Ординатура / Офтальмология / Английские материалы / Ophthalmology A Short Textbook_Lang_2000

12.1 Basic Knowledge 303

Sensitivity of the retina to light intensity: The retina has two types of photoreceptors, the rods and the cones. The 110–125 million rods permit mesopic and scotopic vision (twilight and night vision). They are about 500 times more photosensitive than the cones and contain the photopigment rhodopsin.

Twilight vision decreases after the age of 50, particularly in patients with additional age-related miosis, cataract, and decreased visual acuity. Therefore, glaucoma patients undergoing treatment with miotic agents should be advised of the danger of operating motor vehicles in twilight or at night.

The six to seven million cones in the macula are responsible for photopic vision (daytime vision), resolution, and color perception. There are three types of cones:

blue cones,

green cones,

red cones.

Their photopigments contain the same retinal but different opsins. Beyond a certain visual field luminance, a transition from dark adaptation to light adaptation occurs. Luminance refers to the luminous flux per unit solid angle per unit projected area, measured in candelas per square meter (cd/m2). The cones are responsible for vision up to a luminance of 10 cd/m2, the rods up to 0.01 cd/m2 (twilight vision is 0.01–10 cd/m2; night vision is less than 0.01 cd/m2).

Adaptation is the adjustment of the sensitivity of the retina to varying degrees of light intensity. This is done by dilation or contraction of the pupil and shifting between cone and rod vision. In this manner, the human eye is able to see in daylight and at night. In light adaptation, the rhodopsin is bleached out so that rod vision is impaired in favor of cone vision. Light adaptation occurs far more quickly than dark adaptation. In dark adaptation, the rhodopsin quickly regenerates within five minutes (immediate adaptation), and within 30 minutes to an hour there is a further improvement in night vision (long-term adaptation). An adaptometer may be used to determine the light intensity threshold. First the patient is adapted to bright light for 10 minutes. Then the examining room is darkened and the light intensity threshold is measured with light test markers. These measurements can be used to obtain an adaptation curve (Fig. 12.3).

Sensitivity to glare: Glare refers to disturbing brightness within the visual field sufficiently greater than the luminance to which the eyes are adapted such as the headlights of oncoming traffic or intense reflected sunlight. Because the retina is adapted to a lesser luminance, vision is impaired in these cases. Often the glare will cause blinking or elicit an eye closing reflex. Sensitivity to glare can be measured with a special device. Patients are shown a series of visual symbols in rapid succession that they must recognize despite intense glare.

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Normal and abnormal dark adaptation curves.

cd/m2 3,2 . 10–7

3,2 . 10–5

3,2 . 10–3

3,2

Fig. 12.3 X axis: adaptation time in minutes. Y axis: luminance of the respective test marker in candelas per square meter. The blue curve shows normal progression with Kohlrausch’s typical discontinuity indicating the transition from cone to rod vision. The red curve in retinitis pigmentosa is considerably less steep.

The sensitivity to glare or the speed of adaptation and readaptation of the eye is important in determining whether the patient is fit to operate a motor vehicle.

12.2Examination Methods

Visual Acuity see Chapter 1.

12.2.1Examination of the Fundus

Direct ophthalmoscopy (Fig. 12.4a; see also Fig. 1.13): A direct ophthalmoscope is positioned close to the patient’s eye. The examiner sees a 16-power magnified image of the fundus.

Advantages. The high magnification permits evaluation of small retinal findings such as diagnosing retinal microaneurysms. The dial of the ophthalmoscope contains various different plus and minus lenses and can be adjusted as necessary. These lenses compensate for refractive errors in both the patient

12.2 Examination Methods 305

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and the examiner. They may also be used to measure the prominence of retinal changes, such as the prominence of the optic disk in papilledema or the prominence of a tumor. The base of the lesion is brought into focus first and then the peak of the lesion. A difference of 3 diopters from base to peak corresponds to a prominence of 1 mm. Direct ophthalmoscopy produces an erect image of the fundus, which is significantly easier to work with than an inverted image, and is therefore a suitable technique even for less experienced examiners.

Disadvantages. The image of the fundus is highly magnified but shows only a small portion of the fundus. Rotating the ophthalmoscope can only partially compensate for this disadvantage. Direct ophthalmoscopy also produces only a two-dimensional image.

Indirect ophthalmoscopy (Figs. 12.4b and c): A condensing lens (+ 14 to + 30 diopters) is held approximately 13 cm from the patient’s eye. The fundus appears in two to six-power magnification; the examiner sees a virtual inverted image of the fundus at the focal point of the loupe. Light sources are available for monocular or binocular examination.

Advantages. This technique provides a good stereoscopic, optimally illuminated overview of the entire fundus in binocular systems.

Disadvantages. Magnification is significantly less than in direct ophthalmoscopy. Indirect ophthalmoscopy requires practice and experience.

Contact lens examination: The fundus may also be examined with a slit lamp when an additional magnifying lens such as a three-mirror lens (see Fig. 12.5) or a 78 to 90 diopter lens is used.

Examination of the fundus with a Goldmann three-mirror lens.

Figs. 12.5a and b Principle of the examination: The lens is placed directly on the eye after application of a topical anesthetic. The various mirrors of Goldmann threemirror lens visualize different areas of the retina: 1) posterior pole, 2) central part of the peripheral retina, 3) outer peripheral retina (important in diagnosing retinal tears), 4) gonioscopy mirror for examination of the chamber angle.

12.2 Examination Methods 307

Advantages. This technique produces a highly magnified three-dimensional image yet still provides the examiner with a good overview of the entire fundus. The three-mirror lens also visualizes “blind areas” of the eye such as the angle of the anterior chamber. Contact lens examination combines the advantages of direct ophthalmoscopy and indirect ophthalmoscopy and is therefore the gold standard for diagnosing retinal disorders.

Where significant opacification of the optic media (as in a mature cataract) prevents direct visualization of the retina with the techniques mentioned above, the examiner can evaluate the pattern of the retinal vasculature. The sclera is directly illuminated in all four quadrants by moving a light source back and forth directly over the sclera. Patients with intact retinas will be able to perceive the shadow of their own vasculature on the retina (entoptic phenomenon). They will see what looks like “veins of a leaf in autumn”. Patients who are able to perceive this phenomenon have potential retinal vision of at least 20/200.

Ultrasonography: Ultrasound studies are indicated where opacification of the optic media such as cataract or vitreous hemorrhage prevent direct inspection of the fundus or where retinal and choroidal findings are inconclusive. Intraocular tissues vary in how they reflect ultrasonic waves. The retina is highly reflective, whereas the vitreous body is normally nearly anechoic. Ultrasound studies can therefore demonstrate retinal detachment and distinguish it from a change in the vitreous body. Optic disk drusen are also highly reflective. Ultrasound is also helpful in diagnosing intraocular tumors with a prominence of at least 1.5 mm. The specific echogenicity of the tissue also helps to evaluate whether a tumor is malignant, for example in distinguishing a choroidal nevus from a malignant melanoma (Fig. 12.6).

Ultrasound studies can demonstrate retinal detachment where the optic media of the eye are opacified (due to causes such as cataract or vitreous hemorrhage). This is because the retina is highly reflective in contrast to the vitreous body. Ultrasound can also be used to confirm the presence of malignant choroidal processes.

Fundus photography: Abnormal changes can be recorded with a single-lens reflex camera. This permits precise documentation of follow-up findings. Photographs obtained with a fundus camera in green light provide high-con- trast images of abnormal changes to the innermost layers of the retina such as changes in the layer of optic nerve fibers, bleeding, or microaneurysms.

Fluorescence angiography (with fluorescein or indocyanine green): In fluorescein angiography, 10 ml of 5% fluorescein sodium are injected into one of the patient’s cubital veins. Blue and yellow-green filters are then placed along the optical axis of a single-lens reflex camera. The blue filter ensures that only blue light from the light source reaches the retina. The yellow-green

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Ultrasound examination of the fundus.

Fig. 12.6 Ultrasound findings in malignant melanoma (arrow).

filter blocks the blue components of the reflected light so that the camera records only the image of the fluorescent dye (Fig. 12.7).

Fluorescein angiography is used to diagnose vascular retinal disorders such as proliferative diabetic retinopathy, venous occlusion, agerelated macular degeneration, and inflammatory retinal processes. Where the blood-retina barrier formed by the zonulae occludentes is disturbed, fluorescein will leak from the retinal vessels. Disorders of the choroid such as choroiditis or tumors can also be diagnosed by this method; in these cases indocyanine is better than fluorescein.

12.2.2Normal and Abnormal Fundus Findings in General

Normal fundus: The retina is normally completely transparent without any intrinsic color. It receives its uniform bright red coloration from the vasculature of the choroid (Fig. 12.8). The vessels of the choroid themselves are obscured by the retinal pigment epithelium. Loss of transparency of the retina is a sign of an abnormal process (for example in retinal edemas, the retina appears whitish yellow). The optic disk is normally a sharply defined, yellowish orange structure (in teenagers it is pale pink, and in young children significantly paler) that may exhibit a central depression known as the optic or physiologic cup. Light reflection on the inner limiting membrane will normally produce multiple light reflexes on the fundus. Teenagers will also exhibit a normal foveal reflex and wall reflex surrounding the macula, which is caused by the transition from the depression of the macular to the higher level of the retina (Fig. 12.9).

12.2 Examination Methods 309

Fluorescein angiography of the fundus.

Fig. 12.7 Blue and yellow-green filters are placed along the optical axis of a singlelens reflex camera. a First the blue filter ensures that only blue light from the light source reaches the retina. This excites the previously injected fluorescein dye in the vessels of the fundus. b The excited fluorescein emits yellow-green light, and the blue light is reflected. The yellow-green filter blocks the blue components of the reflected light so that the camera records only the image of the fluorescent dye.

Normal fundus.

Fig. 12.8 The macula lutea lies about 3 – 4 mm temporal to and slightly below the optic disk. The fundus receives its uniform bright red coloration from the vessels of the choroid. Venous diameter is normally 1.5 times greater than arterial diameter.

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Wall reflex surrounding the macula.

Fig. 12.9 Typical highly reflective fundus in a teenager (see wall reflex).

Age-related changes: The optic disk turns pale yellow with age, and often the optic cup will become shallow and will be surrounded by a region of choroidal atrophy. The fundus will become dull and nonreflective. Drusen will be visible in the retinal pigment epithelium and middle peripheral reticular proliferations of pigment epithelium will be present. The arterioles will be elongated due to loss of elasticity with irregular filling due to thickening of the vascular walls. Meandering of the venules will be present with crossing signs, i.e., the sclerotic artery will be seen to compress the vein at the arteriovenous crossing, reducing the diameter of the column of venous blood. In extreme cases venous blood flow will be cut off completely.

Abnormal changes in the fundus: As a rule, loss of transparency of the retina is a sign of an abnormal process. For example in a retinal edema, the retina appears whitish yellow (see Fig. 12.19). A distinctive feature of abnormal retinal and choroidal changes is that the type and appearance of these changes permit precise topographic localization of the respective abnormal process when the diagnosis is made. The ophthalmoscopic image will usually allow one to determine in which of the layers shown in Fig. 12.2 the process is occurring. For example, in Fig. 12.27 (nonexudative age-related macular degeneration) one may see that the drusen and atrophy are located in the retinal pigment epithelium; the structures above it are not affected, as is apparent from the intact vascular structures.

12.2 Examination Methods 311

12.2.3Color Vision

Color vision defects may be congenital (especially in men as they are inherited and X-linked recessive) or acquired, for example in macular disorders such as Stargardt’s disease. Qualitative red-green vision defects are evaluated with pseudoisochromatic plates such as the Ishihara or StillingVelhagen plates. They contain numerals or letters composed of small color dots surrounded by confusion colors (Fig. 12.10) that patients with color vision defects cannot read. The Farnsworth-Munsell tests (Fig. 12.11) can detect blue-yellow color vision defects.

Pseudoisochromatic plates contain numerals that patients with color vision defects cannot read. In the Farnsworth-Munsell test, patients with a color vision defect cannot sort markers with different hues (according to the colors of the rainbow) in the right order.

The Nagel anomaloscope permits quantitative evaluation of color vision defects. The test plate consists of a lower yellow half whose brightness can be adjusted, and an upper half that the patient tries to match to the lower yellow color by mixing red and green. The anomaly ratio is calculated from the final adjustment. Green-blind patients will use too much green, and red-blind patients too much red when mixing the colors.

Perimetry

Ishihara plates for diagnosing red-green vision defects.

Fig. 12.10 Patients with normal color vision will recognize the number 26 on the left and 42 on the right.

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Farnsworth-Munsell test of red-green and blue-yellow color vision defects.

Fig. 12.11 The patient must sort markers of various hues in the right order according to the colors of the rainbow.

12.2.4Electrophysiologic Examination Methods

(electroretinogram, electro-oculogram, and visual evoked potentials; see Fig. 12.2a)

Electroretinogram (ERG): This examination method uses electrodes to record the electrical response of the retina to flashes of light (Fig. 12.12a). Photopic (light-adapted) and scotopic (dark-adapted) electroretinograms are obtained. The electroretinogram (ERG) consists of a negative A wave indicating the response of the photoreceptors and a positive B wave primarily indicating the response of the bipolar cells and the supporting cells of Müller (Fig. 12.12b). A flicker ERG (repeated flashes) isolates pure cone response; a pattern ERG (such as a checkerboard) and oscillating potentials can be used to evaluate the inner layers of the retina. The ERG represents a summation response of the retina. A focal ERG can record the response of isolated areas of the retina.

The classic indication for an electroretinogram is retinitis pigmentosa with early loss of scotopic and photopic potentials.

Electro-oculogram (EOG): The electro-oculogram detects abnormal changes in the retinal pigment epithelium such as macular vitelliform dystrophy. This examination method utilizes the dipole of the eye in which the cornea forms the positive pole and the retinal pigment epithelium the negative pole. The standing potential across cornea and retina in comparison to the cornea is measured indirectly with two temporal electrodes (Fig. 12.13). During the measuring process, the patient performs regular eye movements by alternately focusing on two lights. The standing potential is normally higher in the light-adapted eye than in the dark-adapted eye. The ratio of light-adapted