6.5 Slit-lamp biomicroscopy

What the slit lamp does

The slit lamp is a binocular microscope combined with an adjustable slit of light. Together they give a magnified, stereoscopic, cross-sectional view of the eye from the tear film through the cornea, anterior chamber, iris, and lens, and, with auxiliary lenses, the retina. It is the workhorse of every eye clinic and the instrument on which most anterior-segment findings are documented. Working the joystick, slit controls, magnification, and filters together with both hands is a core technician skill, and smooth control lets the examiner isolate and localize a finding quickly without fatiguing the patient.

Parts of the instrument

• Illumination arm: houses the light source and the slit itself, whose width, height, angle, and color can all be varied. Cobalt-blue and red-free (green) filters live here. The arm pivots around the same axis as the microscope.
• Viewing (observation) arm: the binocular microscope with changeable magnification, commonly 10x and 16x, or a zoom drum.
• Joystick and elevation control: the joystick slides the whole instrument forward and back and side to side to focus, while a knob raises and lowers the beam height.
• Patient support: a chin rest and forehead band, plus an adjustable canthus/fixation marker used to align the patient's eye.

Setup steps

1. Clean the chin rest and forehead band.
2. Seat the patient and adjust the table and chin rest so the lateral canthus lines up with the canthus marker.
3. Set the eyepieces to your own refraction and interpupillary distance.
4. Begin at low magnification with a moderate slit width, then refine.

Illumination techniques

Different illuminations reveal different structures, and skilled examiners switch fluidly between them.

• Diffuse illumination gives a broad, even survey of the surface.
• Direct focal (optic section) is a thin slice used to localize the depth of a corneal or lens finding and, in a darkened room, to grade anterior-chamber cells and flare.
• Sclerotic scatter aims the beam at the limbus so light scatters through the cornea; a central opacity lights up.
• Retroillumination bounces light off the iris or the red fundus reflex to backlight lens or corneal opacities and iris transillumination defects.
• Specular reflection matches the angles of incidence and reflection to display the corneal endothelial mosaic and tear-film debris.

Systematic anterior-segment examination

Examine in a consistent outside-to-inside order so that nothing is missed:

1. Lids and lashes — margins, meibomian glands, lash position.
2. Conjunctiva and sclera — bulbar and palpebral surfaces, injection, follicles or papillae.
3. Cornea — epithelium, stroma, and endothelium; use an optic section to judge depth.
4. Anterior chamber — depth and any cells or flare, viewed with direct focal light in a dark room.
5. Iris — color, nodules, transillumination.
6. Lens — clarity and cataract type or location, best seen with retroillumination.

When inflammation is suspected, grade the anterior chamber in a darkened room using a short, bright 1 mm by 1 mm optic-section beam. Cells are individual white blood cells drifting in the beam and are counted per field; flare is a hazy protein scatter, like a headlight beam in fog, graded by how much it obscures the iris detail. Both are recorded on a standardized 0 to 4+ scale so that change over follow-up visits can be tracked. Document each finding by location and depth as you go rather than relying on memory.

Van Herick angle estimation

The Van Herick technique estimates the anterior-chamber angle without a gonioscopy lens. Place a narrow beam at about 60 degrees near the temporal limbus and compare the width of the dark aqueous gap (the chamber depth) to the thickness of the adjacent corneal section:

• Gap of one corneal thickness or more → grade 4, wide open.
• Gap about one-half corneal thickness → grade 3, open.
• Gap about one-quarter → grade 2, narrow and possibly occludable.
• Gap under one-quarter → grade 1, dangerously narrow.

A shallow gap warns of a narrow, occludable angle at risk for angle closure and should prompt gonioscopy.

Adjuncts

• Fluorescein dye with the cobalt-blue filter stains epithelial defects and abrasions bright green. The same combination drives the Seidel test, in which a stream of clear aqueous diluting the dye reveals a corneal or scleral leak, and it supports applanation tonometry and contact-lens fitting.
• A gonioscopy lens gives a direct, mirrored view of the drainage angle.
• Fundus lenses (78 or 90 D non-contact lenses, or contact lenses) turn the slit lamp into a posterior-segment viewer for the optic disc, macula, and retina.