The eye is one of the most fascinating structures of the body. A great deal of this fascination hinges on the fact that it is the only organ that we can look directly into without having to cut and/or insert a special scope.
The slit lamp is a microscope designed specifically to examine the eye. It is composed of a microscope and a light source. The microscope is binocular; that is, it has two eyepieces, giving the binocular observer a stereoscopic (3-dimensional) view of the eye. Thus, another name for the instrument is the biomicroscope. Because the ocular structures can be examined dimensionally, the location of abnormalities can be determined with great precision.
The slit lamp is used to examine the external ocular adnexa, external eye, anterior chamber, iris, and crystalline lens. The anterior face of the vitreous may also be visible if the lens is clear.
Although there are numerous manufacturers and models of slit lamps, there are basically only two styles. The first utilizes a horizontal prism reflected light source (Figure 1-1). The second has a vertical illumination source (Figure 1-2).
The microscope is mounted on a stage that is designed for movement of the microscope and positioning of the patient. Microscope position is controlled by a joystick. Moving in and out (toward and away from the patient) allows the observer to focus the microscope at various depths. Twisting the joystick (or a wheel at its base) moves the microscope up and down, allowing the examiner to view various structures. The joystick can also be used to move the microscope from side to side, allowing scanning and easy movement from one eye to the other.
The microscope has various magnification settings. Most instruments have magnification abilities ranging from 6X to 40X. Magnification power is changed by flipping a lever or turning a dial (Figures 1-3A and 1-3B). (The power of the oculars is fixed. You are actually changing the objective lens when you change magnification). This can be done during the exam itself and does not interrupt the observer or the patient. Magnifications of 6X, 10X, and 16X are adequate for most exam purposes. Higher magnifications can be achieved by removing the standard oculars and inserting stronger-powered ones.
The actual magnification of what you see through the slit lamp is derived by multiplying the power of the oculars with the power of the objective lens. Thus, if your oculars are 10X and the objective lens is 1.6X, the total magnification will be 16X.
In addition to magnification, some oculars have an internal line or grid for measuring ocular structures. Other eyepieces incorporate an angle scale that is very useful in fitting and evaluating toric contact lenses. An ocular with a cross hair reticule is used in slit lamp photography.
The light source of the slit lamp is unique to this microscope and is the feature that makes it so adaptable for looking at the eye. The light is controlled by a transformer, which provides various voltage settings. The beam of light can be changed in intensity, height, width, direction or angle, and color during the exam with the flick of a lever or turn of a dial (Figure 1-4A and 1-4B). The majority of the microscopic eye exam is done with the light beam set at maximum height but with a narrow width that produces a slit of light, hence the name slit lamp. In addition, the light source (illuminator) moves independently of the microscope unit, making various types of illumination and views of ocular structures possible. A click-stop indicates when the light is directly aligned with the microscope. However, most illumination techniques require that the light be positioned at an angle to the scope. A marked dial (angle scale index) at the base of the arm indicates degrees (Figure 1-5). The microscope
Figure 1-1. Topcon slit lamp model SL-2E with horizontal prism reflected light source. (Photo courtesy Topcon.)
Figure 1-2. Topcon slit lamp model SL-3E with vertical illumination source. (Photo courtesy Topcon.)
Figure 1-3B...or rotating a knob. (Reprinted with permission from Ophthalmic Photography, SLACK Incorporated.)
Figure 1-4B. ...or levers (Reprinted with permission from Ophthalmic Photography, SLACK Incorporated. Photo by Steve Carlton.)
and light are coordinated so that the structure to be viewed is magnified and illuminated. This can be altered by moving the slit image off center, as is required by some illumination techniques (discussed in Chapter 4).
While white light is used for most examinations, there are several colored filters that can be utilized, as well. The cobalt blue filter is used in conjunction with fluorescein dye. The dye pools in areas where the corneal epithelium is broken or absent. The blue light excites the fluorescein, which then takes on a yellowish glow.
The green filter obscures anything that is red (hence the pseudonym red-free light); thus, blood vessels or hemorrhages appear black. This increases contrast, revealing the path and pattern of inflamed blood vessels. Areas of the episclera where lymphocytes (infection-fighting white blood cells) have gathered in response to an inflammatory or immune response will appear as yellow spots under the red-free light. Fleischer ring (seen in keratoconus, see Chapter 5) can also be viewed satisfactorily with the red-free filter.
Some instruments also have a diffuser, which is a piece of frosted glass or plastic that flips in front of the illuminator. The diffuser scatters the light, causing an even spread of light over the entire ocular surface. The filters are placed by flipping a lever (Figure 1-6).
Patient positioning is achieved by an attached head rest unit that includes a moveable chin cup and a stationary forehead band. There may also be a strap that can be fastened behind the patient's head to ensure stability. Most models also include a moveable fixation light, which gives the patient a target to look at while his or her eye is being examined. There may also be grips for the patient to hold on to, mounted on the side of the head rest unit. (Patient positioning is discussed in Chapter 2.) Some units have a breath shield attached to the microscope arm for the convenience and comfort of both operator and patient.
Figure 1-6. Changing filters. (Photo by Mark Arrigoni.)
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