Testing the Potential Resolving Power of the Retina in the Presence of Opacified Ocular Media

Special examination methods are indicated in the presence of opacification of the ocular media of the eye (such as a cataract) to determine the potential visual acuity of the retina. This permits the ophthalmologist to estimate whether optimizing the refractive media with techniques such as cataract surgery or corneal transplantation would achieve the desired improvement.

Laser interference visual acuity testing: Lasers are used to project inference strips of varying widths on to the retina. The patient must specify the direction in which these increasing narrower strips are aligned. This examination can no longer be performed where there is severe opacification of the optic media such as in a mature cataract. The preliminary examination then consists of evaluating the pattern of the transilluminated retinal vasculature.

^ Fig. 16.7 With the retinoscope, the examiner moves a light source (a beam of yellow light) across the pupil (dark spot) at a distance of about 50 cm from the patient. This produces a light reflex (red spot) in the patient's eye. It is important to note how this light reflex (red spot) behaves as the light source of the retinoscope is moved. There are two possibilities:

a "With" motion: the light reflex in the pupil (red spot) moves in the same direction (red arrows) as the light source of the retinoscope (yellow arrows). This means that the far point of the eye is behind the light source. b "Against" motion. The light reflex in the pupil moves in the opposite direction (red arrows) to the light source of the retinoscope (yellow arrows). This means that the far point of the eye lies between the eye and the light source. The examiner places appropriate lenses in front of the patient's eyes (plus lenses for "with" motion and minus lenses for "against" motion) until no further motion of the light reflex is observed. The motion of the retinoscope will then only elicit an infinitely fast reflex (neutral point). This method is used to determine the proper lens for correcting the refractive error.

432 16 Optics and Refractive Errors 16.3 Refractive Anomalies (Table 16.2) 16.3.1 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 30 have refraction less than -1 diopters.

— Refraction in myopia.

Myopia Epidemiology

Fig. 16.8 a The focal point of parallel light rays entering the eye lies anterior to the retina. b Only close objects from which the light rays diverge until they enter the eye are focused on the retina and appear sharply defined. The far point is a finite distance from the eye. c Axial myopia: normal refractive power in an excessively long globe. d Refractive myopia: excessive refractive power in a normal-length globe. e Nuclear cataract with a secondary focal point (patient sees double).

Table 16.2 Overview of the most important refractive anomalies

Table 16.2 Overview of the most important refractive anomalies

Refractive

Focal point of par

Causes

Vision

Possible complica

Optical correction

anomaly

allel incident light

tions

rays

Myopia

Anterior to the

Eyeball too long

Very good near

Increased risk of

Diverging lenses

(nearsightedness)

retina

(axial myopia).

vision.

retinal detach

(minus or concave

* Excessive refrac

*

Poor distance

ment.

lenses).

tive power

vision.

•:• See p. 434 for

(refractive

complications

myopia).

specific to patho

logic myopia.

Hyperopia

Posterior to the

Eyeball too short

Poor near vision.

Disposition to

Converging lenses

(farsightedness)

retina

(axial hyperopia).

*

Accommodation

acute angle clo

(plus or convex

* Insufficient

usually permits

sure glaucoma

lenses).

refractive power

normal distance

(shallow anterior

(refractive hyper

vision (in young

chamber). Cau

opia).

patients and in

tion is advised

slight to mod

with diagnostic

erate hyperopia).

and therapeutic

mydriasis.

Esotropia

Astigmatism

Lack of a focal

Anomalies in the

Patients see every

Risk of refractive

Cylindrical lenses;

point

curvature of the

thing distorted.

amblyopia.

eyeglass correction

normally spherical

is only possible

surfaces of the

where astigmatism

refractive media

is regular.

(cornea and lens).

W Ul

Etiology: The etiology of myopia is not clear. Familial patterns of increased incidence suggest the influence of genetic factors.

Pathophysiology: Whereas parallel incident light rays converge at a focal point on the retina in emmetropic eyes, they converge at a focal point anterior to 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 close objects 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.

H 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 longglobe 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).

H A difference in globe length of 1 mm with respectto a normal eye corresponds 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 (see p. ■). 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 continuously independently of external influences.

Symptoms and diagnostic considerations: The diagnosis is made on the basis 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 by thinning 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 the ciliary muscle is present as it is hardly used. The volume of the vitreous body is too small for the large eye, and it may collapse prematurely. This results in vitreous opacifications that the patient perceives as floaters.

Morphologic fundus changes in myopia, such as maculopathy and Fuchs' spot, are discussed in Section 12.4.6.

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

H 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 regular intervals thereafter.

Glaucoma is more difficult to diagnose in patients with myopia. Measurements of intraocular pressure obtained with a Schi0tz 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 be reduced. This is achieved through the use of diverging lenses (minus or concave 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

— Correction of myopia.

— Correction of myopia.

Fig. 16.9 a Correction with diverging lenses (minus lenses). b Correction with contact lens. c Correction by removing the lens to reduce refractive power of the eye.

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 corrective 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. The reduction in the size of the image is less than with eyeglass correction. Aberrations are also reduced. These advantages are clinically relevant with myopia exceeding 3 diopters.

H 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 absolutely 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 contraction of the atrophic ciliary muscle.

H Myopic patients have "lazy" accommodation due to atrophy of the ciliary muscle. Avery 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 chamber intraocular lens (diverging lens) anterior to the natural lens to reduce refractive power. See Chapter 5 for additional surgical options.

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.

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