Gonioscopy in Glaucoma

Why You Need a Lens to See the Angle at All

The trabecular meshwork sits just behind the corneoscleral limbus, and it's invisible on a plain slit-lamp view for a fixed optical reason: light from the angle strikes the tear-air interface beyond the critical angle and undergoes total internal reflection, a direct consequence of Snell's law. A mirrored gonioscopy lens replaces that interface with one of matched refractive index, so the light escapes the eye instead of bouncing back. That's what makes the angle visible at all, and what lets the clinician classify it as open or closed, look for pathology, and grade its width.

Angle Landmarks, Anterior to Posterior

Going from anterior to posterior, the angle structures run in a fixed order: Schwalbe's line → trabecular meshwork → scleral spur → ciliary body band (see gonioscopic landmark identification for how each is recognized at the slit lamp). Schwalbe's line marks the peripheral termination of Descemet's membrane, a thin ridge of condensed collagen just anterior to where the meshwork begins, and it's the most anteriorly visible landmark in an open angle. It's also the structure displaced anteriorly in Axenfeld-Rieger syndrome, where it produces a visible line known as posterior embryotoxon (see Congenital & Developmental Glaucoma).

Grading & Screening Techniques

The Shaffer classification grades the iridocorneal angle from 0 (closed) to 4 (wide open, >35°) by angular width and is the main predictor of angle-closure risk recorded on a gonioscopy exam. The Van Herick technique offers a faster, non-contact alternative: a narrow oblique slit beam is aimed at the peripheral cornea, and the depth of the peripheral anterior chamber is compared against corneal thickness as a proxy for angle width. It doesn't replace gonioscopy, but it's a quick screen for deciding whether formal gonioscopy is warranted.