Interpretation of the findings on slit-lamp biomicroscopy requires extensive knowledge of normal findings as well as background lesions that occur as incidental findings in the species and breed examined. These include, but are not limited to, embryonic remnants (e.g., persistent pupillary membranes, persistent or absorbing hyaloid arteries), corneal opacities (e.g., corneal scars, corneal degeneration/dystrophy in Sprague-Dawley rats, epithelial dystrophy in Dutch belted rabbits), cataracts, and lesions that may be associated with incidental trauma or inflammation (e.g., synechiae, traumatic cataracts, corneal scars).[1, 2, 10, 16–19] Examples of normal and abnormal slit-lamp findings are noted in Figs. 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, and 2.9. Whenever possible, animals with such background lesions noted at pre-study examinations should not be placed on study. However, in some animals such as the Sprague-Dawley rat, the incidental lesions may be so pervasive that use of affected animals cannot be avoided and randomization will usually ensure the lesions are seen with equal representation across all groups. It is then important to ensure the test article administration does not result in worsening of the lesions. Testing in more than one species is the norm for topical ocular studies and provides additional confirmation of safety.

When slit-lamp examinations are used in systemic toxicology studies, it may be sufficient to simply record abnormal findings, but in topical ocular studies, it is customary to quantify findings and modifications of scoring systems such as the Hackett-McDonald scoring system (Table 2.1) [8, 11] and others such as outlined in the Standardization of Uveitis Nomenclature or SUN system [15]. Such scoring systems are designed to detect very subtle microscopic ocular changes, and they may be commonly employed in both preclinical and clinical studies. When evalua­ting the lens, the examination should include the notation of whether the lens is normal or abnormal and should be accompanied by a comment describing the extent and location of the lens abnormalities as determined by direct and indirect (i.e., retroillumination) illumination. Cataracts may be noted as mild (or incipient invol­ving less than 10% of the lens), moderate (immature), or severe (involving the entire lens), and the location of the opacities may be described or defined by where they are localized in the lens (noted by the positions of the opacities in the slit beam as the light passes through the lens). Localization may thus be defined as anterior capsular, anterior subcapsular, anterior cortical, nuclear, posterior cortical, posterior subcapsular, posterior capsular, or equatorial with combinations of the preceding used if there is more than one opacity. In some instances, drawing of the lens with frontal and cross-sectional views may be used to further document the opacities. With such data recorded, it is thus possible to determine if a cataract is progressing during the study.

In addition to evaluating the solid structures of the eye, the aqueous and anterior vitreous can also be evaluated as the slit beam passes through the anterior chamber and the anterior vitreous just posterior to the lens. Uveitis can result in an increase in protein and cells in the aqueous and vitreous in association with breakdown of the blood-ocular barrier. Increase in protein in the aqueous humor results in a Tyndall effect, referred to as aqueous flare, such that the light of the slit beam can be seen as it passes through the aqueous. As noted in Table 2.1, the flare may be scored from 0 to +3 where 0 equals the normal absence of a visible light beam as it passes through the aqueous, +1 equals a visible light beam that is less intense than the beam as it passes through the normal lens (this includes a barely visible beam which may be further described as trace flare), +2 equals a visible light beam equal in intensity to the beam as it passes through the normal lens, and +3 equals a visible light beam that is greater in intensity than the beam as it passes through the normal lens. As noted in the SUN system (Table 2.2), some systems adapt a slightly different mode for scoring aqueous flare to accommodate noting the presence or absence of fibrin in the aqueous. However, there can be varying amounts of fibrin formation, and some prefer to score fibrin formation in a class by itself (Table 2.3). Since the presence of fibrin in the anterior chamber generally indicates a greater degree of breakdown of the blood-aqueous barrier and hence greater inflammation, this latter practice allows better quantification if fibrin formation is encountered with any frequency in studies.

The presence of cells in the aqueous and anterior vitreous can be observed with slit-lamp biomicroscopy, and the SUN system developed a reasonable method for scoring the quantity of cells observed in the aqueous in a 1 X 1 mm beam (Table 2.4). The observer should concentrate on counting the cells at the same point (usually in the central anterior chamber) without moving the beam since cells are heavier than the surrounding aqueous and will settle inferiorly. In addition to noting the number of cells in the aqueous, it is important to note the types of cells as they may be pigmented arising from the uvea or retinal pigment epithelium (due to injury to or shedding from those structures), nonpigmented (white blood cells arising from an inflammatory response), or red blood cells (arising from damaged vasculature) as may occur with surgical or other invasive procedures (intraocular injections, aqueous or vitresous aspirations, lens extraction, placement of intraocular implants, subretinal injections, etc.). When nonpigmented cells are present in such numbers that frank hypopyon (pus in the anterior chamber) occurs, that should be noted as a comment in addition to the scoring [9, 15]. Likewise, when frank hemorrhage is noted (hyphema with or without clots), that too is worthy of separate comment and clarification.