Emmetropia is absence of refractive error, and ametropia is the presence of refractive error.
The loss of accommodation that comes with aging to all people is called presbyopia (Table 21–2). A person with emmetropic eyes (no refractive error) will begin to notice inability to read small print or discriminate fine close objects at about age 44–46. This is worse in dim light and usually worse early in the morning or when the subject is fatigued. These symptoms increase until about age 55, when they stabilize but persist.
Table 21–2. Table of Accommodation ||Download (.pdf)
Table 21–2. Table of Accommodation
|Age (Years)||Mean Accommodation (Diopters)|
Presbyopia is corrected by use of a plus lens to make up for the lost automatic focusing power of the lens. The plus lens may be used in several ways. Reading glasses have the near correction in the entire aperture of the glasses, making them fine for reading but blurred for distant objects. Half-glasses can be worn to abate this nuisance by leaving the top open and uncorrected for distance vision. Bifocals do the same but allow correction of other refractive errors. Trifocals correct for distance vision by the top segment, the middle distance by the middle section, and the near distance by the lower segment. Progressive power (varifocal) lenses similarly correct for far, middle, and near distances but by progressive change in lens power rather than stepped changes.
When the image of distant objects focuses in front of the retina in the unaccommodated eye, the eye is myopic, or nearsighted (Figure 21–21). If the eye is longer than average, the error is called axial myopia. (For each additional millimeter of axial length, the eye is approximately 3 diopters more myopic.) If the refractive elements are more refractive than average, the error is called curvature myopia or refractive myopia. As the object is brought closer than 6 m, the image moves closer to the retina and comes into sharper focus. The point reached where the image is most sharply focused on the retina is called the “far point.” One may estimate the extent of myopia by calculating the reciprocal of the far point. Thus, a far point of 0.25 m would suggest a 4-diopter minus lens correction for distance. The myopic person has the advantage of being able to read at the far point without glasses even at the age of presbyopia. A high degree of myopia results in greater susceptibility to degenerative retinal changes, including retinal detachment.
Spherical refractive errors as determined by the position of the secondary focal point with respect to the retina.
Concave spherical (minus) lenses are used to correct the image in myopia. These lenses move the image back to the retina.
Hyperopia (hypermetropia, farsightedness) is the state in which the unaccommodated eye would focus the image behind the retina (Figure 21–21). It may be due to reduced axial length (axial hyperopia), as occurs in certain congenital disorders, or reduced refractive error (refractive hyperopia), as exemplified by aphakia.
Hyperopia is a more difficult concept to explain than myopia. The term “farsighted” contributes to the difficulty, as does the prevalent misconception among laymen that presbyopia is farsightedness and that one who sees well far away is farsighted. If hyperopia is not too great, a young person may obtain a sharp distant image by accommodating, as a normal eye would to read. The young hyperopic person may also make a sharp near image by accommodating more—or much more than one without hyperopia. This extra effort may result in eye fatigue that is more severe for near work. The degree of hyperopia a person may have without symptoms is variable. However, the amount decreases with age as presbyopia (decrease in ability to accommodate) increases. Three diopters of hyperopia might be tolerated in a teenager but will require glasses later, even though the hyperopia has not increased. If the hyperopia is too high, the eye may be unable to correct the image by accommodation. The hyperopia that cannot be corrected by accommodation is termed manifest hyperopia. This is one of the causes of deprivation amblyopia in children and can be bilateral. There is a reflex correlation between accommodation and convergence of the two eyes. Hyperopia is therefore a frequent cause of esotropia (crossed eyes) and monocular amblyopia (see Chapter 12).
As explained above, a prepresbyopic person with hyperopia may obtain a clear retinal image by accommodation. The degree of hyperopia overcome by accommodation is known as latent hyperopia. It is detected by refraction after instillation of cycloplegic drops, which determines the sum of both manifest and latent hyperopia. Refraction with a cycloplegic is very important in young patients who complain of eyestrain when reading and is vital in esotropia, where full correction of hyperopia may achieve a cure.
Remember that a moderately “farsighted” person may see well for near or far when young. However, as presbyopia comes on, the hyperope first has trouble with close work—and at an earlier age than the nonhyperope. Finally, the hyperope has blurred vision for near and far and requires glasses for both near and far.
In astigmatism, the eye produces an image with multiple focal points or lines. In regular astigmatism, there are two principal meridians, with constant power and orientation across the pupillary aperture, resulting in two focal lines. The astigmatism is then further defined according to the position of these focal lines with respect to the retina (Figure 21–22). When the principal meridians are at right angles and their axes lie within 20° of the horizontal and vertical, the astigmatism is subdivided into astigmatism with the rule, in which the greater refractive power is in the vertical meridian, and astigmatism against the rule, in which the greater refractive power is in the horizontal meridian. Astigmatism with the rule is more commonly found in younger patients, and astigmatism against the rule more commonly in older patients (Figure 21–23). Oblique astigmatism is regular astigmatism in which the principal meridians do not lie within 20° of the horizontal and vertical. In irregular astigmatism, the power or orientation of the principal meridians changes across the pupillary aperture.
Types of regular astigmatism as determined by the positions of the two local lines with respect to the retina.
Types of astigmatism as determined by the orientation of the principal meridians and the orientation of the correcting cylinder axis.
The usual cause of astigmatism, particularly irregular astigmatism, is abnormalities of corneal shape. The crystalline lens may also contribute. In contact lens terminology, lenticular astigmatism is called residual astigmatism because it is not corrected by a spherical hard contact lens, which does correct corneal astigmatism.
Regular astigmatism often can be corrected with cylindrical lenses, frequently in combination with spherical lenses, or sometimes more effectively by altering corneal shape with rigid contact lenses, which are usually the only optical means of managing irregular astigmatism. Because the brain is capable of adapting to the visual distortion of an uncorrected astigmatic error, new glasses that do correct the error may cause temporary disorientation, particularly an apparent slanting of images.
Natural History of Refractive Errors
Most babies are slightly hyperopic, mean refractive error at birth being 0.5 D. The hyperopia slowly decreases, with a slight acceleration in the teens, to approach emmetropia. The corneal curvature is much steeper (6.59-mm radius) at birth and flattens to nearly the adult curvature (7.71 mm) by about 1 year. The lens is much more spherical at birth and reaches adult conformation at about 6 years. The mean axial length is short at birth (16.6 mm), lengthens rapidly in the first 2 or 3 years (to 21.8 mm), then moderately (0.4 mm per year) until age 6, and then slowly (about 1 mm total) to stability (24 mm) at about 10 or 15 years. Presbyopia becomes manifest in the fifth decade.
Refractive errors are inherited. The mode of inheritance is complex, as it involves so many variables. Refractive error, although inherited, need not be present at birth any more than tallness, which is also inherited, need be present at birth. For example, a child who reaches emmetropia at age 10 years will probably soon become myopic. Myopia usually increases during the teens. Factors influencing progression of myopia are poorly defined but probably include close work. Optical and pharmacological treatments to retard progression of myopia in children have not yet been shown to have long-term benefit.
Anisometropia is a difference in refractive error between the two eyes. It is a major cause of amblyopia because the eyes cannot accommodate independently and the more hyperopic eye is chronically blurred. Refractive correction of anisometropia is complicated by differences in size of the retinal images (aniseikonia) and oculomotor imbalance due to the different degree of prismatic power of the periphery of the two corrective lenses. Aniseikonia is predominantly a problem of monocular aphakia. Spectacle correction produces a difference in retinal image size of approximately 25%, which is rarely tolerable. Contact lens correction reduces the difference in image size to approximately 6%, which can be tolerated. Intraocular lenses produce a difference of less than 1%.
Correction of Refractive Errors
Spectacles continue to be the safest method of refractive correction. To reduce nonchromatic aberrations, the lenses are made in meniscus form (corrected curves) and tilted forward (pantascopic tilt).
The first contact lenses were glass fluid-filled scleral lenses. These were difficult to wear for extended periods and caused corneal edema and much ocular discomfort. Hard corneal lenses, made of polymethylmethacrylate, were the first really successful contact lenses and gained wide acceptance for cosmetic replacement of glasses. Subsequent developments include gas-permeable lenses, made of cellulose acetate butyrate, silicone, or various silicone and plastic polymers, and soft contact lenses, made of various hydrogel plastics, all of which provide increased comfort but greater risk of serious complications.
Rigid (hard and gas-permeable) lenses correct refractive errors by changing the curvature of the anterior surface of the eye. The total refractive power consists of the power induced by the back curvature of the lens, the base curve, together with the actual power of the lens due to the difference between its front and back curvatures. Only the second is dependent on the refractive index of the contact lens material. Rigid lenses overcome corneal astigmatism, including irregular astigmatism, by modifying the anterior surface of the eye into a truly spherical shape.
Soft contact lenses, particularly the more flexible forms, adopt the shape of the patient's cornea. Thus, their refractive power resides only in the difference between their front and back curvature, and they correct little corneal astigmatism unless a cylindrical correction is incorporated to make a toric lens.
Contact lens base curves are selected according to corneal curvature, as determined by keratometry or trial fittings. The front curvature is then calculated from the results of overrefraction with a trial contact lens, or from the patient's spectacle refraction as corrected for the corneal plane.
Rigid contact lenses are specifically indicated for the correction of irregular astigmatism, such as in keratoconus. Soft contact lenses are used for the treatment of corneal surface disorders, but for control of symptoms rather than for refractive reasons. All forms of contact lenses are used in the refractive correction of aphakia, particularly in overcoming the aniseikonia of monocular aphakia, and the correction of high myopia, in which they produce a much better visual image than spectacles. However, the vast majority of contact lenses worn are for cosmetic correction of low refractive errors. This has important implications for the risks that can be reasonably accepted in the use of contact lenses. (Further discussion of therapeutic and cosmetic contact lens use, and the associated complications, is given in Chapter 6.)
Keratorefractive surgery encompasses a range of methods for changing the curvature of the anterior surface of the eye. The expected refractive effect is generally derived from empirical results of similar procedures in other patients and not based on mathematical optical calculations. Further discussion of the methods and outcome of keratorefractive procedures is included in Chapter 6.
Implantation of an intraocular lens has become the preferred method of refractive correction for aphakia, usually being undertaken at the time of cataract surgery but sometimes deferred in complicated cases. A large number of designs are available, foldable lenses, made of silicone or hydrogel plastics, which can be inserted into the eye through a small incision, generally being preferred when available and applicable, but rigid lenses, most commonly consisting of an optic made of polymethylmethacrylate and loops (haptics) made of the same material or polypropylene, also still being used. The safest position for an intraocular lens is within an intact capsular bag following extracapsular surgery.
Intraocular lens power was usually determined by the empirical regression method of analyzing experience with lenses of one style in many patients, from which was derived a mathematical formula based on a constant for the particular lens (A), average keratometer readings (K), and axial length in millimeters (L). A simple example is the SRK(Sanders–Retzlaff–Kraff) equation:
A derivation is the SRK II formula. However, regression formulas are now rarely used. Theoretic formulas utilizing a lens constant, keratometer readings, and axial length, together with estimated anterior chamber depth following surgery, include the SRK/T, Haigis, Holladay, and Hoffer Q formulas. Unfortunately, none of these formulas are based on trigonometric ray tracing methods, which do accurately predict the correct power of intraocular lens for an individual patient. However, satisfactory results are generally obtained with selection of the most reliable formula for the particular axial length. Hoffer Q is indicated for short eyes (axial length less than 22 mm), Holladay for relatively long eyes (axial length 24.6–26 mm), and Haigis or SRK/T for especially long eyes (axial length greater than 26 mm). Because there is a tendency to underestimate the required power in eyes that have previously undergone keratorefractive surgery, calculation of the correct intraocular lens is much more difficult in such cases but is assisted by knowledge of refractive error and keratometer readings prior to the refractive surgery.
An additional (piggyback) intraocular lens is sometimes implanted to correct residual refractive error. Intraocular lenses are occasionally inserted without removal of the crystalline lens (phakic intraocular lens) for treatment of refractive error in young individuals without cataract and prior to onset of presbyopia.
Clear Lens Extraction for Myopia
Extraction of noncataractous lenses may be undertaken for the refractive correction of moderate to high myopia, with reported outcomes comparable to those achieved with laser keratorefractive surgery. The operative and postoperative complications of intraocular surgery, particularly in high myopia, need to be borne in mind.