Refractive Disorders

John F. Doane, Scot Morris, Andrea D. Border, and James A. Denning

CHAPTER CONTENTS

Pathologic Ocular Changes and High Refractive Errors

Refractive disorders, also known as refractive errors, are defined as conditions in which the eye does not refract, or bend, incident light to perfectly focus it onto the retina for the best possible visual acuity. The goal of refractive surgery is the elimination of this optical error for excellent unaided visual acuity.

The total refractive power of the eye is the sum of the refractive surfaces of the eye, which include the cornea (the anterior surface, stroma, and posterior surface), crystalline lens surfaces and substance, interface media (e.g., air, the corneal tear film, and intraocular fluid media), and distance separating the individual components of the eye. When the eye optimally focuses incident light onto the foveolar retina, a person has emmetropia or no refractive error (Figs. 2-1 and 2-2).

Figure 2-1 Schematic illustrating the condition of emmetropia. The optical elements are set for perfect distance vision without the aid of eyeglasses or contact lenses.

AMETROPIA

Ametropia is the condition in which the eye does not optimally focus incident light onto the foveolar retina. Types of ametropias can be classified by measuring the focusing power of the affected eye or comparing the relative powers of an affected individual’s two eyes.

Figure 2-2 Schematics illustrating the five basic refractive states. Upper left: In emmetropia, the patient’s refractive error is piano. The eye is considered to be spherical, meaning that light bends, or refracts, at all meridians to the same focal plane. In this case, the focal plane is directly on the retina, thus providing excellent unaided vision. Upper right: In spherical myopia, either the optical elements (the cornea and crystalline lens) are too refractively powerful or the eyeball is too long, and light is focused in front of the retina (green focal line). Center: In spherical hyperopia, either the focusing elements are too weak or the eyeball is too short, and the focal point for the eye is behind the retina (green focal line). Lower right: In compound myopic astigmatism, the two major meridians focus in front of the retina and, if regular, are oriented 90 degrees apart. The meridian with the greatest diopter power or ability to bend or refract light (green) is more anterior than the less powerful meridian (yellow). Lower left: In compound hyperopic astigmatism, the two major meridians create focal points behind the retina. In this example, the strongest meridian (green) focuses in front of the weaker meridian (yellow).

Whether the refractive error of the eye is axial or refractive depends on the length of the eye (axial) relative to normal length or the curvature of the cornea (refractive) relative to average curvature. In axial ametropia, the eye is too long or short for its given refractive components. In refractive ametropia, the power of the refractive components (cornea and crystalline lens) is too strong or weak for the length of the eye.

Primary Ametropia

congenital
hereditary
environmental
undetermined

Secondary Ametropia

trauma (injury of the cornea leading to induced astigmatism or change in curvature, position, or clarity of the cornea or crystalline lens)

full thickness corneal transplantation (induction of regular or irregular astigmatism, myopia, or hyperopia)

removal of the crystalline lens (may cause change in corneal curvature at site of surgical wound and result in astigmatism)

thickening of crystalline lens during aging (leads to more myopic condition)

clouding or cataract change in crystalline lens (but an intraocular lens may bring the eye close to emmetropia or the desired refraction; small incision surgery results in astigmatically neutral operations)
scleral buckling procedures for retinal detachment (induction of myopia if the eye is compressed enough to cause axial elongation in anterior-posterior orientation)
metabolic imbalance (diabetes mellitus leads to a more myopic state and/or fluctuation of refractive error, but condition resolves with physiologic state)

Myopia and Hyperopia

In myopia, or nearsightedness, light focuses in front of the retina (see Fig. 2-2). Conversely, in hyperopia, or farsightedness, light focuses behind the retina (see Fig. 2-2).

Astigmatism

Astigmatism is present when the refractive status of the eye varies depending on which meridian is evaluated. In effect, the focus points for the different meridians do not coincide with each other. Astigmatism may occur with myopia and hyperopia. The total astigmatism diopter power of the eye typically is measured by refraction.

Causes

corneal astigmatism
crystalline lens or lenticular astigmatism
disparity of the line of sight to the optical axis

Astigmatism may be classified according to where one or both major meridians focus light.

compound myopic astigmatism, in which both major meridians focus in front of the retina (see Fig. 2-2)
compound hyperopic astigmatism, in which both major meridians focus behind the retina with accommodation relaxed (see Fig. 2-2)
mixed astigmatism, in which one major meridian focuses in front of the retina (myopic component) and the other major meridian focuses behind the retina (hyperopic component) (Fig. 2-3)

Astigmatism is also classified according to the regularity of astigmatism.

Figure 2–3 Mixed astigmatism. Upper middle: The two major meridians have opposite powers, one myopic (yellow) and the other hyperopic (green). Lower right: In myopic spherical equivalent (SE) mixed astigmatism, the absolute power of the myopic meridian (yellow) is greater than that of the hyperopic meridian (green). Lower left: In hyperopic SE mixed astigmatism, the absolute power of the hyperopic meridian (green) is greater than that of the myopic meridian (yellow).

Regular Astigmatism

present when the primary (major) strongest and weakest meridians of the eye are 90 degrees apart or at right angles to each other (Fig. 2-4)
correctable with contact lenses, glasses, or refractive surgery

Irregular Astigmatism

primary meridians not at right angles to each other and/or the cornea-air interface is not smooth or symmetrical
less amenable to correction with contact lenses or glasses
best diagnosed by using a hard contact lens, the rings of a photokeratoscope, or the mires of a manual keratometer to ascertain distortion or irregularity (If a patient sees better with a hard contact lens than with eyeglasses, suspect irregular astigmatism. Rule out defects or opacities as a cause of lost best corrected visual acuity.)
may be “regular irregular” (prime meridians of curvature that are not 90 degrees apart, sometimes called nonorthogonal, or meridians that are asymmetrical in power across the visual axis) or “irregular irregular” (nonmeasurable fluctuations to the surface that cannot be compensated by a lens)

Presbyopia

Presbyopia is a condition in which the crystalline lens of the eye loses its elasticity and cannot accommodate (change shape) to facilitate near-vision tasks.

occurs in all patients as they age, typically becoming noticeable after age 40
reduces the range of clear vision so that no one prescription lens provides clear vision at all distances simultaneously
eventually leads to the need for bifocal eyeglasses

Figure 2-4 Regular astigmatism. The two major meridians are 90 degrees apart from each other and are represented on corneal topography by a typical bow-tie appearance with the long axis oriented along the 148-degree meridian.

Anisometropia and Antimetropia

Anisometropia is the condition in which the two eyes of a patient have differing refractive powers. With antimetropia the two eyes of an individual patient are myopic and hyperopic, respectively.

PATHOLOGIC OCULAR CHANGES AND HIGH REFRACTIVE ERRORS

Patients with significant myopia have an increased incidence of some conditions.

retinal thinning
peripheral retinal degeneration
retinal detachment
early cataract development

People with extensive hyperopia have an increased incidence of angle-closure glaucoma. Several factors must be considered regarding the incidence of refractive errors. Heredity, the structure of the eye (e.g., myopic eyes have a large axial length and hyperopic eyes have a smaller one), and environmental factors seem to have the strongest influence on the incidence of refractive disorders.

Genetics

Twin and pedigree studies reveal that heredity is a dominant factor in determining refractive errors and disorders.

Environment

Reading and close work have been studied extensively as contributing factors to the induction and/or progression of myopia. Animal experiments have shown an association between onset of myopia and confined living space.

Age and the Structure of the Eye

Longitudinally over time, the refractive error of an individual is not static. There tends to be a natural progression of refractive error as the body changes from infancy through adulthood. Infants and young children tend to be more hyperopic but progress to less hyperopia or frank myopia as they reach adolescence and adulthood because the eye lengthens as it physically matures. Roughly 25% of the adult U.S. population is myopic, but less than 1% of this group has greater than 10 D of myopia. About half of the entire U.S. population over 50 years of age is hyperopic, and of people with myopia or hyperopia, roughly 30 to 40% have concomitant astigmatism.

Ethnicity

Several ethnic groups (Chinese, Japanese, Egyptians, Germans, Eastern European Jews, and Middle Eastern peoples) tend to have a significantly higher prevalence of myopia.

Gender

Subtle differences between the sexes have been reported.

a slightly higher overall prevalence of myopia in males
a higher prevalence of myopia that is over –6.00 D in females
no difference between the sexes as infants
less difference in children at age 12 than in adults
refractive changes associated with ocular maturation 2 to 3 years earlier in girls than in boys.

CLINICAL INTERVENTION

The clinician has three options for improving the visual function of a patient with ametropia.

eyeglasses
hard or soft contact lenses
surgery

Ento Key