The slit lamp is a fundamental and invaluable tool in the ophthalmologist’s armamentarium. Mastery of the slit-lamp examination is critical in categorizing corneal pathology and formulating a diagnostic (Fig 2-1) and therapeutic plan.
The slit-lamp biomicroscope has 2 rotating arms—one for the slit illuminator and the other for the biomicroscope—mounted on a common axis. The illumination unit is essentially a projector with a light beam that is adjustable in width, height, direction, intensity, and color. The biomicroscope is a binocular Galilean telescope that offers a choice of magnifications. The illumination and microscope arms are parfocal, arranged so that both focus on the same spot, with the slit beam centered on the field of view. This setup provides direct illumination, and purposeful shifting of alignment allows for indirect illumination.
Four reflections of light can be seen when the eye is examined with the slit lamp. These reflections are known as Purkinje images or reflexes, named after the Czech anatomist Jan Evangelista Purkyně. The first Purkinje image is reflection off the surface of the cornea. The second is from the inner surface of the cornea. The third and fourth are from the anterior and posterior surfaces of the lens, respectively.
Figure 2-1 Flowchart for diagnosis of corneal opacity. HSV = herpes simplex virus; HZV = herpes zoster virus.
The slit lamp allows clinicians to examine the eye in a variety of ways, as described in the discussions that follow.
Direct illumination methods
With diffuse illumination, the light beam is broadened, reduced in intensity, and directed at the eye from an oblique angle. Swinging the illuminator arm to produce highlights and shadows can enhance the visibility of raised lesions of the ocular surface and iris (Fig 2-2).
Figure 2-2 Diagram of how light rays interact with the eye in slit-lamp biomicroscopic examination. A, Direct illumination. B, Specular reflection. C, Sclerotic scatter. D, Retroillumination.
(From Tasman W, Jaeger AE, eds. The slit lamp: history, principles, and practice. In: Duane’s Clinical Ophthalmology. Philadelphia: Lippincott; 1995–1999:33. Redrawn by Cyndie C. H. Wooley.)
With focal illumination, the light and the microscope are focused on the same spot, and the slit aperture is adjusted from wide to narrow. Broad-beam illumination, which uses a beam width of about 3 mm, is optimal for visualizing eyelid lesions as well as the corneal opacities seen in dystrophies or scarring. Slit-beam illumination, which uses a beam width of about 1 mm or less, gives an optical section of the cornea (Fig 2-3); this section is essential for evaluation of corneal thinning, edema, stromal infiltrates, and endothelial abnormalities. The examiner can use a very narrow slit beam to help identify differences in refractive index between transparent structures as light rays pass through the cornea, anterior chamber, and lens. The examiner can also reduce the height of a narrow beam to determine the presence and amount of cell and flare in the anterior chamber.
Specular reflections are normal light reflexes bouncing off a surface (see Fig 2-2B). An example is the bright round or oval spot seen reflected from the ocular surface in a typical flash photograph of an eye. These mirror images of the light source can be annoying, and it is tempting to ignore them during slit-lamp examination. However, the clarity and sharpness of these reflections from the tear film give clues to the condition of the underlying tissue.
A faint reflection also comes from the posterior corneal surface. The examiner can enhance this specular reflection by placing a light beam at an appropriate angle, revealing the corneal endothelium. Following are the steps for examining the corneal endothelium with specular reflection:
Begin by setting the slit-beam arm at an angle of 60° from the viewing arm and using a short slit or 0.2-mm spot.
Identify the very bright mirror image of the lightbulb’s filament and the paired epithelial and endothelial Purkinje light reflexes.
Superimpose the corneal endothelial light reflex onto the filament’s mirror image, giving a bright glare.
Use the joystick to move the biomicroscope slightly forward in order to focus the endothelial reflex.
Figure 2-3 Slit section of normal cornea. 1, Tear film. 2, Epithelium. 3, Anterior stroma with a high density of keratocytes. 4, Posterior stroma with a lower density of keratocytes. 5, Descemet membrane and endothelium.
(Reproduced with permission from Krachmer JH, Mannis MJ, Holland EJ, eds. Cornea. 2nd ed. Vol 1. Philadelphia: Elsevier/Mosby; 2005:201. © CL Mártonyi, WK Kellogg Eye Center, University of Michigan.)
Specular reflection is monocular, and 1 eyepiece may require focusing. A setting of 25× to 40× is usually necessary to obtain a clear view of the endothelial mosaic. Cell density and morphology are noted (Fig 2-4); guttae and keratic precipitates appear as nonreflective dark areas.
Indirect illumination methods
Turning a knob on the illumination arm slightly decenters the light beam from its isocentric position, causing the light beam and the microscope to be focused at different but adjacent spots. This technique, proximal illumination, highlights an existing opacity against deeper tissue layers and allows the examiner to see small irregularities that have a refractive index similar to that of their surroundings. Moving the light beam back and forth in small oscillations can help the examiner detect small 3-dimensional lesions, such as a corneal foreign body.
Figure 2-4 Corneal endothelium seen with specular reflection using the slit-lamp biomicroscope at 40× magnification.
(Reproduced with permission from Krachmer JH, Mannis MJ, Holland EJ, eds. Cornea. 2nd ed. Vol 1. Philadelphia: Elsevier/Mosby; 2005:208. © CL Mártonyi, WK Kellogg Eye Center, University of Michigan.)
Total internal reflection in the cornea makes possible another form of indirect illumination, sclerotic scatter. (See BCSC Section 3, Clinical Optics, for a discussion of total internal reflection.) Decentering the isocentric light beam so that an intense beam shines on the limbus and scatters off the sclera causes a very faint glow of the cornea (see Fig 2-2C). Reflective opacities stand out against the dark field, whereas areas of reduced light transmission in the cornea are seen as shades of gray. This technique is effective in demonstrating epithelial edema, mild stromal infiltration, nebulae, and cornea verticillata.
Retroillumination can be used to examine more than one area of the eye. Retroillumination from the iris is performed by displacing the beam tangentially while examining the cornea (see Fig 2-2D). Through observing the zone between the light and dark backgrounds, the examiner can detect subtle corneal abnormalities. Retroillumination from the fundus is performed by aligning the light beam nearly parallel with the examiner’s visual axis and rotating the light so that it shines through the edge of the pupil. Opacities in the cornea, phakic or pseudophakic lens, or posterior capsule (Fig 2-5) are highlighted against the red reflex, and iris defects can be transilluminated.
Mártonyi CL, Maio M. Slit lamp examination and photography. In: Mannis MJ, Holland EJ, eds. Cornea. Vol. 1. 4th ed. Philadelphia: Elsevier; 2017:79–109.
The slit-lamp examination should be performed in an orderly fashion, beginning with direct illumination of the eyelids (margin, meibomian glands, and eyelashes), conjunctiva, and sclera. A broad beam illuminates the cornea and overlying tear film in the optical section. Details are examined with a narrow beam. The examiner estimates the height of the tear meniscus and looks for mucin cells and other debris in the tear film. Discrete lesions are measured with a slit-beam micrometer or an eyepiece reticule. All indirect illumination methods, including retroillumination, accentuate fine changes. The examiner then uses specular reflection to inspect the endothelium and has the patient shift gaze in different directions so that each corneal quadrant can be surveyed. A slit beam is used to estimate the thickness of the cornea and the depth of the anterior chamber. A short beam or spot will show flare or cells in the aqueous humor. Direct and indirect illumination and retroillumination techniques are used to identify abnormalities of the iris and lens.
Figure 2-5 Retroillumination reflex from the fundus, highlighting an Nd:YAG laser opening in the posterior capsule with Elschnig pearl formation.
(Courtesy of Stephen E. Orlin, MD.)
With all illumination methods, the examiner actively controls the light beam to sweep across the eye, using shadows and reflections to bring out details. Having the patient blink can help the examiner distinguish ocular surface changes from tiny opacities floating in the tear film. After initial low-power screening, much of the slit-lamp examination is performed using higher magnifications.
Visualization of deeper and peripheral intraocular structures, except for the anterior vitreous humor, requires special lenses. A contact lens allows examination of the intermediate and posterior portions of the eye; its use is often combined with angled mirrors and prisms for gonioscopy and peripheral fundus examination.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.