BASICS

DESCRIPTION

• Refractive error exists when parallel light rays from a distant object do not come to focus on the retina of an unaccommodating eye. In refractive error the focusing power of the unaccommodating eye does not match its axial length.

• Myopia: The focusing power is very strong and the image of a distant object is focused in front of the retina.

• Hyperopia: The focusing power (cornea and lens) is insufficient to bring parallel light rays to focus on the retina in the unaccommodated eye and the distant object is focused behind the retina.

• Astigmatism: The curvature of the corneal surface is not spherical. In regular astigmatism one meridian has a maximal curvature while the meridian 90° away has a minimum curvature. (Explain to patients as a circle cut from the side of a football compared to the side of a basketball.) In an eye with astigmatism, light rays from an object are focused with an amount of power determined by which meridian of the cornea they pass through. Irregular astigmatism is a pathologic condition most commonly associated with keratoconus or corneal scarring where the principal meridians are not 90° apart.

EPIDEMIOLOGY

Incidence

Clinically significant refractive error increases with age. ∼5% of children 0.5 to 6 years of age need glasses (1). 50% of the U.S. population 20 years and older require correction (2).

Prevalence

• Of the 50% of those aged 20 and older in the USA requiring correction

• Myopia 33.1%

• Hyperopia 3.6%

• Astigmatism 36.2%

RISK FACTORS

Genetics

Strong heritability. Up to 90% in twin studies (3)

GENERAL PREVENTION

Many theories have been advanced to suggest environmental factors involved in myopia development and progression from increased reading and near work to use of children’s night-lights. Some suggest accommodative exercise, under-correcting myopes, or use of plus lenses for reading. None are generally accepted.

PATHOPHYSIOLOGY

Most refractive error (between –4.00 and +4.00) occurs in eyes with axial length and focal power considered within normal range. Greater refractive error is usually associated with abnormal axial length.

COMMONLY ASSOCIATED CONDITIONS

• High myopia:

– Down’s syndrome

– Stickler’s syndrome

– Ehlers–Danlos syndrome

– Retinopathy of prematurity

– Marfan’s syndrome

– Weill Marchesani syndrome

DIAGNOSIS

HISTORY

• Symptoms depend on the type and degree of error and patient age. Any refractive error can cause headaches and asthenopia (eye strain). Specifically:

– Myopia: Distance blur, clearer at near. (The higher the myopia the more distance is blurred and the closer is the near focus.)

– Hyperopia: Distance is clearer than near. (Hyperopia can be corrected by accommodation.) If accommodative amplitude is greater than degree of hyperopia, distance and near vision may be clear. In older patients, especially if presbyopic, or high degrees of hyperopia, both distance and near are blurred.

– Astigmatism: Distance and near blur.

PHYSICAL EXAM

• Distance and near unaided acuity will usually suggest type of error.

• Improvement with pinhole indicates that decreased vision is refractive as opposed to other pathology.

• Uncorrected hyperopia can be associated with esotropia (accommodative esotropia), especially with high degrees of hyperopia or high AC/A ratio.

DIAGNOSTIC TESTS & INTERPRETATION

Diagnostic Procedures/Other

• Subjective refraction: Using a phoropter or loose trial lenses various lens combinations are introduced before the eye to determine the prescription giving best-corrected visual acuity (BCVA). Starting point is usually the habitual prescription or determined by an objective test such as follows:

– Retinoscopy

– Automated refractor

– Keratometry – measures corneal curvature allowing an estimate of the astigmatic error

• Jackson cross cylinder (JCC) test: Used to subjectively measure the axis and power of the astigmatic error. A JCC is a spherocylindrical lens with spherical equivalent power of plano.

• Manifest refraction: A “dry” refraction performed without drops. More natural state of the eye, but the patient can accommodate causing overcorrection of myopes and under correction of hyperopes. The amount of hyperopia found with a dry refraction is the “manifest hyperopia.”

• Cycloplegic refraction: A “wet” refraction done with cycloplegic drops such as cyclopentolate or tropicamide which paralyzes the ciliary body preventing accommodation. Not the normal physiologic state of the eye. The additional hyperopia uncovered with a wet refraction is the “latent hyperopia.”

TREATMENT

ADDITIONAL TREATMENT

General Measures

• Correcting myopia: A patient with myopia can generally be given the least minus power while still maintaining BCVA. With manifest refraction must be careful not to over minus young myopes who can accommodate and “eat minus.” Accommodation makes an image appear smaller and darker, and may be erroneously perceived as clearer. With cycloplegic refraction –0.25 D can be added to adjust for return of ciliary body tone (4)[C].

• Correcting hyperopia: A happy patient with uncorrected hyperopia without complaints of blur, fatigue, or headaches may not yet need correction. If symptomatic, can generally be given maximum plus power that still maintains BCVA found by manifest refraction. Amount of latent hyperopia to be corrected depends on age, symptoms, and needs. As a rule, up to half of accommodative amplitude can be used for extended periods comfortably. As accommodative amplitude decreases with age more plus will be needed to maintain comfortable vision even though total amount of hyperopia present remains unchanged. Children with esotropia should be given full cycloplegic refraction. If reducing plus, must reduce plus equally in each eye to keep accommodation balanced.

• Correcting astigmatism: Children need full astigmatic correction to prevent amblyopia. They will usually adapt to large amount of new astigmatic correction (5)[C]. Adults are less adaptable. If no previous cylinder correction has been worn glasses may be rejected. New oblique cylinder is especially hard to accept. If wearing cylinder correction, difficulty may be experienced in adapting to increased power or axis change. If reducing cylinder power found on refraction in order to increase chance of acceptance, the spherical equivalent should be maintained.

SURGERY/OTHER PROCEDURES

• Laser correction: Lasik and PRK for correcting up to ∼10 D of myopia, 6 D of hyperopia, and 4 D of astigmatism

• Intrastromal corneal rings: Used to correct mild to moderate degrees of myopia to approximately 3 D and astigmatism up to 1 D. Advantage of being reversible.

• Clear lens extraction and phakic IOLs: Alternative to laser correction for high degrees of refractive error or thin corneas.

ONGOING CARE

FOLLOW-UP RECOMMENDATIONS

• If glasses rejected:

– Verify glasses made to prescription

– Check patient and spectacle PD

– Check frame fit looking for vertex distance, position of optic centers, pantoscopic and face form tilt

– Double check refraction especially if prescribed from cycloplegic

– Remake Rx closer to habitual

– Decrease or eliminate cylinder power

– Rotate axis closer to habitual or toward 90/180

PROGNOSIS

• Myopia generally progresses through teenage years and then stabilizes. Later, myopic shift can occur as nuclear sclerosis develops.

• Hyperopia generally lessens through childhood, but hyperopic symptoms can develop as greater near demands are placed on school children and as accommodative amplitude decreases into adulthood.

• Astigmatism generally changes less through the years. There is a slow increase in with-the-rule astigmatism in the aging eye.

COMPLICATIONS

• Myopia:

– Retinal detachment

– Posterior vitreous detachment

– Open angle glaucoma

– Myopic degeneration with choroidal neovascular membrane

• Hyperopia:

– Amblyopia

– Strabismus

– Narrow angle glaucoma

• Astigmatism:

– Keratoconus

– Amblyopia

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