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Low Vision Assessment and Rehabilitation

Anatomy


We first discuss some fundamentals of ocular structure. Figure 1 shows a cross-sectional schematic of the eye. Vision is detected by the retina, a complex light-sensitive structure that lies on the inner surface of the eyeball wall (sclera) and that communicates back to the brain via the optic nerve. Just outside the retina lies a critical single-cell thick layer called the retinal pigment epithelium, separating the retina from a thick highly vascular layer called the choroid, and its forward extension, the iris. The remainder of the eye is comprised largely of transparent structures, such as the corneal window on its front surface, the lens situated behind the pupil, and the anterior and posterior chambers of the eye which are filled with clear fluids.

Figure 2 illustrates the internal appearance of the normal eye as seen by your eye doctor. Detailed vision is provided by the central 5.5mm (1/5 inch) macula lying in the center of the visual axis, just to the outside of the optic nervehead, and extending upwards and downwards to the blood vessel arcades coursing laterally from the optic nerve. Most of the retina, the 95% outside the macula, detects motion and night vision and does not provide detailed vision.

Disease of the retina is commonly investigated by special color photographs confined to the central part of the retina (e.g. 30° camera), which is the critical area of retinal disease causing loss of detailed vision. Figure 3 [scroll down] shows a fundus photograph and fluorescein angiogram of a patient with diabetic retinopathy.

The eye is like a camera. Light enters through the curved front window, the cornea. The lens of the eye focuses the light on the retina (film), forming an image. When the space between the front and the back of the eye is too long a person is myopic (nearsighted) and the image from distant objects falls short of the retina (film). In hyperopic eyes (farsightedness), the eye length is too short and the image falls behind the retina.

Glasses merely bend the light rays a specific amount, depending on the strength of the glasses, and cause the image to move onto the retina (Figure 4 shows the retina of a person looking at a candle.) If the retina (film) is damaged, no matter how well the picture is focused, it will not be clear - like a picture on damaged film. "Stronger glasses" will only pull the focused image forward in front of the retina and blur the image. This explains why "stronger glasses" are not the answer to low vision--it is a very common misconception. This "logical sounding" suggestion is unfortunately wrong.


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Last Update: Nov 29 2012