Glaucoma isn’t a single disease. It’s a group of eye conditions that damage the optic nerve, and the two main types — open angle and narrow angle (also called closed angle) — work very differently, produce different symptoms, and require different treatment approaches.
If you’ve been diagnosed with glaucoma, or told you’re at risk, understanding which type you have makes a significant difference in how you manage it. One type progresses silently over years. The other can become a medical emergency within hours.
This guide breaks down the key differences between open angle and narrow angle glaucoma in plain language, so you know what to expect, how each is treated, and when to be concerned.
What Is Open Angle Glaucoma?
Open angle glaucoma is by far the most common type, accounting for roughly 90% of all glaucoma cases. In this form, the drainage angle between the iris and cornea remains physically open, but the microscopic drainage channels within the angle (called the trabecular meshwork) don’t work as efficiently as they should.
Think of it like a sink with a wide-open drain that has a slow, partial clog deep inside the pipe. Water can still get through, but not as fast as it should. Over time, the slow backup causes pressure to rise gradually.
How it progresses: Open angle glaucoma develops slowly over months and years. Eye pressure rises gradually, and the optic nerve sustains incremental damage. Because the pressure increase is slow and painless, most patients have no symptoms at all until significant, irreversible vision loss has already occurred. This is why open angle glaucoma is often called the “silent thief of sight.”
Who it affects: Open angle glaucoma is more common in people of African descent, those over age 60, and people with a family history of glaucoma. It affects both eyes, though often at different rates.
What Is Narrow Angle Glaucoma?
Narrow angle glaucoma (also called angle-closure glaucoma or closed-angle glaucoma) is less common but potentially more urgent. In this form, the drainage angle is physically narrow or can close completely, creating a mechanical blockage that prevents fluid from draining.
Using the sink analogy, this is like having a drain that’s so narrow it can get completely plugged by debris at any moment. When it plugs, the sink overflows rapidly.
How it progresses: Narrow angle glaucoma can follow two patterns. The chronic form develops gradually as the angle slowly closes over time — similar in pace to open angle glaucoma. The acute form, however, is a medical emergency. The angle closes suddenly and completely, causing eye pressure to spike to dangerous levels within hours. Acute attacks cause severe symptoms that require immediate treatment.
Who it affects: Narrow angle glaucoma is more common in people of Asian and Inuit descent, women (2-4 times more likely), people over 50, and those who are farsighted. The condition runs in families because eye anatomy — particularly the depth of the anterior chamber — is genetically determined.
Side-by-Side Comparison
This table summarizes the key differences between the two types at a glance:
| Feature | Open Angle Glaucoma | Narrow Angle Glaucoma |
|---|---|---|
| How common | ~90% of glaucoma cases | ~10% of glaucoma cases |
| Drainage angle | Open, but drainage is sluggish | Narrow or closed, blocking drainage |
| Onset | Very gradual, over years | Can be sudden (acute) or gradual (chronic) |
| Symptoms | None until advanced — painless | Acute: severe pain, blurred vision, nausea. Chronic: often none |
| Pain | No | Yes (during acute attacks) |
| Vision loss pattern | Slow peripheral loss, then central | Can be rapid and severe during acute attack |
| Emergency risk | Low — progresses slowly | High — acute attacks need immediate treatment |
| First-line treatment | Daily eye drops (prostaglandins) | Laser iridotomy (creates drainage hole in iris) |
| Surgical options | SLT laser, MIGS, trabeculectomy | Laser iridotomy, cataract surgery, trabeculectomy |
| Medication concerns | Few medication restrictions | Must avoid anticholinergic drugs that dilate pupil |
| Both eyes affected | Yes, usually at different rates | Yes — if one eye attacks, the other is at high risk |
| Higher risk groups | African descent, age 60+ | Asian/Inuit descent, women, age 50+, farsighted |
| Prognosis with treatment | Good — most maintain useful vision | Good if caught early; acute attacks can cause rapid damage |
How Each Type Is Diagnosed
Both types of glaucoma are diagnosed through a comprehensive eye exam, but the specific tests emphasized differ slightly.
Shared Diagnostic Tests
Every glaucoma evaluation includes measuring intraocular pressure (tonometry), examining the optic nerve for signs of damage, and mapping the visual field to detect areas of vision loss. These tests apply to both types.
The Critical Test: Gonioscopy
Gonioscopy is what distinguishes the two types. During this painless test, your eye doctor places a special mirrored contact lens on the surface of your eye to directly view the drainage angle. This is the only way to determine whether your angles are open, narrow, or closed — and therefore which type of glaucoma you have or are at risk for.
If your eye doctor has never performed gonioscopy during your exam, it’s worth asking about, especially if you have risk factors for narrow angles.
Additional Tests for Narrow Angles
For patients with narrow or borderline angles, anterior segment OCT provides a detailed cross-sectional image of the front of the eye, allowing precise measurement of the angle width and anterior chamber depth. This imaging helps your doctor determine how narrow your angles actually are and whether preventive treatment is warranted.
Treatment Differences
The treatment approach for each type reflects the underlying problem — slow drainage vs. mechanical blockage.
Treating Open Angle Glaucoma
Because the angle is open but the internal drainage channels are sluggish, treatment focuses on either reducing the amount of fluid the eye produces or improving outflow through the existing channels:
- Prescription eye drops — The most common first-line treatment. Prostaglandin analogues (like latanoprost) are typically prescribed first, used once daily. Other drop classes include beta-blockers, alpha-agonists, and carbonic anhydrase inhibitors.
- Selective Laser Trabeculoplasty (SLT) — A laser treatment applied to the drainage meshwork to improve outflow. Increasingly used as a first-line alternative to drops.
- MIGS (Minimally Invasive Glaucoma Surgery) — Small implantable devices that create new drainage pathways, often performed alongside cataract surgery.
- Trabeculectomy — Traditional filtering surgery for advanced cases that don’t respond to other treatments.
Treating Narrow Angle Glaucoma
Because the problem is a physical blockage (or risk of blockage), treatment focuses on creating an alternative drainage pathway or physically opening the angle:
- Laser Peripheral Iridotomy (LPI) — The first-line treatment. A laser creates a tiny hole in the iris, allowing fluid to bypass the pupil and flow directly to the drainage angle. This 5-10 minute outpatient procedure is often curative for preventing acute attacks.
- Cataract Surgery (Lens Extraction) — Removing the thickened natural lens and replacing it with a much thinner artificial lens physically opens the drainage angle. This is increasingly recognized as one of the most effective treatments for narrow angle glaucoma.
- Pressure-lowering eye drops — Used to manage elevated pressure, often in combination with laser or surgical treatment.
- Trabeculectomy or MIGS — For advanced cases that don’t respond to iridotomy and lens extraction.
Key Treatment Difference
The most important distinction is urgency. Open angle glaucoma is typically managed on an outpatient schedule — you have time to try different drops, assess results, and adjust. Narrow angle glaucoma, particularly the acute form, requires rapid intervention. An acute angle closure attack is treated as an emergency, with the goal of lowering pressure within hours and performing laser iridotomy as soon as the eye is stable.
Can You Have Both Types?
Yes. Mixed-mechanism glaucoma occurs when a patient has both a sluggish trabecular meshwork (open angle component) and a narrow drainage angle (angle closure component). This isn’t uncommon, and it’s one reason why gonioscopy is so important — the treatment approach needs to address both problems.
Additionally, a patient can have open angle glaucoma in one eye and narrow angles in the other, though this is less common since the anatomical predisposition for narrow angles usually affects both eyes.
Frequently Asked Questions
Which type of glaucoma is worse?
Neither is universally “worse” — both can cause permanent blindness if untreated. However, narrow angle glaucoma carries a unique danger because acute attacks can cause severe vision loss within hours, while open angle glaucoma typically progresses over years. On the other hand, open angle glaucoma’s lack of symptoms means it’s often diagnosed later. The best outcome for either type comes from early detection and appropriate treatment.
Can open angle glaucoma turn into narrow angle glaucoma?
No — the two types have different underlying mechanisms. Open angle glaucoma is caused by sluggish drainage channels, while narrow angle glaucoma is caused by the physical anatomy of the eye. However, as the eye’s natural lens thickens with age, previously borderline angles can become narrow enough to pose a risk, which is why ongoing monitoring includes angle assessment.
I was told I have “narrow angles” but not glaucoma. Should I be worried?
Having narrow angles is an anatomical finding, not a disease diagnosis. It means you’re at higher risk for angle closure, but it doesn’t mean you have glaucoma. Your eye doctor will monitor your angles regularly and may recommend preventive laser iridotomy if the angles are particularly narrow or other risk factors are present. Think of it as a risk factor that’s being managed, not a diagnosis to fear.
How do I know which type I have?
Your eye doctor determines this through gonioscopy — the test where a mirrored lens is placed on your eye to view the drainage angle directly. If you’ve been diagnosed with glaucoma and aren’t sure which type, ask your doctor. The type determines your treatment plan and what precautions you should take.
Does narrow angle glaucoma require more frequent monitoring?
Generally, yes — at least initially. Narrow angle patients who haven’t had laser iridotomy may need monitoring every 6 to 12 months. After successful laser treatment, annual exams are standard. Open angle glaucoma patients typically need visits every 3 to 6 months depending on how well-controlled their pressure is.
Know Your Type — Get the Right Diagnosis
Whether you’re newly diagnosed, have a family history of glaucoma, or simply haven’t had a comprehensive eye exam recently, understanding which type of glaucoma affects you is the foundation of effective treatment.
At Garibaldi Eye Care, every glaucoma evaluation includes gonioscopy to assess your drainage angles, along with pressure testing, optic nerve imaging, and visual field mapping. We diagnose the specific type and create a personalized treatment plan based on your anatomy, risk factors, and lifestyle.
Contact Garibaldi Eye Care today to schedule your comprehensive glaucoma evaluation.
Unsure which type applies to you? Your optometrist in Squamish at Garibaldi Eye Care can assess your risk.
