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Diseases of the Retina

T3 C ra

The receptors of the retina (^ A1 b) are rods (Rs) and three different types of cones (Cs). The latter mediate the color sense (red, green, blue; see below) and are particularly numerous at the site of sharpest vision (fovea centra-lis). The rods mediate black and white vision and particularly predominate in the retinal periphery. The light-sensitive outer segments of the photoreceptors are renewed regularly, while the residues of the pigment epithelial cells are phagocytized. The photoreceptors transmit their excitation via bipolar cells (Bps) to the ganglion cells (Gs). Amacrine cells (Ams) and horizontal cells (Hcs) form cross-connections between photoreceptors, bipolar cells and ganglion cells (^ A1 a).

If phagocytosis of the pigment epithelial cells is impaired, metabolic products accumulate and the photoreceptors degenerate (retinitis pigmentosa; ^A2). Macular degeneration that occurs in childhood (Stargardt's disease) is due to a genetic defect of an ATP-binding transport protein (ABCR) that is normally expressed in the outer segment of the photoreceptors. A defect of this transporter can disturb the normal turnover of the outer segments. Heterozygote carriers of the genetic defect suffer from increasing macular degeneration as they grow older.

Electroretinogram (ERG). When light falls on the retina, potential differences can be recorded between the cornea and an indifferent electrode on the ear (^A3). Sudden exposure to light at first generates an a-wave, the summation of potential changes at the receptors. It is followed by a b-wave due to potential changes in the bipolar cells and glial cells, and a c-wave due to potential changes in the pigment epithelium. When the light is turned off, a d-wave is registered (off-effect), the sum of the potential changes in the photoreceptor and bipolar cell membranes (reversed potential).

Occlusion of the central artery causes death of the amacrine cells, bipolar cells and ganglion cells and thus blindness. However, the receptors and pigment epithelium survive because they are supplied with adequate oxygen by the choroid vessels. In the ERG the b-wave is thus absent, but the a-wave and c-wave are preserved. In retinal detachment from the pigment epithelium no deflections are registered in the ERG. If the retina is completely detached, the patient is totally blind.

Diabetic retinopathy (^B) is the mostcom-mon disease of the retina. The cells around the thin retinal blood vessels (pericytes) produce sorbitol from the increased supply of glucose (^ p. 290), swell up, and thus narrow the vessels. Additionally, the vessel walls are thickened by glycosylation (AGE; ^ p. 290). This results in ischemia of the tissues, formation of angiotrophic mediators, increase in vascular permeability, formation of new vessels, and hemorrhages. This bleeding opacifies the vitreal body, the ischemia destroys the retina and may ultimately lead to blindness.

Night blindness. The visual pigment consists of 11-cis-retinol, a metabolite of vitamin A and a protein that is different in the rods and the three types of cones (^ C1). In vitamin A deficiency the formation of visual pigment in rods and cones is impaired, resulting in reduced light perception especially at low light intensity.

The function of the cones is to provide color vision. The pigments of the red, green, and blue cones each have different spectral sensitivities. Mutations of the genes for the respective pigments impair color vision. Partial or complete loss of the particular pigment (^ C2) leads to weak red color vision or red color blindness (protanomaly or protanopia, respectively), green color weakness or blindness (deuter-anomaly or deuteranopia), or blue color weakness or blindness (tritanomaly or tritanopia). As the genes for the red and green pigments are located on the X chromosome, many more men than women suffer from red or green color blindness.

If there are no cones, not only is there no color vision, but visual acuity is also greatly reduced, because the person can see only with much fewer rods in the fovea (rod monochro-masia).

Color vision can be tested e.g. with tables in which the numbers can be correctly recognized only by means of the corresponding cones (^ C3).

A. Diseases of the Retina b

Electroretinogram c b a

|— B. Diabetic Retinopathy

Capillary

Diabetes mellitus

Normal fundus

Pericyte

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