Las Reservas Naturales Urbanas

The slit lamp, also known as a biomicroscope, is an instrument that provides light and magnification for examining the eye. The name comes from the fact that the light is often in the form of a long, narrow slit, which allows the structures of the eye through which it passes to be seen in cross section. Thus, the slit lamp provides excellent depth perception, permitting abnormalities in the eye to be localized precisely. A variety of sophisticated techniques— shining the light in from different angles or changing the size, shape, or brightness of the light—can be employed to allow subtle problems to come into view more easily.

For the slit lamp examination, you will be asked to sit forward and place your chin on the chin rest, which should be covered with some disposable paper for hygienic purposes. Your forehead must also come forward to touch the forehead rest. Although this procedure sounds easy, it can be difficult for elderly (inflexible), stout, or full-chested individuals, who have a hard time getting close enough to position their head properly. Here are some hints. Instead of just leaning forward while you are sitting back in the exam chair, slide your bottom forward so that you are sitting near the edge of the seat. If you are very short, try sitting on a thick phone book to get up to the required height if the seat cannot be adequately adjusted. Make sure that when you place your chin on the chin rest, your forehead doesn’t tilt back away from the forehead rest; if it does, it will be difficult for the examiner to focus on your eyes properly. Most slit lamps also have bars to hold on to, much like the han- dlebars on a bicycle. Use them to stabilize yourself.

We examine the cornea, noting how clear it is, whether any scars or for- eign bodies are present, and whether there are any scratches or other defects in the surface. We also look for evidence of fluid (edema) and for any other signs of dysfunction. Fluid in the cornea, for example, may be caused by a loss of the cells lining the inner surface of the cornea, by high pressure in the eye, or by inflammation. Sloughing off of epithelial cells on the outer surface of the cornea may be caused by the irritative effects of infections, by dry eye syn- drome, or by medications used in the eye. Since contact lenses rest on the cornea, we can check the fit of the lenses and make sure they are not harming the cornea. We also look at the deeper layers of the cornea, especially in the area of the endothelium, the cells that line the back surface of the cornea. Tiny

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dots called guttata and wrinkles are signs of possible degeneration of the endothelium, which could ultimately result in edema of the cornea.

We check the conjunctiva, the clear membrane over the white of the eye, for redness, pigmentation, and growths. An infection or other inflammation will make the eye look red by causing the blood vessels in the conjunctiva to become dilated. Bleeding from the blood vessels will create a solid red spot on the eye.

We then look deeper into the eye, checking the anterior chamber, the fluid- filled space bounded by the cornea in front and the iris from behind. With the slit lamp, we can determine how deep the chamber is and whether any blood, inflammatory cells, or protein is present. We can see cells in the fluid using high magnification. Protein in the fluid causes flare, a cloudiness seen in the slit of light that travels through it, much like what you see in the light coming from a movie projector in a slightly dusty room. Flare and cells are caused by inflammation. Blood may occur after injury or may represent bleeding from abnormal blood vessels on the iris in people who have diabetes or other dis- orders affecting blood vessels.

Next, we check the iris, the brown or blue ring of the eye. The color depends on the amount of pigmentation present. We look for the presence of rubeosis iridis, the abnormal blood vessels mentioned earlier that can bleed and cause other problems. We also look for changes in the color of the iris, for growths, and for areas where the pigmentation has been lost. If there has been previous inflammation or surgery in the eye, the iris may be scarred down to the lens behind it in some places.

The lens of the eye lies right behind the iris. We look for cloudiness in the lens, the condition called cataract. The slit lamp helps us determine in exactly which layer of the lens any cloudiness is located. We also make sure that the lens has not been dislocated, that is, moved slightly out of position as the result of injury or some other factor.

We examine the vitreous, the gel that fills the large chamber of the eye behind the lens, to see whether any cells are present. Such cells may indicate inflamma- tion or, rarely, a malignancy. Tiny pigment flecks may signal the presence of a recent tear in the retina. If bleeding from any cause occurs, that can easily be seen as well. The front part of the vitreous can easily be seen with the slit lamp, but to examine the back part, a special focusing lens must be placed in front of the eye. This lens may simply be held in place, or it may be placed on the front surface of the eye as a contact lens would be.

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