exercises; optical lenses can correct the refractive error.
COMMONLY ASSOCIATED CONDITIONS
  • Patients with diabetes mellitus have fluctuating myopia as a result of poorly controlled blood glucose and concomitant swelling of the crystalline lens.
  • Those who have a pathologic steepening of the cornea due to structural weakness (keratoconus) may have increasing myopia and astigmatism throughout life.
image DIAGNOSIS
PHYSICAL EXAM
Snellen eye chart (developed by Dr. Hermann Snellen in 1862)
  • Test each eye separately.
  • The smallest row of letters that the patient can read determines visual acuity in the uncovered eye.
  • The standard convention is visual acuity measured at 20 feet with a Snellen chart. A person with “normal” acuity will see letters clearly at 20 feet as recorded 20/20 (6/6 in meters). A person with 20/200 vision would be considered legally blind and able only to see a letter at 20 feet that a person with normal vision could see clearly at 200 feet.
DIAGNOSTIC TESTS & INTERPRETATION
  • Pinhole vision test: To distinguish a refractive error from an organic cause of visual blurring, have the patient look through a pinhole in a card without a corrective lens.
  • Patients with a pure refractive error have improved vision because the pinhole blocks nonparallel and unfocused rays of light.
  • There are no lab tests for measuring visual acuity.
  • A number of digital imaging systems exist for measuring refractive error.
    • Ultrasound or Laser Interferometry can determine the length of the eye (A-Scan or Laser Biometry).
    • Autorefractors can measure myopia, hyperopia, and astigmatism using a projected light into the eye and measuring the quality of its image upon its return.
    • Corneal topography or Orbscan quantitatively and qualitatively measures the amount of curvature on the surface of the cornea that affects the bending and focusing of light on the retina.
    • Wavefront corneal analysis measures the reflected rays of light as they pass into and out of the entire optical system (cornea, lens, and retina).
  • Most corneal and refractive imaging techniques are reserved for individuals who are contemplating refractive surgical correction (intraocular lenses or excimer laser surgery “LASIK”).
Follow-Up Tests & Special Considerations
Corneal and refractive imaging are used to monitor changing refractive errors postsurgically and to follow pathologic changes such as in keratoconus.
Diagnostic Procedures/Other
  • Methods
    • Objective streak retinoscopy may be used to measure the degree of refractive error in spherocylinder correction for the proper spectacle/contact lens.
    • Antimuscarinic agents, such as cyclopentolate (Cyclogyl) or tropicamide (Mydriacyl), applied topically paralyze the ciliary body, preventing accommodation. Cycloplegic refraction can then be performed.
    • &agr;-Adrenergic agents, such as phenylephrine (Neo-Synephrine), can be used concomitantly to stimulate and dilate the pupillary dilator muscle, making measurements easier. They have no effect on cycloplegia.
  • Age-related testing
    • Newborns should be examined for general eye health; ophthalmologic evaluation is indicated for any problems discovered.
    • Vision screening should occur at each well-child visit.
    • Visual acuity testing and motility testing should be performed at ˜3 to 5 years of age or at any time that a malposition of the eyes, squinting, or difficulty following objects is observed.
    • Visual acuity must be performed by age 5 years or prior to the child entering school to screen for and pick up refractive errors early before they lead to amblyopia.
    • Visual acuity should be retested prior to obtaining a driver’s license, at age 40 years, and every 2 to 4 years until age 65 years, after which age evaluations are recommended every 1 to 2 years.
DIFFERENTIAL DIAGNOSIS
  • Corneal disease
  • Cataract
  • Retinal abnormalities
  • Diseases of the optic nerve
  • Neuromuscular disorders
  • Neurologic impairment
image TREATMENT
GENERAL MEASURES
  • Spectacle lenses (glasses)
  • Soft/hard contact lenses
  • Surgery
MEDICATION
Precaution: Antimuscarinic agents may induce acute glaucoma via acute-angle closure in susceptible individuals. The elderly and those with high hyperopia or a history of angle closure should not be tested with these agents.
First Line
No medical treatment for refractive error exists.
Second Line
Cycloplegia is sometimes used to “fog/blur” the better seeing eye in cases of amblyopia.
ISSUES FOR REFERRAL
Any individual who fails a basic screening visual acuity test should be promptly referred to an eye care professional for evaluation. Children do not outgrow refractive errors. Left untreated, refractive errors can lead to permanent visual loss.
P.901

ADDITIONAL THERAPIES
Eye exercises and visual training may improve hand-to-eye coordination and visual awareness but do not treat refractive error.
SURGERY/OTHER PROCEDURES
  • Astigmatic keratotomy with a surgical diamond blade has been performed either as an isolated procedure or as a part of the cataract surgical procedure to effectively flatten the steep corneal meridian (1,2)[C].
  • Manual astigmatic keratotomy is being superseded by the femtosecond laser whereby the surgeon applies the laser to perform the limbal corneal relaxing incisions (LRIs) instead of the surgical diamond blade (3)[C].
  • Laser-assisted in-situ keratomileusis (LASIK) was approved by the FDA in 1997. A superficial corneal flap is created with a mechanical keratome/femtosecond laser, under which the excimer laser removes a small amount of tissue corresponding to the correction of refractive error, thus reshaping the cornea. LASIK can be used for all refractive errors but not in all patients. Healing is rapid because reepithelialization is not required. Surgery is reserved for those with a stable refractive error and a cornea that is thick enough to allow safe removal of tissue. The procedure is permanent.
  • Implantable contact lens (ICL): A thin plastic lens is permanently implanted either in the anterior chamber and anchored to the iris (FDA approved in 2005, VERISYSE) or in the posterior chamber (Visian ICL, FDA approved in 2006) between the iris and the human crystalline lens. All refractive errors can be corrected. Risks include damage to the natural lens during surgery, cataract formation, and intraocular inflammation/infection.
  • Excimer laser photorefractive keratectomy: The laser photoablates corneal tissue from the central visual axis, flattening it. Following the procedure, the cornea must reepithelialize. Healing takes several months, during which there is a mild haze and blurring of vision; FDA approved in October 1995.
  • Other modifications:
    • LASIK (laser in-situ keratomileusis), which involves removing of the superficial epithelium before laser application (Epi-LASIK).
    • IntraLASIK is a femtosecond laser that is used to create the flap as an alternative to a mechanical keratome.
    • Custom ablation: uses the information from a wavefront map of the cornea and eye to tailor the ablation to remove higher orders of refractive errors
  • Older methods now superseded by LASIK:
    • Radial keratotomy: With topical anesthesia, a surgical keratome is used to incise multiple (4 to 8) radial incisions into the peripheral cornea to a depth of ≥85%. The incisions cause gaping and thus flatten the central, optical zone of the cornea. Radial keratotomy is safe and effective, corrects nearsightedness and astigmatism, but has been replaced by excimer laser surgery for its improved accuracy and stability of correction.
    • Photorefractive keratectomy: Surface corneal tissue is removed with the excimer laser to reshape the cornea, correcting nearsightedness, farsightedness, or astigmatism. Risks and disadvantages include pain, keratitis, and potential scarring. Healing requires 3 months with topical antibiotics and steroids, with associated risks of glaucoma, cataract, chronic inflammation, and scarring.
  • Several scleral expansion procedures have been under investigation to attempt to increase the space within the ciliary body by surgical means to restore lens movement and accommodation. The results have been unsatisfactory.
  • Bifocal intraocular lenses (several types have been approved by the FDA: AcrySof IQ ReSTOR, ReZOOM-AMO, TECNIS-AMO) are implanted in the posterior chamber. They increase the depth of field to allow viewing objects both at near and at distance. Side effects include decreased contrast, glare, ghosting of images, and problems with night vision.
  • Accommodative intraocular lenses (Crystalens): first FDA-approved intraocular lens; several others under clinical investigation. The accommodative intraocular lens moves in response to ciliary body movement to allow focusing at different focal lengths. Side effects are minimal. Present lenses have limited range of focus.
image ONGOING CARE
FOLLOW-UP RECOMMENDATIONS
Patient Monitoring
  • Exams recommended when starting school, entering college, with any change in vision quality, and at the age of onset of presbyopia (the need for reading glasses)
  • Individuals with any of the known risks of eye disease or conditions that could affect the eyes (e.g., diabetes, thyroid disease, hypertension, macular degeneration) need annual ocular examinations.
  • Family history of eye diseases/refractive errors necessitates annual reexam.
DIET
Although refractive error is unaffected by diet, diet has been shown to be important for retinal health. Diets high in fats, cholesterol, along with cigarette smoking or diabetes can lead to an increased risk of age-related macular degeneration (AMD), a potentially blinding disease of the retina. Diets high in vegetables, low in meats, and high in antioxidants (e.g., lutein, &bgr;-carotene, vitamins A, C, and E, zinc, and omega-3 fatty acids) have been shown to be protective for retinal degeneration. The age-related eye disease study (AREDS) formulation, which has been proven effective in numerous clinical trials, includes a specific daily amount of 500-mg vitamin C, 400 IU of vitamin E, 15 mg of &bgr;-carotene (often labeled as the equivalent of 25,000-IU vitamin A), 80 mg of zinc as zinc oxide, and 2 mg of copper as cupric oxide to prevent copper deficiency anemia and is taken with meals in two equally divided doses in the morning and evening. This therapy has been superseded by the AREDS2 formulation, which eliminated vitamin E due to its associated increased risk of lung cancer in smokers and replacing &bgr;-carotene with lutein and zeaxanthin.
PATIENT EDUCATION
  • All patients should have their eyes examined when starting school and periodically thereafter.
  • The options of eyeglasses, contact lenses, and permanent surgical correction of refractive errors can be considered.
  • Encourage aggressive control of diabetes mellitus and annual exams by an ophthalmologist. These individuals are at increased risk of blindness from their disease and should not undergo refractive corrective surgery.
PROGNOSIS
  • Good, if discovered early and corrected appropriately.
  • It is not unusual for refractive errors to be temporarily worsened during pregnancy because of hormonal changes in tear function and corneal swelling.
  • It is common that refractive errors worsen as a child ages until complete adult growth has been achieved (this is around age 18 to 20 years for girls and 20 to 25 years for boys). This is a common source of concern for parents. Reassurance that refractive change is often seen with growth spurts, especially at puberty, is advised.
  • Any refractive change that is increasing/worsening each year should prompt further investigation by an ophthalmologist.
REFERENCES
1. Kershner RM. Keratolenticuloplasty. In: Gills JP, Martin RG, Thornton SP, et al, eds. Surgical Treatment of Astigmatism. Thorofare, NJ: Slack; 1994: 143-155.
2. Grabow HB. Intraocular correction of refractive errors. In: Kershner RM, ed. Refractive Keratotomy for Cataract Surgery and the Correction of Astigmatism. Thorofare, NJ: Slack; 1994:141.
3. Abbey A, Ide T, Kymionis GD, et al. Femtosecond laser-assisted astigmatic keratotomy in naturally occurring high astigmatism. Br J Ophthalmol. 2009;93(12):1566-1569.
Additional Reading
&NA;
  • American Academy of Ophthalmology. Policy Statement: Frequency of Ocular Exams. San Francisco, CA: American Academy of Opthalmology; 2009.
  • American Academy of Ophthalmology, American Academy of Pediatrics, American Association for Pediatric Ophthalmology and Strabismus. Joint Policy Statement: Vision Screening for Infants and Children. San Francisco, CA: American Academy of Ophthalmology; 2007. http://www.aao.org/clinicalstatement/vision-screening-infants-children--2013.
  • Fax-on-Demand service of American Academy of Ophthalmology for policy statement and patient education materials: (908) 935-2761.
  • National Eye Institute. Facts About Refractive Errors. Bethesda, MD: National Eye Institute; 2009. http://www.nei.nih.gov/health/errors/errors/.asp.
Codes
&NA;
ICD10
  • H52.7 Unspecified disorder of refraction
  • H52.00 Hypermetropia, unspecified eye
  • H52.10 Myopia, unspecified eye
Clinical Pearls
&NA;
  • Vision screening should occur at each well-child visit.
  • Visual acuity testing should be performed at ˜3.5 years of age or at any time that the physician or parent is concerned about the child’s eye movements, ability to see, or track objects.
  • Children never grow out of a refractive error. All suspected refractive errors should be referred to the appropriate eye care professional for evaluation to prevent permanent loss of vision from amblyopia.