Myopia (Shortsightedness)

Refractive Anomalies (Table 16.2)

Myopia (Shortsightedness)

Definition

A discrepancy between the refractive power and axial length of the eye such that parallel incident light rays converge at a focal point anterior to the retina (Fig. 16.8a).

Epidemiology: 

Approximately 25% of persons between the ages of 20 and 30have refraction less than –1 diopters.

Etiology: 

The etiology of myopia is not clear. Familial patterns of increasedincidence suggest the influence of genetic factors.

Pathophysiology: 

Whereas parallel incident light rays converge at a focalpoint on the retina in emmetropic eyes, they converge at a focal point anteriorto the retina in myopic eyes (Fig. 16.8a). This means that no sharply defined images appear on the retina when the patient gazes into the distance (Fig. 16.8a). The myopic eye can only produce sharply defined images of closeobjects from which the light rays diverge until they enter the eye (Fig. 16.8b).The far point moves closer; in myopia of –1 diopter it lies at a distance of 1 m.

In myopia, the far point (distance from the eye = A) can be calculated using the formula: A (m) = 1/D, where D is myopia in diopters.

Possible causes include an excessively long globe with normal refractive power (axial myopia; Fig. 16.8c) and, less frequently, excessive refractive power in a normal-length globe (refractive myopia; Fig. 16.8d).

A difference in globe length of 1 mm with respect to a normal eye corre-sponds to a difference of about 3 diopters in refractive power.

Special forms of refractive myopia:

❖ Myopic sclerosis of the nucleus of the lens (cataract) in advanced age. This causes a secondary focal point to develop, which can lead to monocular diplopia (double vision).

❖ Keratoconus (increase in the refractive power of the cornea).

❖ Spherophakia (spherically shaped lens).

Forms: 

These include:

❖ Simple myopia (school-age myopia): Onset is at the age of 10 – 12 years.Usually the myopia does not progress after the age of 20. Refraction rarely exceeds 6 diopters. However, a benign progressive myopia also exists, which stabilizes only after the age of 30.

❖ Pathologic myopia: This disorder is largely hereditary and progresses con-tinuously independently of external influences. 

Symptoms and diagnostic considerations: 

The diagnosis is made on thebasis of a typical clinical picture and refraction testing. Myopic patients have very good near vision. When gazing into the distance, they squint in an attempt to improve their uncorrected visual acuity by further narrowing the optic aperture of the pupil. The term “myopia” comes from this squinting; the Greek word “myein” means to squint or close the eyes. Older myopic patients can read without corrective lenses by holding the reading material at about the distance of the far point.

The typical morphologic changes occurring in myopia are referred to as myopia syndrome. Progressive myopia in particular is characterized bythin-ning of the sclera. The elongation of the globe causes a shift in the axes of the eye.

This often simulates esotropia. The anterior chamber is deep. Atrophy of theciliary muscle is present as it is hardly used. The volume of the vitreous body istoo small for the large eye, and it may collapse prematurely. This results in vit-reous opacifications that the patient perceives as floaters.

Morphologic fundus changes in myopia, such as maculopathy and Fuchs’ spot..

The risk of retinal detachment is increased in myopia. However, it does not increase in proportion to the severity of the myopia.

Because of the increased risk of retinal detachment, patients with myopia should be examined particularly thoroughly for prodromal signs of retinal detachment, such as equatorial degeneration or retinal tears. Therefore, examination of the fundus with the pupil dilated is indicated both when the first pair of eyeglasses is prescribed and at reg-ular intervals thereafter.

Glaucoma is more difficult to diagnose in patients with myopia. Measure-ments of intraocular pressure obtained with a Schiøtz tonometer will be lower than normal due to the decreased rigidity of the sclera.

Applanation tonometry yields the most accurate values in patients with myopia because the rigidity of the sclera only slightly influences results.

The optic cup is also difficult to evaluate in patients with myopia because the optic nerve enters the eye obliquely. This also makes glaucoma more difficult to diagnose.

Treatment: 

The excessive refractive power of the refractive media must bereduced. This is achieved through the use of diverging lenses (minus or con-cave lenses; Fig. 16.9a). These lenses cause parallel incident light rays to diverge behind the lens. The divergent rays converge at a virtual focal point in front of the lens. The refractive power (D) is negative (hence the term “minus lens”) and is equal to 1/f, where f is the focal length in meters. 

Previously, biconcave or planoconcave lens blanks were used in the manufacture of cor-rective lenses. However, these entailed a number of optical disadvantages. Today lenses are manufactured in a positive meniscus shape to reduce lens aberrations.

Correction with contact lenses (Fig. 16.9b) offers optical advantages. Thereduction in the size of the image is less than with eyeglass correction. Aber-rations are also reduced. These advantages are clinically relevant with myopia exceeding 3 diopters.

The closer the “minus” lens is to the eye, the weaker its refractive power must be to achieve the desired optic effect.

Minus lenses to be used to correct myopia should be no stronger than abso-lutely necessary. Although accommodation could compensate for an overcor-rection, patients usually do not tolerate this well. Accommodative asthenopia (rapid ocular fatigue) results from the excessive stress caused by chronic con-traction of the atrophic ciliary muscle.

Myopic patients have “lazy” accommodation due to atrophy of the cili-ary muscle. A very slight undercorrection is often better tolerated than a perfectly sharp image with minimal overcorrection.

In certain special cases, removal of the crystalline lens (Fig. 16.9c) may be performed to reduce the refractive power of the myopic eye. However, this operation is associated with a high risk of retinal detachment and is rarely performed. There is also the possibility of implanting an anterior chamberintraocular lens (diverging lens) anterior to the natural lens to reduce refrac-tive power.

Popular health books describe exercises that can allegedly treat refractive errors such as nearsightedness without eyeglasses or contact lenses. Such exercises cannot influence the sharpness of the retinal image; they can only seemingly improve uncorrected visual acuity by training the patient to make better use of additional visual information. However, after puberty no late sequelae of chronically uncorrected vision are to be expected.