Slit Lamp Examination and Photography

Broad-beam illumination

The term broad-beam illumination is variably used and highly subject to interpretation. It can vary from a beam width of 1 mm to its full size of approximately 11 mm. In this discussion a flexible width is assumed. As with the recommended oscillation of the slit illuminator, a dynamically altered beam width is also beneficial. In this application, the beam is intended only as a source of bright focal illumination, with the width adjusted to maximize information within the area under study. As the light strikes tissue interfaces, it is reflected, refracted, transmitted, scattered, and absorbed in a highly variable fashion. Thus a given width of beam with its corresponding overall intensity may provide good information to confirm a particular entity but may overpower findings associated with another. A beam width of 2–3 mm can provide a good starting point ( Fig. 7.6 ). Although width is an important factor in the beam’s efficiency in specific applications, its intensity also affects its usefulness. A beam that is too bright will produce scatter, reducing the examiner’s ability to discriminate. Conversely, a beam intensity that is too “gentle” (in deference to the patient’s comfort, for example) may preclude detection of mild departures from the normal.

As various tissues are examined, suspected alterations from the normal will either be confirmed or will prompt the examiner to use more or less light in their continued pursuit. As a general rule, forms of illumination should be exaggerated in both directions beyond the ideal setting, especially with the use of broad-beam illumination. The beam should be narrowed to the point of diminished width and then increased in width beyond the ideal setting to the point at which information loss occurs once again. Only by testing these limits will optimum size and intensity become apparent.

Many conditions are seen best using broad-beam illumination ( Box 7.2 ). All changes that are fairly opaque and reflect or absorb considerable amounts of light can be visualized easily. The light should be applied tangentially for maximum effectiveness. Topographic changes will become dramatically sculpted by the raking light. Additionally, oblique illumination will obviate the dazzling specular reflections resulting from axial lighting ( Fig. 7.7 ). Tangentially applied broad-beam illumination is one of the most effective forms for examining the iris surface ( Figs. 7.8–7.10 ) and certain changes within the lens ( Fig. 7.11 ).

When no abnormalities are seen with this technique, more selective forms of illumination are indicated. The absence of findings under broad-beam illumination should never encourage the conclusion that abnormalities are not present. However useful in the applications described earlier, broad-beam illumination can be quite counterproductive to the detection of many alterations of subtle expression.

Optical section

This narrowest slit beam is, in effect, a fine blade of light that makes possible the virtual serial sectioning of transparent tissues in the living eye. The tangential presentation of these “light slices” facilitates an essentially cross-sectional view of the cornea and lens even though these structures are largely parallel with the plane of observation. The sharply focused light is completely confined to the optical section, reducing scatter and maximizing contrast between the illuminated section and the dark, unilluminated surround. The result is a clear, basically uncompromised view of the tissue within the beam. As the slit beam is projected from an increasingly lateral position (away from the axis of the biomicroscope), the greater angular presentation has the effect of increasing the distance between the anterior and posterior surfaces of the structure under study. This increase serves to clarify intrastructural relationships and localization of abnormalities within.

This capability represents the most selective and most isolating manner in which such tissue may be illuminated and observed ( Box 7.3 ). For maximum effectiveness, the light intensity is set to maximum and the slit beam is diminished in width to a point just before the optical section loses structural integrity. The thinner the beam, the more selective the optical section, thus producing finer delineation of information within that section. Beam width, however, should never be reduced to the point at which information is compromised because of light loss.

The transparency of the cornea coupled with its propensity for both primary and secondary expressions of numerous diseases makes it the most important component of the eye to section with light. Although remarkably transparent, normal corneal tissue is sufficiently relucent to reflect the narrow slit beam and articulate the optical section.

A high-magnification view of the optical section will produce excellent discrimination of the substantial corneal layers. Beginning with the tear film, each layer may be selectively examined for departures from the normal ( Fig. 7.12 ). The normal tear film will “flow” dynamically within the slit beam following each blink of the eyelids. Its reflectivity will alter with the amount of refreshed, protective lipid on its surface, but it will maintain a constant thickness and a smooth anterior face. The epithelium is seen as a line of nonreflectance or greatly diminished reflectance between reflections from the tear film and the Bowman layer, which is contiguous with (and largely indistinguishable from) the anteriormost reflection from the stroma. The stroma itself is quite transparent. By encroaching on the zone of specular reflection, however, its reflectance can be enhanced considerably for better visualization of its structure. The optical section will terminate with the heightened reflection from the endothelium.

As corneal transparency is lost to disease or injury, an increased amount of light is reflected. (The condition of a sclerotic, totally opaque cornea represents the extreme, or terminal, end of this transmission spectrum. In this condition, much of the light is reflected by the surface, limiting visual access to deeper layers. In such cases, moderate amounts of illumination will be considerably more informative, and a thorough examination will necessitate the use of indirect techniques, as discussed later in this chapter.)

An inadequate tear film will present as a compromise to the normally smooth, unbroken reflection of the beam ( Fig. 7.13 ). An edematous epithelium will become dimensional in the optical section and reflect increasing amounts of light ( Fig. 7.14 ). Changes in focal density will be isolated to the anterior, middle, or posterior cornea ( Figs. 7.15 and 7.16 ). The Descemet membrane will become visible when abnormal mechanical forces alter its normal topography ( Fig. 7.17 ). The endothelium will reflect increased amounts of light when it is affected by changes such as Fuchs dystrophy ( Fig. 7.18 ). Within the visual axis, relatively mild expressions of tissue compromise can cause notable symptoms. Determining exact location and distribution is significant for arriving at a diagnosis.

As the cornea is scanned in optical section, the relationship of the anterior to the posterior surface is also observed for changes in normal thickness and curvature. Ectatic changes, such as keratoconus or keratoglobus, will become readily apparent ( Fig. 7.19 ). Focal elevations or depressions also become obvious as the slit beam deviates toward or away from the light source ( Fig. 7.20 ).

It is important to apply the narrow slit beam to all ocular surfaces. A thorough scan of the lid margins, the bulbar and palpebral conjunctiva, the plica, caruncle, and corneal limbus will provide confirmatory information or add to what was gleaned using the modalities described earlier. The narrow slit beam is most effective in detecting topographic changes in these structures ( Fig. 7.21 ). Follicles, papillae, or other dimensional alterations are well stated in this manner. All tissue will demonstrate some penetration by the beam. The degree of penetration is variably limited by the optical density of the tissue under study. Although actual penetration may be minimal, the information produced can be valuable. Of equal or greater importance is the indirect, proximal illumination simultaneously achieved (see the later section titled “Indirect Illumination”).

The relationship between the cornea and iris is also evaluated with a narrow slit beam. By projecting the light from a moderate angle and observing the distance between the reflections from the cornea and the iris, a good estimate of anterior chamber depth can be obtained. Observing this relationship at the limbus provides information regarding the grade of the angle. A completely closed segment of the angle is indicated by a contiguous presentation of corneal and iris reflections ( Fig. 7.22 ). In a similar fashion, anterior synechiae may present as focal areas of contact between the reflected beams at the posterior corneal surface. This condition is confirmed by observing the slit beam coursing up the side of the tented iris tissue to make contact with the reflection from the posterior corneal surface ( Fig. 7.23 ).

Tyndall light/anterior chamber cells and flare

Based on the Tyndall phenomenon, pinpoint illumination is maximally effective at isolating aqueous cells and flare. The anterior chamber is considered optically empty, as its contents do not reflect sufficient light to express the beam in the normal state. The cells and protein that present in response to local inflammation, therefore, can be seen readily when isolated within the well-defined, narrow tunnel of light produced by pinpoint illumination.

A small round beam of high intensity is directed tangentially through the anterior chamber, and the focal point of the light (and the biomicroscope) is swept through the aqueous to determine the presence and density of cells and flare. For maximum contrast, when conditions permit, cells and flare should be observed against the dark background of a dilated pupil while minimizing the light striking the iris.

Although the “pinpoint” or “pencil of light” configuration represents the most discriminating technique ( Fig. 7.24 ), the standard grading system used to describe the concentration of cells and flare assumes the use of a beam approximately 1 by 3 mm in size. The number of actual cells seen simultaneously within that beam is the stated degree of the condition. The amount of abnormal protein is determined by the examiner’s impression of the reflectivity (or Tyndall effect) of the aqueous. The degree of expression of these two conditions is stated in terms of one to four or more cells and/or flare ( Fig. 7.25 ).

Specular reflection

The bright mirrored reflections of light sources are considered regular, or specular, reflections as opposed to the more common irregular reflections whereby most objects are seen. Specular reflections are subject to Snell’s law of optics, which states that the angle of reflection equals the angle of incidence. This suggests a level of difficulty associated with the location of such a reflection that is simply not present when the eye is being examined. On the curved ocular surfaces, specular reflections are easily elicited. The convex cornea and the high reflectivity of its anteriormost layer, the tear film, combine to make such reflections more or less ever-present companions during the course of an examination. Although they may sometimes be annoying, these reflections are, in fact, of great value for gathering information about the condition of the eye.

As the eye is viewed in direct illumination, the zone of specular reflection is studied as an expression of surface integrity. This evaluation should be performed under a fairly low light level so as to diminish scatter and make visible detail within the zone of specular reflection. The area of specular reflection is not only a mirror image of the source but also faithfully mirrors the topographic condition of the surface on which it rests. Therefore a compromised corneal surface will produce an abnormal reflection of the light source. Broken or granular reflections may indicate an inadequate tear film, the presence of foreign material, or a compromise to underlying tissue expressed as an alteration from normal topography. A greatly diminished or irregular reflection is always a clear sign of abnormality ( Fig. 7.26 ).

The most important application of the specular reflection is in the evaluation of the corneal endothelium. While not expressly difficult, this technique may prove initially challenging. The reflectivity of the endothelial surface is so much lower than that of the tear film layer that the specular reflection may not be appreciated even when present. An angular difference of 30–40 degrees between the slit illuminator and the biomicroscope will make the task easier by producing greater separation between the two reflections. Moving the incident beam laterally across the face of the cornea will elicit the bright specular reflection from the tear film layer. By observing the adjacent area on the side opposite the light source, the more demure endothelial reflection will be seen. Moving the biomicroscope forward approximately 0.5 mm will bring into focus the endothelial layer, and cellular detail should become apparent. To obtain a clear view of endothelial cells, especially those that populate the uncompromised young cornea, a magnification of 25× to 40× is required ( Fig. 7.27 ). When such levels of magnification are not available, the endothelium can still be evaluated with this technique. The reflection is observed for continuity and uniform intensity as the light is played across the endothelium. When conditions such as guttae are present, the homogeneity of the reflection is interrupted ( Fig. 7.28 ).

In severe expressions of disease-related endothelial compromise, such as advanced Fuchs dystrophy, the reflection may become totally altered from the normal. Coalesced guttae may cause it to appear patchy or quite dark overall and, even at high magnification, information about individual cell borders may not be present ( Fig. 7.29 ). In such conditions, even specular micrography may fail to produce satisfactory information regarding cell morphology or density.

The specular reflection can also be used to examine the surface of the conjunctiva and the anterior and posterior surfaces of the lens.