2020–2021 BCSC Basic and Clinical Science Course™
3 Clinical Optics
Chapter 8: Optical Instruments
Anterior- and Posterior-Segment Imaging
Direct focal illumination. This is the most commonly used examination technique, in which the examiner focuses on the area directly illuminated by the slit (Fig 8-11A).
Retroillumination. The beam can be decentered, so that it is striking the iris, with the examiner focusing with the microscope in front, on the cornea, for example. This enables the observer to see corneal opacities illuminated against the black pupil by the reflected light from the iris (iris retroillumination). As another example, by shining the beam through the edge of the pupil, one can observe opacities in the cornea, iris, and lens retroilluminated by the reflected light from the fundus (fundus retroillumination; Fig 8-11B).
Sclerotic scatter. The slit beam is decentered so that it strikes the junction between sclera and cornea, causing light to be totally internally reflected, like a fiber-optic light pipe (Fig 8-11C). In this manner, light follows its longest possible path through the cornea and makes nebular opacities more visible against a dark pupil background.
Specular reflection. By looking directly at the bright reflection of the slit beam under high magnification, the examiner can observe irregularities in the corneal surface and see the endothelial cell pattern (Fig 8-11D; this is the principle underlying specular microscopy).
Figure 8-11 Examination techniques with the slit-lamp biomicroscope. A, Direct illumination. B, Retroillumination. C, Sclerotic scatter. D, Specular reflection.
(Illustration by Kristina Irsch, PhD. Insets in A–D reproduced with permission from Krachmer JH, Mannis MJ, Holland EJ, eds. Cornea. 2nd ed. Vol 1. Philadelphia: Elsevier/Mosby; 2005:201-217. © CL Mártonyi, WK Kellogg Eye Center, University of Michigan.)
There are several attachments for the slit lamp that extend its use beyond these examination techniques, some of which are described below.
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.