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Gonioscopy: looking into the drainage angle

  • Examination

A mirrored contact lens shows the drainage angle, which cannot be seen directly. It tells open from closed angles and guides glaucoma treatment.

Updated

See it in 3D: Drainage angle · Iris · Cornea · Ciliary body

What it is

Light from the angle between the iris and the cornea is reflected back inside the eye (total internal reflection), so the angle cannot be seen directly. A goniolens placed on the cornea removes that barrier. Indirect gonioscopy at the slit lamp with a mirrored lens is routine, and indentation with a four-mirror lens shows whether a closed angle is only touching or stuck by adhesions. It is needed in every glaucoma suspect, a shallow anterior chamber, raised eye pressure, uveitis, blunt trauma and suspected new vessels.

Normal values and scales

  • Shaffer grading runs from 4 (wide open) to 0 (closed); grades 2 or less are narrow.
  • An angle is called occludable when the posterior trabecular meshwork cannot be seen in at least half of its circumference.
  • The Van Herick slit-lamp estimate is a screen, not a substitute for gonioscopy.
  • Angle recession after blunt trauma (a widened ciliary body band) carries a lifelong glaucoma risk.
  • Heavy pigment suggests pigment dispersion or pseudoexfoliation; new vessels on the meshwork suggest neovascular glaucoma.

How it is done

  1. Explain the test and instil an anaesthetic drop; dim the room and use a short, narrow slit kept off the pupil so that it does not constrict.
  2. Place the goniolens on the cornea (with coupling gel if the lens needs it) while the patient looks straight ahead.
  3. Examine all four quadrants: each mirror shows the angle opposite to it, so the upper mirror shows the lower angle.
  4. Identify the structures from the iris root forwards: ciliary body band, scleral spur, trabecular meshwork and Schwalbe line.
  5. Indent with a four-mirror lens: an angle that opens is appositionally closed; one that stays shut has peripheral anterior synechiae.
  6. Record the grade and width per quadrant, pigment, new vessels, synechiae, recession or foreign bodies, often as a drawing.
  7. Clean the lens and check the cornea with fluorescein afterwards.

How to interpret it

  • Open-angle glaucoma cannot be diagnosed without gonioscopy, because secondary and angle-closure causes look the same on the disc.
  • Too much light or pressing on the lens opens the angle artificially and gives a falsely reassuring result.
  • A confirmed occludable angle is usually treated with laser iridotomy to prevent an acute attack.

Next step / Initial management

  • Shallow anterior chamber, narrow Van Herick or symptoms of intermittent closure (halos, headache in dim light): refer to ophthalmology for gonioscopy.
  • Occludable angle confirmed: ophthalmology discusses laser peripheral iridotomy.
  • Blunt trauma with hyphaema: gonioscopy once the eye has settled, then lifelong pressure checks if the angle is recessed.
  • Diabetes or vein occlusion with new iris vessels: urgent gonioscopy and retinal laser by ophthalmology.
  • Any glaucoma suspect or raised pressure: gonioscopy as part of the first assessment.

Test yourself

Think of the answer, then tap the question to see it.

Why can the drainage angle not be seen without a goniolens?

Light from the angle undergoes total internal reflection at the corneal surface.

Which angle structures are seen from the iris root forwards?

Ciliary body band, scleral spur, trabecular meshwork and Schwalbe line.

During indentation gonioscopy, a closed angle does not open. What does it mean?

Peripheral anterior synechiae (adhesions), rather than appositional closure.

What late risk follows angle recession after blunt trauma?

Glaucoma, which can appear years later, so pressure is checked for life.

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See it in 3D4

The structures of Durgical's 3D anatomy where it is found.

In Pharmacology1

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Educational content; not a clinical guideline and no substitute for assessment by a health professional. For sudden visual loss, severe eye pain, a chemical burn or an eye injury, go to an emergency department at once.

Sources: Gonioscopy (EyeWiki (American Academy of Ophthalmology), 2025); Acute Angle-Closure Glaucoma (StatPearls Publishing (NCBI Bookshelf), 2026); Pigment Dispersion Glaucoma (StatPearls Publishing (NCBI Bookshelf), 2023).

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Representative model, for educational purposes only.