Michigan; part B courtesy of John E. Sutphin, MD.)
Indirect Illumination Methods
Proximal illumination
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.
Sclerotic scatter
Total internal reflection in the cornea makes possible another form of indirect illumination, sclerotic scatter. (See BCSC 3, Clinical Optics, for 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. 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 (Fig 2-3).
Figure 2-3 Cornea verticillata in Fabry disease demonstrated by sclerotic scatter against the dark background of a well-
dilated pupil. (Reproduced with permission from Krachmer JH, Mannis MJ, Holland EJ, eds. Cornea. 2nd ed. Vol 1. Philadelphia: Elsevier/Mosby; 2005:212. © CL Mártonyi, WK Kellogg Eye Center, University of Michigan.)
Retroillumination
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. 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 it shines through the edge of the pupil. Opacities in the cornea or lens (such as corneal dystrophies) are highlighted against the red reflex, and iris defects are transilluminated (Fig 2-4).
Farrell TA, Alward WLM, Verdick RE. Fundamentals of slit-lamp biomicroscopy. In: The Eye Exam and Basic Ophthalmic Instruments [DVD]. San Francisco: American Academy of Ophthalmology; 1993. (Reviewed for currency 2007.)
Mártonyi CL. Slit lamp examination and photography. In: Krachmer JH, Mannis MJ, Holland EJ, eds. Cornea. 3rd ed. Vol 1. Philadelphia: Elsevier/Mosby; 2011:89–123.
Figure 2-4 Epithelial fingerprint dystrophy is best visualized in retroillumination from the fundus. (Reproduced with permission from
Krachmer JH, Mannis MJ, Holland EJ, eds. Cornea. 2nd ed. Vol 1. Philadelphia: Elsevier/Mosby; 2005:217. © CL Mártonyi, WK Kellogg Eye Center, University of Michigan.)
Clinical Use
The slit-lamp examination should be done 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. Retroillumination and indirect illumination 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, slit, and retroillumination techniques are used to identify abnormalities of the iris and lens.