Ординатура / Офтальмология / Английские материалы / Pocket Textbook Atlas Of Ophthalmology_Lang, Thieme_2000

444 16 Optics and Refractive Errors

Treatment of irregular astigmatism: This form cannot be corrected with eyeglasses. External astigmatism may be managed with a rigid contact lens (Fig. 16.13b), keratoplasty, or surgical correction of the refractive error. Irregular internal astigmatism is usually lens-related. In this case, removal of the lens with implantation of an intraocular lens is indicated.

Only regular astigmatism can be corrected with eyeglasses.

16.3.4Anisometropia

Definition

In anisometropia, there is a difference in refractive power between the two eyes.

Epidemiology: Anisometropia of at least 4 diopters is present in less than 1% of the population.

Etiology: The reason for the varying development of the two eyes is not clear. This primarily congenital disease is known to exhibit a familial pattern of increased incidence.

Pathophysiology: In anisometropia, there is a difference in refractive power between the two eyes. This refractive difference can be corrected separately for each eye with different lenses as long as it lies below 4 diopters. Where the difference in refraction is greater than or equal to 4 diopters, the size difference of the two retinal images becomes too great for the brain to fuse the two images into one. Known as aniseikonia, this condition jeopardizes binocular vision because it can lead to development of amblyopia (anisometropic amblyopia). The aniseikonia, or differing size of the retinal images, depends not only on the degree of refractive anomaly but also depends significantly on the type of correction. The closer to the site of the refraction deficit the correction is made, the less the retinal image changes in size. Correction with intraocular lenses results in almost no difference in image size. Contact lenses produce a slight and usually irrelevant difference in image size. However, eyeglass correction resulting in a difference of more than 4 diopters leads to intolerable aniseikonia (see Table 7.4).

Symptoms: Anisometropia is usually congenital and often asymptomatic. Children are not aware that their vision is abnormal. However, there is a tendency toward strabismus as binocular functions may remain underdeveloped. Where the correction of the anisometropia results in unacceptable aniseikonia, patients will report unpleasant visual sensations of double vision.

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16.4 Impaired Accommodation 445

Diagnostic considerations: Anisometropia is usually diagnosed during routine examinations. The diagnosis is made on the basis of refraction testing.

Treatment: The refractive error should be corrected. Anisometropia exceeding 4 diopters cannot be corrected with eyeglasses because of the clinically relevant aniseikonia. Contact lenses and, in rare cases, surgical treatment are indicated. Patients with unilateral aphakia or who do not tolerate contact lenses will require implantation of an intraocular lens.

Correction of unilateral aphakia with unilateral glasses is usually contraindicated because it result in aniseikonia of approximately 25%.

16.4Impaired Accommodation

16.4.1Accommodation Spasm

Definition

An accommodation spasm is defined as inadequate protracted contraction of the ciliary muscle.

Etiology: Accommodation spasms are rare. They may occur as functional impairment or they may occur iatrogenically when treating young patients with parasympathomimetic agents (miotic agents). The functional impairments are frequently attributable to heightened sensitivity of the accommodation center, which especially in children (often girls) can be psychogenic. Rarely the spasm is due to organic causes. In these cases, it is most often attributable to irritation in the region of the oculomotor nuclei (from cerebral pressure or cerebral disorders) or to change in the ciliary muscle such as in an ocular contusion.

Symptoms: Patients complain of deep eye pain and blurred distance vision (lenticular myopia).

Diagnostic considerations and differential diagnosis: The diagnosis is made on the basis of presenting symptoms and refraction testing, including measurement of the range of accommodation. This is done with an accommodometer, which determines the difference in refractive power between the near point and far point. A differential diagnosis should exclude latent hyperopia. In children, this will frequently be associated with accommodative esotropia and accommodative pupil narrowing.

Treatment: This depends on the underlying disorder. Cycloplegic therapy with agents such as tropicamide or cyclopentolate may be attempted in the presence of recurrent accommodation spasms.

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446 16 Optics and Refractive Errors

Prognosis: Iatrogenic spasms are completely reversible by discontinuing the parasympathomimetic agents. The prognosis is also good for patients with functional causes. Spasms due to organic causes require treatment of the underlying disorder but once treatment is initiated the prognosis is usually good.

16.4.2Accommodation Palsy

Definition

Failure of accommodation due to palsy of the ciliary muscle.

Etiology: This rare disorder is primarily to one of the following causes:

Iatrogenic drug-induced palsy due to parasympatholytic agents such as atropine, cyclopentolate scopolamine, homatropine, and tropicamide.

Peripheral causes: Oculomotor palsy, lesions of the ciliary ganglion, or the ciliary muscle.

Systemic causes: Damage to the accommodation center in diphtheria, diabetes mellitus, chronic alcoholism, meningitis, cerebral stroke, multiple sclerosis, syphilis, lead or ergotamine poisoning, medications such as isoniazid or piperazine, and tumors.

Symptoms: The failure of accommodation leads to blurred near vision and may be associated with mydriasis where the sphincter pupillae muscle is also involved. The clinical syndromes listed below exhibit a specific constellation of clinical symptoms and therefore warrant further discussion.

Post-diphtheria accommodation palsy: This transitory palsy is a toxic reaction and occurs without pupillary dysfunction approximately four weeks after infection. Sometimes it is associated with palsy of the soft palate and/or impaired motor function in the lower extremities.

Accommodation palsy in botulism: This is also a toxic palsy. It does involve the pupil, producing mydriasis, and can be the first symptom of botulism. It is associated with speech, swallowing, and ocular muscle dysfunction accompanied by double vision.

Tonic pupillary contraction is associated with tonic accommodation.

Sympathetic ophthalmia is characterized by a decrease in the range of accommodation, even in the unaffected eye.

Measurement of the range of accommodation is indicated whenever sympathetic ophthalmia is suspected.

Diagnostic considerations: In addition to measuring the range of accommodation with an accommodometer, the examiner should inquire about other ocular and general symptoms.

Treatment: This depends on the underlying disorder.

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Prognosis: The clinical course of tonic pupillary contraction is chronic and results in irreversible loss of accommodation. The toxic accommodation palsies are reversible once the underlying disorder is controlled.

16.5Correction of Refractive Errors

16.5.1Eyeglass Lenses

Monofocal Lenses

There are two basic types.

Spherical lenses refract light equally along every axis.

Toric lenses (known as cylindrical lenses) refract light only along one axis. Spherical and toric lenses can be combined where indicated.

The refractive power of the lenses is measured manually or automatically with an optical interferometer. The measured refraction is specified as spherocylindrical combination. By convention, the specified axis of the cylindrical lens is perpendicular to its axis of refraction (Fig. 16.15c and d). The orientation of this axis with respect to the eye is specified on a standardized form (Fig. 16.16).

Example: + 4.00 diopters –2.00 diopters/90 degrees means that the lens represents a combination of converging lens (+ 4 diopters) and cylindrical lens (–2 diopters) with its axis at 90 degrees.

Eyeglass lenses exhibit typical characteristics when moved back and forth a few inches in front of one’s eye. Objects viewed through minus lenses appear to move in the same direction as the lens; objects viewed through plus lenses move in the opposite direction. A cylindrical lens produces image distortions when turned.

Multifocal Lenses

Multifocal lenses differ from the monofocal lenses of uniform refractive power discussed in the previous section in that different areas of the lens have different refractive powers. These lenses are best understood as combinations of two or more lenses in a single lens.

Bifocals: The upper and middle portion of the lens is ground for the distance correction; the lower portion is ground for the near-field correction

(Fig. 16.17a and b). Patients are able to view distant objects in focus and read using one pair of eyeglasses, eliminating the need to constantly change glasses. The gaze is lowered and converged to read. This portion of the lens contains the near-field correction. This near-field correction can be placed in a different part of the lens for special applications.

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450 16 Optics and Refractive Errors

Special Lenses

The following types of lenses have been developed for special applications:

Plastic lenses: These lenses reduce the weight of eyeglasses where severe ametropia must be corrected. Another advantage is that these lenses are largely shatterproof, which is why they are preferred for children. However, they are easily scratched.

Absorption lenses: These lenses are indicated in patients with increased sensitivity to glare.

Operating motor vehicles in twilight or at night with eyeglasses that absorb more than 20% of incident light is dangerous because of the resulting reduction in visual acuity.

Photochromatic lenses: These lenses darken in response to the intensity of ultraviolet light. The lenses become darker at low temperatures than at high temperatures; they lighten more slowly at low temperatures and more rapidly at high temperatures. Light attenuation ranges between 15 and 50% in some lenses and between 30 and 65% in others.

Photochromatic lenses pose problems for patients operating motor vehicles. The lenses darken only slightly in a warm car with the windows closed due to the lack of ultraviolet light. Dark lenses lighten too slowly when the car enters a tunnel.

Coated lenses: Extremely thin coatings of magnesium fluoride can be applied to lenses to reduce surface reflection on the front and back of the lens.

Subjective Refraction Testing for Eyeglasses

While the patient looks at vision charts, the examiner places various combinations of lenses in front of the patient’s eye. The patient reports which of two lenses produces the sharper image. The better of the two is then compared with the next lens. This incremental method identifies the optimal correction. It is expedient to use the patient’s objective refraction as the starting point for subjective testing. Refraction testing is performed either with a series of test lenses from a case or with a Phoroptor, which contains many lenses that can be automatically or manually placed before the patient’s eye.

The examination proceeds in three stages:

Monocular testing: The optimal refraction for achieving best visual acuity is determined separately for each eye. The weakest possible minus lens is used in myopic patients, and the strongest possible plus lens in hyperopic patients. The red-green chromatic aberration test can be used for fine refraction. In this test, the patient compares optotypes on green and red backgrounds. Fine adjustment of refraction permits precise shifting of the

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focal point of the light on the retina. Optotypes on both red and green backgrounds then appear equally sharply defined.

Binocular testing: The objective of this stage is to achieve a balance between both eyes.

Near point testing: The final stage of the examination determines the patient’s near visual acuity, and, if necessary, the presbyopic addition

(“Add”). Allowance is made for the patient’s preferred reading and working

position.

The values determined by this examination are entered in the eyeglass prescription (see Fig. 16.16). The vertex distance at which refraction was performed is an important additional parameter for the optician. This is the distance between the back surface of the test lens and the anterior surface of the cornea. If the manufactured eyeglasses have a different vertex distance, then the strength of the lenses should be altered accordingly. This is because the optical effect of eyeglass lenses varies according to the distance from the eye.

Before the lenses are fitted into the frame, the distance between the pupils must be measured to ensure that the lenses are properly centered. The center of the lens should be in front of the pupil. The prismatic effects of eccentric lenses might otherwise cause asthenopic symptoms such as headache or a burning sensation in the eyes.

To facilitate early detection of glaucoma, intraocular pressure should be measured in any patient over the age of 40 presenting for refraction testing for eyeglasses.

16.5.2Contact Lenses

16.5.2.1 Advantages and Characteristics of Contact Lenses

Contact lenses are in immediate contact with the cornea. Although they are foreign bodies, most patients adapt to properly fitted contact lenses. Contact lenses differ from eyeglasses in that they correct the refractive error closer to the location of its origin. For this reason, the quality of the optical image viewed through contact lenses is higher than that viewed through eyeglasses. Contact lenses have significantly less influence on the size of the retinal image than does correction with eyeglasses. Lenses do not cloud up in rainy weather or steam, and peripheral distortion is minimized. The cosmetic disadvantage of thick eyeglasses in severe ametropia is also eliminated. Severe anisometropia requires correction with contact lenses for optical reasons, i.e., to minimize aniseikonia.

Contact lenses are defined by the following characteristics:

Diameter of the contact lens.

Radius of curvature of the posterior surface.

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452 16 Optics and Refractive Errors

Geometry of the posterior surface, i.e., spherical, aspherical, complex curvature, or toric.

Refractive power.

Material.

Oxygen permeability of the material (Dk value).

The cornea requires oxygen from the precorneal tear film. To ensure this supply, contact lens materials must be oxygen-permeable. This becomes all the more important the less the contact lens moves and permits circulation of tear fluid. Contact lenses may be manufactured from rigid or flexible materials.

Rigid Contact Lenses

These contact lenses have a stable, nearly unchanging shape. Patients take some time to become used to them and should therefore wear them often. The goal is to achieve the best possible intimacy of fit between the posterior surface of the lens and the anterior surface of the cornea (Fig. 16.19). This allows the contact lens to float on the precorneal tear film. Every time the patient blinks, the lens is displaced superiorly and then returns to its central position. This permits circulation of the tear film.

Previously, polymethyl methacrylate (PMMA) was used as a material. However, this is practically impermeable to oxygen. The lenses were fitted in small diameters with a very shallow curvature; the central area maintained contact with the cornea while the periphery projected. This allowed excellent tear film circulation, and patients were able to wear the lenses for surprisingly long periods. Today, highly oxygen-permeable materials such as silicone

Fit of a rigid contact lens.

Fig. 16.19 A tear film lies between the anterior surface of the cornea and the posterior surface of the lens (visualized by fluorescein dye).

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