Gonioscopy: What It Is, What to Expect and What Open or Closed Angles Mean
Updated: 2 days ago
Written and medically reviewed by Dr. Baraa Alnahhal, MD · Last reviewed: November 2026 · How we review
Quick Answer
Gonioscopy is an eye exam that uses a contact lens and a slit-lamp microscope to look at the drainage angle, where the iris meets the white of the eye and fluid drains out. It shows whether the angle is open, narrow or closed, which helps diagnose and classify glaucoma. It takes 5 minutes or less and is painless.
What the research says
Imaging hasn't replaced gonioscopy: a meta-analysis of 23 studies found anterior segment optical coherence tomography (AS-OCT) detected angle closure with a median sensitivity of 87% and specificity of 84% compared with gonioscopy, but concluded it is not yet able to replace gonioscopy (Desmond T et al., Graefe's Archive for Clinical and Experimental Ophthalmology, 2022) [2]
AI on eye scans is closing the gap: across 6 studies and 5,269 patients, deep-learning algorithms reading AS-OCT images identified closed angles with 94.0% sensitivity and 93.6% specificity using gonioscopy as the reference standard (Olyntho MAC et al., Journal of Glaucoma, 2024) [3]
Key takeaways
Shows the eye's drainage angle to tell open-angle from narrow- or closed-angle glaucoma.
Indirect gonioscopy (mirrored lens, in the office) is the usual type; direct gonioscopy is usually done under anesthesia, often during surgery.
Numbing drops, a gel-coated lens and a slit lamp; about 5 minutes or less.
Gonioscopy alone does not diagnose glaucoma; it is combined with eye pressure, optic nerve and visual field tests.
Sudden severe eye pain with blurred vision, headache or vomiting can mean acute angle closure, an emergency.
What is gonioscopy?
Clear fluid called aqueous humor constantly flows into the front of the eye and drains out through a meshwork in the angle between the iris and cornea. If the angle is narrow or blocked, pressure can build and damage the optic nerve. Because the angle is hidden behind the edge of the cornea, it can't be seen directly with a normal light; the gonioscopy lens uses mirrors or a special curve to bring it into view. Sources: [1]
Gonioscopy takes about 5 minutes or less.
Why is it done?
Checking for and classifying glaucoma, including angle-closure and open-angle glaucoma.
Detecting pseudoexfoliation syndrome, pigment deposits or new blood vessels in the angle.
Looking for adhesions between the iris and cornea (synechiae).
Checking for iris or nearby tumors.
Planning laser or surgical glaucoma treatment. Sources: [1]
Types of gonioscopy
Type | How it works | Where it's done |
Indirect gonioscopy | Mirrored lens reflects the angle into view | In the office with a slit lamp |
Direct gonioscopy | Magnifying lens views the angle straight on | Usually in hospital under general anesthesia, often during surgery or in infants |
Dynamic (indentation) gonioscopy | Gentle pressure with the lens shows whether a closed angle can open | In the office during indirect gonioscopy |
Sources: [1]
How do I prepare?
Usually no preparation is needed for indirect gonioscopy. If you wear contact lenses, ask how long to leave them out. Dilating drops may or may not be used, so bring sunglasses just in case. Sources: [1]
What happens during the exam?
Numbing drops are put in your eye.
You rest your chin and forehead on the slit lamp.
A small amount of wetting gel is placed on the lens to remove air bubbles.
The provider places the lens gently on the front of your eye and looks straight ahead or in set directions while examining the angle all the way around.
The provider may press gently (dynamic gonioscopy) to see whether a narrow angle opens. Sources: [1]
The indirect exam usually takes 5 minutes or less. Your eye is numb, so you only feel slight pressure, and watering afterward is common. Sources: [1]
Understanding your results
Where fluid drains out of the eye between the iris and cornea.
Results are usually explained during or right after the exam. Your provider describes how much of the drainage structure is visible around the angle and notes any pigment, adhesions or abnormal vessels. A narrow or closed angle, or other signs of blocked outflow, suggests risk of angle-closure glaucoma and may lead to treatment such as a laser iridotomy. An open angle doesn't rule out glaucoma; open-angle glaucoma is diagnosed with tonometry, optic nerve exams and visual field tests. Sources: [1]
Gonioscopy remains the reference standard for angle closure.
Imaging such as AS-OCT is quicker and contact-free, with a median sensitivity of 87% for angle closure, and AI tools reading these images reach about 94%. But gonioscopy remains the reference standard against which these tools are measured. Sources: [2, 3]
Risks
The exam itself has no listed side effects. The numbing or dilating drops can cause temporary stinging, blurred vision or light sensitivity. Sources: [1]
When to seek emergency care
Get emergency care for sudden vision loss or sudden severe eye pain, especially with headache, nausea or vomiting. These can signal acute angle-closure glaucoma, which can cause rapid, permanent vision loss without immediate treatment. Sources: [1]
Conclusion
Gonioscopy is a short, painless exam that answers a key glaucoma question: is the eye's drainage angle open or closed? Most glaucoma cases (about 90%) develop slowly, so regular eye exams every 1–2 years, and at least yearly after age 40 or with risk factors, help catch problems early. Sources: [1]
Keep learning: see our guide to glaucoma tests and glaucoma surgery.
Frequently asked questions
Does gonioscopy hurt?
No. Numbing drops are used, so you feel only slight pressure from the lens.
How long does gonioscopy take?
The indirect exam usually takes about 5 minutes or less.
Can I drive afterward?
Usually yes, unless dilating drops were used and your vision is blurry.
What does a narrow angle mean?
The iris sits close to the drainage meshwork, raising the risk of angle-closure glaucoma. Laser treatment may be recommended.
Does gonioscopy diagnose glaucoma?
Not alone. It classifies the type and risk, while eye pressure, optic nerve and visual field tests confirm glaucoma.
Can an OCT scan replace gonioscopy?
Not yet. AS-OCT detects angle closure with about 87% sensitivity, but gonioscopy remains the reference standard.
What is dynamic gonioscopy?
Gentle pressure with the lens to see whether a closed angle can open, which shows whether adhesions are present.
Do I need to remove contact lenses?
Yes. Ask your provider how long to leave them out before the exam.
What are the warning signs of acute angle closure?
Sudden severe eye pain, blurred vision, halos around lights, headache, nausea or vomiting. Seek emergency care.
How often should I have my eyes checked?
Every 1–2 years, and at least yearly after 40 or if you have risk factors such as diabetes or family history of glaucoma.
Related Reading
Glaucoma Tests: Types, What to Expect and What Results Mean all the tests used together to diagnose glaucoma.
Angle-Closure Glaucoma: An Eye Emergency Caused by Blocked Fluid Drainage the emergency gonioscopy helps predict.
What Is Open-Angle Glaucoma? Symptoms, Causes, and Treatment the common slow form of glaucoma.
Tonometry (Eye Pressure Test): Types, What to Expect and Normal Eye Pressure the eye pressure test done alongside gonioscopy.
Eye Exam: What Happens, Tests Included and How Often to Go the routine exam where gonioscopy may be included.
References
Cleveland Clinic. Gonioscopy: What It Is, Procedure Details & Results. Cleveland Clinic. Last updated July 23, 2024. https://my.clevelandclinic.org/health/diagnostics/22421-gonioscopy
Desmond T, Tran V, Maharaj M, et al. Diagnostic accuracy of AS-OCT vs gonioscopy for detecting angle closure: a systematic review and meta-analysis. Graefe's Archive for Clinical and Experimental Ophthalmology. 2022;260(1):1-23. PMID 34223989. https://doi.org/10.1007/s00417-021-05271-4
Olyntho MAC, Jorge CAC, Castanha EB, et al. Artificial intelligence in anterior chamber evaluation: a systematic review and meta-analysis. Journal of Glaucoma. 2024;33(9):658-664. PMID 38747721. https://doi.org/10.1097/IJG.0000000000002428
Medical disclaimer
This article is for general education only and does not replace advice from a qualified healthcare professional. Talk to your doctor about your own diagnosis and treatment, and seek emergency care right away for severe or sudden symptoms.
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