Gonioscopy Technique
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Direct answer
The drainage angle is the only anterior-segment structure invisible to ordinary examination — its light undergoes total internal reflection at the cornea — and gonioscopy defeats the physics: a contact lens (Goldmann single- or three-mirror with coupling gel, or the four-mirror Zeiss/Posner without gel) redirects the angle image to the observer. Reading proceeds from Schwalbe's line through the trabecular meshwork (the pigmented posterior band being the outflow pathway), to scleral spur and ciliary body band, graded by Shaffer: grade 4 open (35-45 degrees), grade 3 (25-35), grade 2 about 20 degrees where closure becomes possible, grade 1 about 10 degrees at high risk, and 0 closed. Indentation with the four-mirror lens distinguishes appositional closure, which opens under pressure, from peripheral anterior synechiae, which do not; van Herick's slit-lamp estimate — peripheral chamber depth under one-quarter corneal thickness suggesting an occludable angle — is the screening surrogate.
What you must remember
- Physics in one line: angle rays exceed the critical angle at the tear-air interface; the goniolens's higher refractive index lets them escape — hence the gel eliminating the cornea-lens gap.
- Structures, anterior to posterior: Schwalbe's line (termination of Descemet's), non-pigmented then pigmented trabecular meshwork, scleral spur, ciliary body band (visible only in wide angles), iris root.
- Shaffer grading: 4 = 35-45° open; 3 = 25-35° open; 2 ≈ 20° closure possible; 1 ≈ 10° high risk; 0 = closed — grade 2 or less over roughly 180 degrees or more of invisible trabecular meshwork defines the occludable angle.
- Lens families: Goldmann (indirect, gel-coupled) for stable viewing; Zeiss four-mirror or Posner for indentation — no gel, and gentle pressure opens an appositional angle; Koeppe for direct surgical viewing.
- Technique discipline: dim room, narrow short slit, minimal gel, patient looks toward the mirror being used, avoid pressing — pressure falsely narrows a Goldmann view or falsely widens a Zeiss view.
- Interpretation musts: angle recession after trauma (asymmetric ciliary body band beyond 180 degrees carries glaucoma risk), peripheral anterior synechiae in uveitis and closure, heavy pigmentation in pseudoexfoliation and pigment dispersion, neovascularisation as an emergency.
- Van Herick screen: peripheral depth under one-quarter corneal thickness at the temporal limbus (grade 1-2) suggests an occludable angle and earns gonioscopy; grade 4, equal to corneal thickness, is broadly safe.
- Indian relevance: angle-closure glaucoma contributes a higher share of blindness in Asian eyes — gonioscopy before dilatation in every hyperopic elderly eye, especially before fundus photography or cycloplegia.
Reading the angle, quadrant by quadrant
A 62-year-old hyperope presents after a dusk headache with haloes. Van Herick grade 1 temporally, both eyes. Gonioscopy with the Goldmann lens and methylcellulose, four quadrants in a dim room with a narrow slit — inferior quadrant first, the most pigmented and easiest to learn on. Schwalbe's line is visible; below it, non-pigmented meshwork only; no scleral spur, no ciliary body band in any quadrant: Shaffer grade 1 throughout — an occludable angle. Switching to the Posner lens, gentle indentation opens all quadrants to reveal the spur: appositional, not synechial — attack risk without permanent damage yet. She leaves with pilocarpine and a laser peripheral iridotomy referral, and the fellow eye gets prophylaxis, because primary angle closure is a bilateral disease.
Contrast the routine post-trauma review: the left angle shows a widened, torn ciliary body band across three clock hours against a normal right — an angle recession documented now, with annual pressure checks for life, since post-traumatic glaucoma can declare itself years later.
What the viva examiner hunts
The distinctions carry the marks: appositional versus synechial closure (indentation opens the former only); Shaffer 2 versus 3 as the risk watershed; and the trap of pressing — a Goldmann lens pressed hard folds Descemet's and artificially narrows, while the same pressure through a Zeiss opens. Expect also why gonioscopy precedes every dilatation in a suspect, what "occludable" means (pigmented meshwork invisible for the defining arc), and the recognition pearls — blood in Schlemm's canal, Sampaolesi's line, iris bombé in pupillary block.
Frequently asked questions
Why can the angle not be seen without a goniolens?
Light from the angle strikes the cornea-air interface beyond the critical angle and is totally internally reflected; the lens-gel optics abolish the interface so the image escapes.
Grade the Shaffer system's risky angles.
Grade 2 (about 20 degrees) is where closure becomes possible, grade 1 (about 10 degrees) carries high risk, and 0 is closed; grades 3 and 4 are open.
What does indentation gonioscopy distinguish?
Whether closure is appositional — opening when the four-mirror lens presses — or synechial, where peripheral anterior synechiae hold it shut regardless of pressure.
What does a van Herick grade 1 finding imply?
Peripheral chamber depth under one-quarter of corneal thickness, suggesting an occludable angle that requires gonioscopy before any pupillary dilatation.
Why is gonioscopy repeated after ocular trauma?
To detect angle recession — a torn, widened ciliary body band — which predisposes to post-traumatic glaucoma years later and mandates lifelong pressure surveillance.