Gonioscopy Technique

Why the Angle Needs a Lens

The anterior chamber angle contains the trabecular meshwork, Schwalbe's line, scleral spur, and iris root, none of which a routine slit-lamp exam can see. Light reflected off these structures undergoes total internal reflection at the anterior corneal surface before it ever reaches the examiner's eye, because the refractive index drops too sharply going from cornea to air. A goniolens defeats that reflection by replacing the air interface with a surface whose refractive index is close to the cornea's own. With the interface neutralized, angle structures become visible, either directly or by way of an angled mirror.

Pre-Exam Preparation and Sequencing

Because the lens makes direct contact with the cornea, a topical anesthetic goes in first, to eliminate the discomfort and blink reflex that would otherwise interfere with the exam. Room illumination should stay dim. A dim room keeps the pupil physiologically larger; bright light constricts the pupil first and artifactually widens a narrow angle before you ever get a look at it. Dilation carries the same distortion risk and is worse: mydriatic and cycloplegic drops act on the iris directly and reshape the angle configuration, and in a narrow angle can crowd it shut. For that reason, gonioscopy should ideally happen before pupil dilation, so the exam reflects the eye's true baseline anatomy.

At the slit lamp, use a short, narrow beam aimed away from the pupil. A wide beam striking the pupil triggers reflex constriction, with the same false-widening effect as bright ambient light, and the same downstream risk: underestimating true angle-closure risk.

Direct vs. Indirect Goniolenses

Goniolenses fall into two families. Direct gonioscopy uses a dome-shaped Koeppe-type lens with no mirrors, held on the anesthetized cornea while the examiner views the angle through a separate handheld light source and viewing system, typically a portable slit lamp or an operating microscope. The patient lies supine rather than sitting upright, which is why direct gonioscopy is the technique of choice in the operating room or during an exam under anesthesia, settings where an upright slit lamp isn't practical. Its wide contact area and mirror-free design give a broad, panoramic view of the angle in a single placement.

Indirect (mirrored) gonioscopy is performed at the standard slit lamp with the patient upright, and is by far the more common approach in clinic. Angled mirrors built into the lens redirect the line of sight so that angle structures 180° away from the mirror come into view. The classic Goldmann lens carries three mirrors of differing angulation, so a single lens can show the angle itself as well as the peripheral retina and pars plana depending on which mirror is used. Its contact surface is flat, with a larger radius than the cornea, which is why it needs a viscous coupling agent (methylcellulose or a similar gonioscopic solution) on its concave surface to fill that gap and remove the air interface. The four-mirror family (Zeiss, Posner, Sussman) takes the opposite design: a small corneal contact diameter and steep curvature close enough to the cornea's own that no coupling fluid is needed, and all four mirrors can be read without rotating the lens.

Performing the Exam

Each mirror on a mirrored lens shows only the angle structures directly opposite it. Full 360° coverage means rotating the lens slowly and systematically, pausing at each mirror position long enough to assess that quadrant before moving on, rather than holding the lens still. Too much pressure on a flat-contact lens like the Goldmann distorts the cornea, inducing folds or striae in Descemet's membrane that degrade the view. A routine, non-indentation exam calls for minimal, controlled contact pressure throughout.

Indentation (Compression) Gonioscopy

Indentation, or compression, gonioscopy is a deliberate variant of the technique, reserved for working up a narrow or apparently closed angle. A four-mirror lens (Zeiss, Posner, or Sussman) is the right tool for it: the same small contact diameter and steep curvature that let it skip coupling fluid also let it transmit gentle pressure through the cornea without inducing the corneal folds a larger flat-contact lens would produce under equivalent force.

The maneuver pushes aqueous peripherally into the angle recess, transiently deepening the angle. What happens next reveals the mechanism of closure. An appositional closure, iris pushed against the trabecular meshwork by pupillary block but not adherent to it, opens under indentation. A synechial closure does not: here the iris is permanently fused to the trabecular meshwork by peripheral anterior synechiae, and no amount of pressure will separate it. The distinction drives management directly, since an appositional closure may respond to laser iridotomy while a synechial one will not.

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