Examine / Gonioscopy
Gonioscopy
Why the drain cannot be seen without a lens, and what the lens shows.
The angle between the iris and the cornea, where the aqueous drains, lies behind the limbus and is hidden by total internal reflection at the cornea's front surface. A goniolens on the cornea, with a mirror or a steep curve, lets the light out, and the angle is examined at the slit lamp.
Why a lens
Light leaving the angle strikes the cornea's front surface at an angle beyond the critical angle for a cornea-to-air interface and is reflected back inside. A contact lens on the cornea replaces the air with plastic of a similar refractive index, and the light escapes, either through a mirror in the lens (indirect gonioscopy: the Goldmann three-mirror, the Zeiss four-mirror) or directly through a steep lens (direct: the Koeppe, used in the operating theatre and in children).
The landmarks, front to back
Schwalbe's line (the end of Descemet's membrane, the first landmark), the trabecular meshwork (a band, pigmented in its posterior part, over Schlemm's canal), the scleral spur (a white line, the most reliable landmark), the ciliary body band (grey-brown), and then the iris root. The more of these seen, the more open the angle. The Shaffer grades run from 4 (wide open, the ciliary body visible) to 0 (closed, nothing behind Schwalbe's line).
| Seen | Grade | Means |
|---|---|---|
| Ciliary body band | 4, wide open | Closure impossible |
| Scleral spur | 3, open | Closure unlikely |
| Trabecular meshwork only | 2, narrow | Closure possible |
| Schwalbe's line only | 1, very narrow | Closure likely |
| Nothing | 0, closed | Closed, at least where looked at |
What it finds
Whether an angle is open or narrow, which decides whether a glaucoma is open-angle or angle-closure and whether a dilating drop is safe. Peripheral anterior synechiae, where the iris has stuck to the meshwork after closure or inflammation. New vessels in the angle (neovascular glaucoma, after a vein occlusion or diabetes). Pigment, pseudoexfoliation material, angle recession after blunt trauma, and a foreign body hiding in the inferior angle.
- The bedside estimate before a goniolens is the van Herick test: a narrow slit at the limbus compares the peripheral chamber depth with the corneal thickness. A depth less than a quarter of the corneal thickness is an angle at risk of closure.
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