The examiner actively controls the light beam with multiple illumination methods to sweep across the eye, using shadows and reflections to bring out details. Having the patient blink can also help the examiner distinguish changes of the ocular surface from tiny opacities floating in the tear film. After initial low-power screening, much of the slit-lamp examination is performed using higher magnifications.
Except for the anterior vitreous humor, deeper and peripheral intraocular structures require special lenses. A contact lens allows examination of the intermediate and posterior portions of the eye and is often combined with angled mirrors and prisms for gonioscopy and peripheral fundus examination.
Stains
Fluorescein
Topical fluorescein is a nontoxic, water-soluble hydroxyxanthene dye that is available in several forms: as a 0.25% solution with an anesthetic (benoxinate or proparacaine), an antiseptic (povidoneiodine), and a preservative; as a 2% nonpreserved unit-dose eyedrop; and in impregnated paper strips. Fluorexon is a related macromolecular compound available as a 0.35% nonpreserved solution that will not stain most contact lenses. Staining is easily detected with a cobalt blue filter.
Fluorescein is most commonly used for applanation tonometry and evaluation of the tear film, including filaments. Tear breakup time (TBUT) is measured by instilling fluorescein, asking the patient to hold the eyelids open after 1 or 2 blinks, and counting the seconds until a dry spot appears. The appearance of dry spots in less than 10 seconds is considered abnormal. TBUT is further discussed in Chapter 3. Fluorescein detects disruption of intercellular junctions and will stain punctate and macroulcerative epithelial defects (positive staining) such as herpetic dendritic lesions or dysplastic epithelium. It can also highlight nonstaining lesions that project through the tear film (negative staining), such as basement membrane dystrophy or Thygeson superficial punctate keratitis. Different disease states can produce various punctate staining patterns (Fig 2-5). Fluorescein that collects in an epithelial defect will diffuse into the corneal stroma and cause a green flare in the anterior chamber. Pooling of the dye due to an indentation or thinning of the cornea must be distinguished from actual staining. In the dye disappearance test, the tear meniscus is observed for the disappearance of fluorescein. Prolonged presence of the dye suggests a blockage of the drainage system.
Figure 2-5 Punctate staining patterns of the ocular surface. (Illustration by Joyce Zavarro.)
The Seidel test is used to detect seepage of aqueous humor through a corneal perforation. The examiner applies fluorescein using a moistened strip or concentrated drop to the site of suspected leakage and looks for a flow of clear fluid streaming through the orange dye under cobalt blue light (Fig 2-6).
Figure 2-6 Leakage of aqueous from the anterior chamber (arrow) following a corneal laceration. Concentrated fluorescein on the edge of the aqueous rivulet (Seidel test) indicates an active flow of fluid from a leaking anterior chamber.
Rose Bengal and Lissamine Green
Rose bengal and lissamine green (both available as a 1% solution or in impregnated strips) are other water-soluble dyes. They stain the epithelial cells of the cornea and conjunctiva when a disruption occurs in the protective mucin coating. These dyes are routinely used for evaluating tear-deficiency states and for detecting and assessing various epithelial lesions, such as the extent of corneal intraepithelial neoplasia. Rose bengal is toxic to the epithelium. Lissamine green is better tolerated and has fewer toxic effects on cultured human corneal epithelial cells (Fig 2-7). See also Chapter 3.
Faulkner WJ, Varley GA. Corneal diagnostic techniques. In: Krachmer JH, Mannis MJ, Holland EJ, eds. Cornea. 3rd ed. Vol 1. Philadelphia: Elsevier/Mosby; 2011:131–138.