Slit Lamp Examination

The slit lamp biomicroscope is the primary tool for detailed, magnified evaluation of the anterior segment, combining a variable-width illumination beam with binocular magnification (typically 6x to 40x). With accessory lenses — gonioscopy lenses held on the eye, or indirect ophthalmoscopy lenses positioned 5-10 cm from the face — it also extends to the angle and posterior segment.

The examination follows a fixed sequence to avoid missing pathology: begin at low magnification with diffuse illumination for a general overview, then move systematically through lids and lashes, conjunctiva and sclera, cornea (medium-to-high magnification, direct focal illumination), anterior chamber (narrow slit beam at 30-45 degrees to grade cells and flare), iris, lens, and anterior vitreous when visible. Skipping structures or examining out of order is one of the most common ways pathology gets missed, as is relying only on diffuse illumination rather than also using oblique, indirect, or retroillumination techniques — each highlights different types of pathology (optical sectioning localizes depth, retroillumination silhouettes anterior structures against the red reflex, sclerotic scatter reveals subtle corneal edema).

Fluorescein with the cobalt blue filter is essential for detecting epithelial defects — omitting it, or applying it inadequately, is a recognized way to miss corneal pathology. A shallow anterior chamber on exam should prompt gonioscopy rather than being noted and left uninvestigated, given the implications for angle-closure risk.

One clinically useful distinction the slit lamp helps make is between episcleritis and scleritis: superficial vessel injection that blanches with topical phenylephrine favors episcleritis, while deeper, bluish, non-blanching vessel injection favors scleritis.¹ This vessel-blanching distinction, along with the patient's history, is what a source specifically addressing episcleritis-versus-scleritis diagnosis describes as the basis for differentiating the two conditions — it's worth noting this framework comes from a single source rather than being validated across a broad evidence base, and that source describes scleritis as often (not universally) associated with systemic rheumatologic disease and needing prompt anti-inflammatory treatment, rather than stating this applies to every case.¹ In practice, this means a bluish, non-blanching, painful scleral injection should prompt consideration of a systemic workup and specialist input rather than being treated as a purely local finding — while recognizing that not every scleritis case will turn out to have identifiable systemic disease.

Infection control is a real, distance-driven consideration specific to this exam: slit lamp examination occurs at 0.25 to 0.5 meters of face-to-face proximity, closer than most other parts of a clinical encounter, which has been highlighted as a respiratory-transmission risk for the examiner.² Patient-sided breath shields attached to the chin rest have been specifically tested (rather than assumed) and shown to substantially reduce forward droplet spread in simulated testing, and are a reasonable adjunct alongside standard hand hygiene and surface disinfection between patients.³