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

copolymers are available. This eliminates the time limit for daily wearing. These lenses may also remain in the eye overnight in special cases, such as aphakic patients with poor coordination (prolonged wearing).

Rigid contact lenses can be manufactured as spherical lenses and toric lenses. Spherical contact lenses can almost completely compensate for corneal astigmatism of less than 2.5 diopters. This is possible because the space between the posterior surface of the spherical contact lens and the anterior surface of the astigmatic cornea is filled with tear fluid that forms a “tear lens.” Tear fluid has nearly the same refractive index as the cornea. More severe corneal astigmatism or internal astigmatism requires correction with toric contact lenses. Rigid contact lenses can even correct severe keratoconus.

Soft Contact Lenses

The material of the contact lens, such as hydrogel, is soft and pliable. Patients find these lenses significantly more comfortable. The oxygen permeability of the material depends on its water content, which may range from 36% to 85%. The higher the water content, the better the oxygen permeability. However, it is typically lower than that of rigid lenses. The material is more permeable to foreign substances, which can accumulate in it. At 12.5–16 mm, flexible lenses are larger in diameter than rigid lenses. Flexible lenses are often supported by the limbus. The lens is often displaced only a few tenths of a millimeter when the patient blinks. This greatly reduces the circulation of tear film under the lenses. This limits the maximum daily period that patients are able to wear them and requires that they be removed at night to allow regeneration of the cornea. Deviation from this principle is only possible in exceptional cases under the strict supervision of a physician.

As the lenses are almost completely in contact with the surface of the cornea, corneal astigmatism cannot be corrected with spherical soft lenses. This requires toric soft lenses.

Special Lenses

The following types of special lenses are available for specific situations:

Therapeutic contact lenses: In the presence of corneal erosion, soft ultrathin (0.05 mm) contact lenses act as a bandage and thereby accelerate reepithelialization of the cornea. They also reduce pain. Soft contact lenses may also be used in patients receiving topical medication as they store medication and only release it very slowly.

Corneal shields: These are collagen devices that resemble contact lenses. These shields are gradually broken down by the collagenase in the tear film. They are used as bandages and substrates for topical medication in the treatment of anterior disorders, such as erosion or ulcer.

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454 16 Optics and Refractive Errors

Iris print lenses: These colored contact lenses with a clear central pupil are used in patients with aniridia and albinism.

They produce good cosmetic results, reduce glare, and can correct a refractive error where indicated.

Bifocal contact lenses: These lenses were developed to allow the use of contact lenses in presbyopic patients. As in eyeglasses, a near-field correction is ground into the lens. This near-field portion is always located at the bottom of the lens because the lens is heavier there. When the patient gazes downward to read, the immobile lower eyelid pushes this near-field portion superiorly where it aligns with the pupil and becomes optically effective. Another possibility is diffraction (bending of light rays as opposed to refraction) through concentric rings on the posterior surface of the contact lens. This produces two images, a distant refractive image and a near-field diffractive image. The patient chooses the image that is important at the moment. It is also possible to correct one eye for distance vision and the fellow eye for near vision (monocular vision).

Disadvantages of Contact Lenses

Contact lenses exert mechanical and metabolic influences on the cornea. Therefore, they require the constant supervision of an ophthalmologist.

Mechanical influences on the cornea can lead to transient changes in refraction. “Spectacle blur” can result when eyeglasses suddenly no longer provide the proper correction after removing the lens. Contact lenses require careful daily cleaning and disinfection. This is more difficult, time-consuming, and more expensive than eyeglass care and is particular important with soft lenses.

Metabolic influences on the cornea: The macromolecular mesh of material absorbs proteins, protein breakdown products, low-molecular-weight substances such as medications and disinfectants, and bacteria and fungi. Serious complications can occur where daily care of the contact lenses is inadequate. With their threshold oxygen permeability, soft contact lenses interfere with corneal metabolism. Contact lenses are less suitable for patients with symptoms of keratoconjunctivitis sicca.

Contact Lens Complications

Complications have been observed primarily in patients wearing soft contact lenses. These include:

Infectious keratitis (corneal infiltrations and ulcers) caused by bacteria, fungi, and protozoans.

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Acanthamoeba keratitis is a serious complication affecting wearers of soft contact lenses and often requires penetrating keratoplasty.

Giant papillary conjunctivitis: This is an allergic reaction of the palpebral conjunctiva of the upper eyelid to denatured proteins. It results in proliferative “cobblestone” conjunctival lesions.

Corneal vascularization may be interpreted as the result of insufficient supply of oxygen to the cornea.

Severe chronic conjunctivitis: This usually makes it impossible to continue wearing contact lenses.

16.5.3Prisms

Prisms can change the direction of parallel light rays. The optical strength of a prism is specified in prism diopters. Prism lenses can be combined with spherical and toric lenses. When prescribing eyeglasses, the ophthalmologist specifies the strength and the position of the base of the prism. Prism lenses are used to correct heterophoria (latent strabismus) and ocular muscle palsies, and in preparation for surgery to correct strabismus.

A 1 diopter prism deflects a ray of light 1 cm at a distance of 1 m from the base of the prism.

16.5.4Magnifying Vision Aids

The reduction in central corrected visual acuity as a result of destruction of the fovea with a central scotoma requires magnifying vision aids. However, magnification is always associated with a reduction in the size of the visual field. As a result, these vision aids require patience, adaptation, motivation, and dexterity. Cooperation between ophthalmologist and optician is often helpful. The following systems are available in order of magnification.

Increased near-field corrections: The stronger the near-field correction, the shorter the reading distance. Magnification (V) is a function of the refractive power of the near-field correction (D) and is determined by the equation V = D/4.

Example: Eyeglasses with a 10 diopter near-field correction magnify the image two and one-half powers. However, the object must be brought to within 10 cm of the eye.

Magnifying glasses are available in various strengths, with or without illumination.

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456 16 Optics and Refractive Errors

Monocular and binocular loupes, telescopes, and prism loupes: An optical magnifying system is mounted on one or both eyeglass lenses. The optical system functions on the principle of Galilean or Keplerian optics.

Closed-circuit TV magnifier: This device displays text at up to 45 power magnification.

16.5.5Aberrations of Lenses and Eyeglasses

Optical lens systems (eyeglasses or lenses) always have minor aberrations. These aberrations are not material flaws, rather they are due to the laws of physics. Expensive optical systems can reduce these aberrations by using many different lenses in a specific order.

Chromatic Aberration (Dispersion)

This means that the refractive power of the lens varies according to the wavelength of the light.

Light consists of a blend of various wavelengths. Light with a short wavelength such as blue is refracted more than light with a long wavelength such as red (Fig. 16.20). This is why monochromatic light (light of a single wavelength) produces a sharper image on the retina.

Chromatic aberration is the basis of the red-green test used for fine refraction testing.

Chromatic aberration.

Fig. 16.20 Chromatic aberration splits white light into its component spectral colors. Red is refracted least, and blue is refracted most.

Spherical Aberration

This means that the refractive power of the lens varies according to the location at which the light ray strikes the lens.

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Patients may report being able to see better when looking through a disk with a pinhole (a stenopeic aperture) than without it. This usually is a sign of an uncompensated refractive error in the eye.

The further peripherally the light ray strikes the lens, the more it will be refracted (Fig. 16.21). The iris intercepts a large share of these peripheral light rays. A narrow pupil will intercept a particularly large share of peripheral light rays, which improves the depth of field. Conversely, depth of field is significantly poorer when the pupil is dilated.

Patients who have received mydriatic agents should refrain from driving.

Spherical aberration.

Fig. 16.21 Due to spherical aberration the refraction of light rays increases the further peripherally they strike the lens.

Astigmatic Aberration

A punctiform object viewed through a spherical lens appears as a line.

If one looks through a lens obliquely to its optical axis, it will act as a prism (Fig. 16.22a). A prism refracts a light ray toward its base (Fig. 16.22b). In addition to this, the light is split into its component spectral colors. Light with a short wavelength (blue) is refracted more than light with a long wavelength (red). Astigmatic aberration is an undesired side effect that is present whenever one looks through a lens at an oblique angle.

This phenomenon should be distinguished from astigmatic or toric lenses, which correct for astigmatism of the eye when the patients looks through them along the optical axis.

Curvature of Field

This means that the magnification of the image changes as one approaches the periphery. The result is a sharp image with peripheral curvature. Convex or plus lenses produce pincushion distortion; concave or minus lenses produce barrel distortion (Fig. 16.23).

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458 16 Optics and Refractive Errors

Astigmatic aberration.

Apex

Fig. 16.22 a Lenses may be regarded as composed of many prisms, which explains many of the optical phenomena of lenses such as dispersion. b A prism refracts a light ray toward its base twice (solid line). However, it appears to the observer that the object is shifted toward the apex of the prism (dotted line).

Curvature of field.

Fig. 16.23 Viewing an object through plus lenses produces pincushion distortion of the image, whereas minus lenses produce barrel distortion.

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459

17 Ocular Motility and Strabismus

Doris Recker, Josef Amann, and Gerhard K. Lang

Definition

Strabismus is defined as deviation of an eye’s visual axis from its normal position.

There are two major types of manifest strabismus or heterotropia:

1.Concomitant strabismus (from the Latin “comitare”, accompany). The deviating eye accompanies the leading eye in every direction of movement. The angle of deviation remains the same in all directions of gaze. This form of strabismus may occur as monocular strabismus, in which only one eye deviates, or as alternating strabismus, in which both eyes deviate alternately.

2.Paralytic strabismus results from paralysis of one or more eye muscles. This form differs from concomitant strabismus in that the angle of deviation does not remain constant in every direction of gaze. For this reason, this form is also referred to as incomitant strabismus.

Epidemiology: The incidence of strabismus is about 5–7%. Esotropia (convergent strabismus) occurs far more frequently than exotropia (divergent strabismus) in Europe and North America. Concomitant strabismus usually occurs in children, whereas paralytic strabismus primarily affects adults. This is because concomitant strabismus is generally congenital or acquired within the first few years of life, whereas paralytic strabismus is usually acquired, for example as a post-traumatic condition.

17.1Basic Knowledge

Ocular motility: The movements of the eyeballs are produced by the following extraocular muscles (Fig. 17.1):

The four rectus muscles: the superior, inferior, medial, and lateral rectus muscles.

The two oblique muscles: the superior and inferior oblique muscles.

All of these muscles originate at the tendinous ring except for the inferior oblique muscle, which has its origin near the nasolacrimal canal. The rectus muscles envelope the globe posteriorly, and their respective tendons insert

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Figs. 17.1a and b The two oblique ocular muscles insert on the temporal aspect posterior to the equator. The four rectus muscles insert on the superior, inferior, nasal, and temporal sclera.

into the superior, inferior, nasal, and temporal sclera. The oblique muscles insert into the temporal globe posterior to the equator. The insertion of the muscles determines the direction of their pull (see Table 17.1).

The connective tissue between the individual ocular muscles is incorporated into the fascial sheath of the eyeball (Tenon’s capsule). Other important anatomic structures include the lateral and medial check ligaments comprising the lateral connections of the orbital connective tissue and the ligament of Lockwood. This is comprised of the ligamentous structures between the inferior rectus and inferior oblique that spread out like a hammock to the medial and lateral rectus muscles.

These anatomic structures and the uniform nerve supply to the extraocular muscles (like acting muscles have like nerve supply) ensure ocular balance. Changes that disturb this balance, such as ocular muscle paralysis that limits or destroys the affected muscle’s ability to contract, cause strabismus. The angle of deviation is a sign of abnormal imbalance.

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462 17 Ocular Motility and Strabismus

Table 17.1 Function of the extraocular muscles with the gaze directed straight ahead

Continued !

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