INTRODUCTION — Refractive errors and presbyopia are common, correctable causes of impaired vision throughout the world.
The normal eye creates a clear image by bending (refracting) light in order to focus it onto the retina. Refractive errors occur when a component of the eye's optical system fails to focus the optical image. Presbyopia ("aging sight") is a non-refractive error that also affects visual acuity. Presbyopia occurs when the lens loses its normal accommodating power and can no longer focus on objects viewed at arm's length or closer.
This topic will cover the epidemiology, diagnosis, and treatment of refractive errors and presbyopia in adults. Refractive errors in children and laser refractive treatment are discussed separately. (See "Refractive errors in children" and "Laser refractive surgery".)
FUNCTIONAL ANATOMY OF THE EYE — The eye's ability to focus is dependent on the eye's optical system, which consists of two refractive surfaces working in tandem: the cornea and the crystalline lens (figure 1).
The refractive power of the eye and of corrective lenses is measured in diopters, where a diopter is the reciprocal of the focal length measured in meters. The focal length is the distance from a lens to its focus or focal point.
The cornea is the more powerful refractive element, accounting for two-thirds of the eye's refractive power. The lens provides the remaining one-third, for a total refractive power of 60 diopters (D).
The cornea has a fixed amount of refractive power while the lens can vary its power by altering its shape . Accommodation is the term used to describe this change in lens shape, which can increase the power of the lens to enable the eye to focus on objects at arm's length or closer.
PATHOPHYSIOLOGY — Refractive errors are a developmental disorder. There are causes of adult-onset refractive errors, such as cataracts, diabetes, and alteration in shape of the eye, which are not discussed in this section. The developing eye in childhood through early adulthood maintains focus on the retina by a process known as emmetropization. This process involves detection of a blurred image on the retina with subsequent diffusion of signals into the outer layers of the eye, including the choroid and sclera (figure 1). This results in alteration of the size of the eye in order to maintain clear vision throughout development . Emmetropization is accomplished by modulating scleral growth and choroidal thickness. With refractive errors, the emmetropization process goes awry for reasons that remain largely unknown.
Cellular and/or physiologic mechanisms are thought to cause retinal defocusing with resulting altered axial length and myopia during the developmental process. In addition, eye growth is sensitive to an individual's visual experience. The mechanisms thought to cause retinal defocusing include [3,4]:
●Form deprivation, which can lead to alterations in the ocular lens system, with elongation of the visual axis
●Scleral stretching (secondary to increased intraocular pressure)
●Autonomic deficits during accommodation
DEFINITIONS — Emmetropia (normal refraction) describes the refractive state in which parallel light rays emanating from an object located 20 feet or more from the eye form a focused image on the retina of an eye that has not accommodated. (figure 2 and movie 1). Ametropia refers to the refractive state in which the image is not focused on the retina.
Abnormal refraction (ametropia) — Refractive disorders are ametropic states in which the eye is unable to bring light rays emanating from an object viewed from a distance into clear focus on the retina, without the aid of a refractive device (eg, spectacles, contact lenses). Myopia, hyperopia, and astigmatism are the three ametropic states. They are caused by abnormalities in the axial length (distance from the posterior corneal surface to the retina) of the eye or in the shape of the cornea.
Myopia — Myopia ("nearsightedness") is a common refractive disorder in which the axial length of the eye is either too long or the refractive power of the eye's optical system is too great (generally due to corneal protrusion resulting in steep corneal curvature). The image is focused in front of the retina (figure 3 and movie 2). This results in blurred distance vision unless optical correction is achieved with a refractive device. A refractive device can provide a more concave surface which reduces the excessive focusing power of the eye's optical system (movie 3).
Hyperopia — Hyperopia ("farsightedness") is a refractive disorder in which the axial length of the eye is too short or the power of the eye's optical system is insufficient (due to a flat cornea) to produce a focused image on the retina. The image is focused behind the retina (figure 4 and movie 4). Hyperopia is corrected with a refractive device that provides a more convex refracting surface in order to increase the deficient focusing power of the eye's optical system (movie 5).
Astigmatism — Astigmatism ("lack of a pinpoint" in Greek) refers to the refractive condition in which a warped corneal surface causes light rays entering the eye along different planes to be focused unevenly (movie 6). The patient reports blurred vision at all viewing distances. Unlike myopia and hyperopia, which are corrected by spectacles containing a spherical surface, astigmatism is corrected by spectacles containing a cylindrical optical surface (movie 7). A spherical lens is a device with optical symmetry which can either converge or scatter light rays, while a cylindrical lens does not have optical symmetry. One meridian (a closed curve on the surface of a sphere) in the cylindrical lens is more curved than the opposite meridian. Soft (toric) or rigid gas permeable contact lenses can also be used to correct astigmatism. Contact lenses are discussed in detail separately. (See "Overview of contact lenses".)
Presbyopia — Presbyopia ("aging sight") is a non-refractive error that also affects visual acuity. Presbyopia occurs when the lens loses its normal accommodating power and can no longer focus on objects viewed at arm's length or closer. As such, it is not considered an ametropic state, but rather one in which the normal physiologic function of lens accommodation has been lost.
In youth, the eye is able to easily accommodate or increase the curvature of the lens by contracting the ciliary muscle (figure 1). The ciliary muscle surrounds the lens and is connected to it by zonular fibers. The normally taut zonular fibers stretch the lens and keep it from assuming a fully rounded state. The desire to focus on reading material automatically stimulates ciliary smooth muscle contraction, which loosens the zonular fibers and allows the lens to become more rounded. In the fully rounded state, the lens provides the additional refractive power needed to bring reading material into focus.
During the natural aging process, the crystalline lens loses its elasticity and therefore its ability to become more rounded when the zonular fibers loosen their grip. Presbyopia usually begins after age 40 when patients start to appreciate the inability to focus on objects at reading distance. In patients with presbyopia, the eye's focusing power for reading is lost progressively and fully by age 65 years.
EPIDEMIOLOGY — The prevalence and risk factors associated with myopia are well-described. The epidemiologic data for other refractive errors have been less studied.
Prevalence — Refractive disorders are common, affecting approximately one-third of persons age ≥40 years in the United States and Western Europe . Worldwide, a total of 123 million people are estimated to be visually impaired from uncorrected refractive errors, of whom approximately four million are blind (as defined by best-corrected visual acuity <20/400) .
Prevalence rates vary depending on the type of refractive error, with myopia and astigmatism being the most common.
●Myopia – The global prevalence of myopia was estimated at 23 percent in 2000 and is increasing , including in the United States and Europe [8,9]. In the United States, the prevalence of myopia from between 1971 and 1972 to between 1999 and 2004 nearly doubled among persons age 12 to 54 years (25 versus 42 percent) . In Europe, myopia prevalence is higher among more recent birth decades; age-standardized myopia prevalence is 18 percent in those born between 1910 and 1939 compared with 23.5 percent among those born between 1940 and 1979 . This difference, in part, may reflect changes in education and consequent demands for close-up tasks such as reading, population shifts, and changes in the classification of myopia. (See 'Risk factors' below.)
The prevalence of myopia decreases with increasing age, from about 40 percent in adults age 20 to 59 years to about 20 percent in adults age ≥60 years . The decrease in myopia may be secondary to progressive cataract formation. In contrast to childhood-onset myopia where the degree of myopia tends to worsen rapidly, the degree of myopia stabilizes in late adolescence and is subject to a slow rate of myopic change after age 20.
Prevalence rates of myopia also vary by race and ethnicity. In the United States, White Americans have the highest rates of myopia, followed by African Americans, then Hispanic Americans . Worldwide, the prevalence of myopia is especially high among Asian persons [11-13].
●Hyperopia – In contrast to myopia, the prevalence of hyperopia increases with age, from 1 to 2 percent in those age 20 to 59 years to 10 percent in those age ≥60 years .
●Astigmatism – Similar to hyperopia, the prevalence of astigmatism increases with age from approximately 25 percent among adults age 20 to 59 to 50 percent among adults age ≥60 years .
●Presbyopia – It is estimated that, in 2015, there were 1.8 billion people with presbyopia worldwide. Approximately 826 million of those had uncorrected or undercorrected vision . In the United States, presbyopia is the most common cause of visual impairment due to aging of the "baby boomer" generation, the 76 million Americans born between 1946 and 1964 .
●Myopia — The development of myopia is associated with higher education levels and intelligence test scores, as well as occupations requiring close-up tasks of high accommodative demand (eg, reading, writing, computer work) . Known causes include:
•Genetic – Multiple studies have shown that hereditary factors are associated with the development of myopia [16,17]. Lack of complete concordance between myopia and monozygotic twins, as well as between generations in families, suggests a polygenic inheritance model with influence of gene-environment effects .
•Reading – An association in children between myopia and prolonged reading or reading at close range is well-documented [18,19]. Continuous hyperopic defocusing that occurs during prolonged periods of reading may lead the emmetropization mechanism to increase the axial length of the eye, leading to myopia .
•Medications – Myopia can develop rapidly following use of certain medications. Sulfa-derived medications (eg, sulfamethoxazole, topiramate) and diuretics (eg, furosemide, acetazolamide) can induce transient myopia through forward displacement and thickening of the lens by relaxation of the zonules . Cholinergic medications (eg, neostigmine, pilocarpine) can also result in accommodative spasm with subsequent transient myopia. Withdrawal of the medication usually results in complete resolution of myopia .
•Diabetes mellitus – Transient refractive changes in patients with diabetes mellitus are well-documented [23,24]. Alterations in serum osmolarity secondary to changes in blood glucose levels can cause an influx of osmotic fluid into the lens with subsequent lens swelling and a transient increase in refractive power or myopia. As a result, patients with type I or type II diabetes often present with transient blurred vision, particularly those with poorly controlled glucose.
The prevalence of myopia is higher in diabetic patients compared with non-diabetic patients . Patients with diabetes who have poorly controlled hemoglobin A1C are also more likely to have myopia due to lenticular changes.
•Trauma – Ocular trauma can cause forward displacement of the lens with subsequent myopia.
Scleral laceration repair and use of scleral encircling elements in retinal detachment repair can temporarily deform the globe, inducing astigmatism and myopia. In cases where this deformation continues postoperatively, the increased axial length leads to persistent myopia .
•Excessive accommodation – Patients with pathologically excessive accommodation may have "accommodative spasm." The symptoms may include double vision and myopia. Anxiety and patient behaviors (forced excessive convergence by focusing on a near object) are common causes, but traumatic brain injury and parasympathomimetic medications can also lead to excessive accommodation [27,28].
•Increased intraocular pressure – Although the rate of myopia progression may be associated with increased intraocular pressure , data from the Correction of Myopia Evaluation Trial have shown no correlation between intraocular pressure and myopic progression .
•Maternal factors – Myopia is associated with greater maternal age at birth and maternal smoking during pregnancy .
•Light exposure – The role of light exposure in the development of myopia is unclear. After adjusting for confounders, children who spend more time in outdoor activities have a lower prevalence of myopia [31,40,41]. Persons who are near-sighted have higher serum melatonin levels, suggesting a role for light exposure and circadian rhythm in the myopic growth mechanism . Results are mixed as to whether night lights (dim light left on during sleep) are associated with development of myopia [32,33].
●Hyperopia — Several risk factors may lead to a posterior shift of the crystalline lens or shortening of the eye's axial length and result in hyperopia. These include ocular trauma, mass effect of an orbital tumor posterior to the retina, scleral inflammation with subretinal thickening, and surgical removal of the crystalline lens without replacing it with a synthetic intraocular lens.
Anticholinergic medications (eg, oxybutynin, scopolamine) exert parasympatholytic effects on the ciliary muscle, decreasing accommodation and inducing hyperopia. Patients treated for hyperglycemia who have a rapid decrease in blood glucose levels experience transient hyperopia that can last for several weeks [34,35].
●Astigmatism — Risk factors for astigmatism are largely unknown but may be related to genetic and/or developmental factors .
Daily functioning — Several observational studies have shown that refractive errors are associated with limitations in instrumental activities of daily living (IADLs), falls, decreased ability to drive or work, and depression [43-47]. In one cohort study of noninstitutionalized older adult subjects, visual impairment was associated with limitations in ADLs, one-fifth of which were estimated to be preventable by use of best optical correction .
Although visual correction may improve ability to read and perform other tasks, it does not appear to reduce risk or rate of falls among older adults. In one randomized trial, visual correction including new eyeglasses, glaucoma management, and cataract surgery led to an increased rate of falls compared with usual care , possibly related to an adjustment period needed to adapt to new corrective eyeglasses, and a less sedentary lifestyle.
Associated eye disorders — Refractive errors have also been associated with other ocular pathology [49-54].
High degrees of refractive error (myopia >6.0 diopters [D], hyperopia >3.0 D, astigmatism >3.0 D) are associated with pathologic ocular changes . Highly myopic patients have an increased incidence of retinal thinning, peripheral retinal degeneration, retinal detachment , cataract , and glaucoma [52,53]. Among patients with myopic refractive errors of 1 to 3 D, more than half of non-traumatic retinal detachments are attributable to myopia . (See "Retinal detachment" and "Cataract in adults" and "Open-angle glaucoma: Epidemiology, clinical presentation, and diagnosis" and "Angle-closure glaucoma".)
Hyperopia is associated with ocular pathology but to a lesser extent than myopia. Hyperopia is associated with angle-closure glaucoma, age-related macular degeneration, and cataracts [54,55].
SCREENING AND DIAGNOSTIC TESTS — There is no consensus on how and when to perform routine screening of visual acuity. Different organizations have different recommendations:
●The US Preventive Services Task Force states that there is insufficient evidence to assess the benefits and harms of screening for impaired vision in adults aged 65 and older without vision problems .
●The Canadian Task Force on the Periodic Health Examination recommends against screening in primary care settings for impaired vision in community-dwelling adults aged 65 and older without risk factors for impaired vision .
●The American Academy of Ophthalmology (AAO) recommends comprehensive eye examinations by ophthalmologists for adults with no symptoms or risk factors , although there is little evidence supporting this recommendation.
Visual screening tests are recommended in children, which is discussed separately. (See "Vision screening and assessment in infants and children", section on 'Vision screening'.)
There are a variety of visual charts to examine visual acuity, although few data are available showing superiority of one method over another . The Snellen eye chart is considered one of the clinical standards for evaluating visual acuity and can be used in the primary care setting (figure 5) . Patients should have their vision examined at 20 feet and while reading. While there is no explicit evidence regarding appropriate cutoffs for referral, patients with unexplained vision loss in either eye should be referred to an eye specialist for further evaluation.
Although refractive errors may be suspected on history of visual blurriness or by visual acuity testing, diagnosis of all refractive disorders is confirmed with use of a phoropter by the eye specialist. A phoropter is an instrument containing different lenses used to bring the focus of objects onto the retina (picture 1). In addition to diagnosis, a phoropter is also used to measure severity of refractive error for prescribing corrective lenses. Automated devices may also allow assessment of refractive errors which may provide a technological advancement in addressing global blindness .
A retinoscope and pinhole occluder are additional devices that can be used to assess refractive error by an eye specialist. A retinoscope is an objective measure of refractive error that is not dependent upon patient response (picture 2). The retinoscope projects a beam of light into the patient's eye through the pupil. Through the peephole in the scope, the observer sees a light reflex coming from the patient's pupil. By observing the behavior of the reflex under certain conditions, the observer can objectively determine the refractive error of the patient's eye.
A pinhole occluder may also be used as a screening tool for refractive errors (picture 3). The occluder is a simple way to focus light, as in a pinhole camera, temporarily removing the effects of refractive errors such as myopia. Because light passes only through the center of the eye's lens, defects in the shape of the lens (errors of refraction) have no effect while the occluder is used. This can allow estimation of the maximum improvement in a patient's vision that can be attained by lenses to correct errors of refraction. Squinting works similarly to a pinhole occluder, by blocking light through the outer parts of the eye's lens.
TREATMENT — The decision to treat refractive disorders depends on the individual patient's symptoms and needs. Treatment is aimed to improve visual acuity, visual comfort (eg, visual distortion, polycoria, decreased stereopsis), and other visual function (eg, color discrimination, motion detection, peripheral vision).
First-line treatments include corrective lenses, such as glasses and contact lenses, or refractive surgery.
Corrective lenses — Corrective lenses work by altering the overall power of the lens system of the eye. Concave lenses for myopia act to weaken the lens system, or reduce its power in diopters. Convex lenses for hyperopia act by strengthening the lens system, or increasing its power in diopters. Cylindrical lenses are used to correct astigmatism.
Presbyopia is treated with use of convex lenses as "reading glasses" or in combination with a correction for distance viewing in which the lenses may be lined (bifocals, trifocals) or unlined (multifocals, progressive).
Spectacles, or glasses, are the most common method of treating refractive errors, followed by contact lenses. (See "Overview of contact lenses".)
Interpreting the prescription — By convention, lenses that increase divergence and decrease the eye's focusing power are considered "minus" lenses; lenses that increase convergence and increase focusing power are considered "plus" lenses. Accordingly, myopic refractive error is corrected with concave ("minus", "divergent") lenses (movie 3), and hyperopia is corrected with convex ("plus", "convergent") lenses (movie 5). Astigmatism is corrected by cylindrical lenses which even out the eye's focusing power (movie 7).
In spectacle prescription notation, the first number is the refractive power, also known as the sphere. A minus number indicates a concave lens; a plus number indicates a convex lens. The second number denotes the cylinder, which corrects any astigmatic component. The "x" that follows the astigmatism power denotes the axis (in radial degrees) of the cylinder. Spectacle prescriptions are then written in increments of 0.25 diopters (D). For example, a prescription for a patient with myopia and astigmatism may read -4.50 + 2.50 x 090 and a prescription for a patient with hyperopia and astigmatism may read +3.25 + 1.50 x 180.
OD signifies the right eye (oculus dexter in Latin). OS signifies the left eye (oculus sinister). OU signifies both eyes (oculi uterque).
Surgical correction — The most common surgical procedure performed to correct refractive errors is laser in situ keratomileusis (LASIK). Overall outcomes with LASIK are favorable, leading to rapid recovery of vision and minimal pain after surgery. Other corneal surgical procedures include photorefractive keratectomy (PRK), and laser epithelial keratomileusis (LASEK). Laser refractive procedures are discussed in detail elsewhere. (See "Laser refractive surgery".)
Intraocular lens implants, similar to those used to correct vision with cataract surgery, can correct high degrees of myopia. However, intraocular lens implants can result in cataract formation, increased intraocular pressure, and damage to the cornea over time. Thus, lens implants are not routinely recommended for correction of refractive errors.
Eye drops: Pilocarpine — A modified ophthalmic solution of pilocarpine hydrochloride is available in the United States for treatment of presbyopia in adults ages 40 to 55 . Pilocarpine causes constriction of the pupil, resulting in an increase in the depth of field and improvement in near and intermediate visual acuity. The onset of effect occurs within 15 minutes and last for up to six hours. Headache is the most common side effect, and patients may experience difficulty changing focus from near vision to distance vision. There is also a small increased risk of retinal tears and retinal detachment, especially in patients with pre-existing retinal disease, and examination of the retina prior to treatment initiation should be considered . Patients should be advised to avoid driving at night or performing other hazardous tasks in poorly illuminated spaces while the drug is active.
Evidence for efficacy comes from two trials, GEMINI-1 and GEMINI-2, in which pilocarpine resulted in improved ability to read at low light compared with placebo [66,67]. Although no cases of retinal tears or detachments were reported in the trials, rare cases of retinal detachment have been reported with other miotics when used in susceptible individuals and those with preexisting retinal disease. Patients should be advised to seek immediate medical care with sudden onset of vision loss .
INFORMATION FOR PATIENTS — UpToDate offers two types of patient education materials, "The Basics" and "Beyond the Basics." The Basics patient education pieces are written in plain language, at the 5th to 6th grade reading level, and they answer the four or five key questions a patient might have about a given condition. These articles are best for patients who want a general overview and who prefer short, easy-to-read materials. Beyond the Basics patient education pieces are longer, more sophisticated, and more detailed. These articles are written at the 10th to 12th grade reading level and are best for patients who want in-depth information and are comfortable with some medical jargon.
Here are the patient education articles that are relevant to this topic. We encourage you to print or e-mail these topics to your patients. (You can also locate patient education articles on a variety of subjects by searching on "patient info" and the keyword(s) of interest.)
●Basics topics (see "Patient education: Age-related vision loss (The Basics)" and "Patient education: Open-angle glaucoma (The Basics)" and "Patient education: Presbyopia and refractive errors (The Basics)")
SUMMARY AND RECOMMENDATIONS
●Refractive errors occur when a component of the eye's optical system fails to focus an optical image onto the retina. The optical system consists of two refractive surfaces working in tandem: the cornea and the crystalline lens (figure 1). (See 'Functional anatomy of the eye' above.)
●Refractive errors include myopia (nearsightedness), hyperopia (farsightedness), and astigmatism (abnormal corneal shape). Presbyopia (aging sight) is a non-refractive error that occurs when the lens loses its normal accommodating power. (See 'Definitions' above.)
●Abnormalities in refraction are common, affecting one-third of persons age ≥40 years in the United States and Western Europe. Worldwide, uncorrected refractive errors are a common cause of poor visual acuity and blindness. (See 'Epidemiology' above.)
●Assessment of visual acuity in the primary care setting can be performed with a standard Snellen chart. Patients with visual acuity less than 20/25 in either eye have impaired visual acuity and should be referred to an eye specialist for further evaluation. (See 'Screening and diagnostic tests' above.)
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