Understanding Glaucoma
Glaucoma is a group of eye diseases characterised by progressive damage to the optic nerve, typically associated with raised intraocular pressure (IOP). It is a leading cause of irreversible blindness worldwide — “irreversible” is the critical word. Unlike cataract, vision lost to glaucoma cannot be restored, which is why early detection and pressure control are the entire foundation of management.
All forms of glaucoma share two features: characteristic structural damage to the optic nerve head and corresponding functional loss of vision (visual field defects). What differs between the types is the mechanism by which fluid drainage fails — and that mechanism determines presentation, urgency, and treatment.
Aqueous Humor Dynamics
To understand glaucoma, start with the fluid inside the eye:
- Aqueous humor is a clear fluid produced by the ciliary body behind the iris. It nourishes the lens and cornea and maintains the eye’s shape and pressure.
- The fluid flows from the posterior chamber (behind the iris), through the pupil, into the anterior chamber (between the iris and cornea).
- It drains out of the eye mainly through the trabecular meshwork, located at the drainage angle — the junction where the iris meets the cornea — and then into Schlemm’s canal and the venous system.
- Intraocular pressure is a balance: production rate versus drainage rate. Glaucoma arises when drainage becomes impaired while production continues normally.
Think of a sink with the tap running: if the drain is partially blocked, water (pressure) builds up. The elevated pressure compresses the delicate nerve fibres where they exit the eye through the optic nerve head, and over time those fibres die.
Open-Angle Glaucoma — Mechanism
In primary open-angle glaucoma (POAG), the drainage angle itself is anatomically open and looks normal on examination — but the trabecular meshwork is functionally resistant to fluid outflow. Fluid reaches the drain but cannot pass through it easily. Pressure rises slowly, over months to years.
POAG is the most common form of glaucoma in most populations. Its insidious onset is what makes it dangerous: the patient feels nothing while peripheral nerve fibres die.
Angle-Closure Glaucoma — Mechanism
In angle-closure glaucoma, the problem is anatomical: the iris physically blocks the drainage angle, sealing the trabecular meshwork off from the anterior chamber. Fluid cannot reach the drain at all.
- This typically happens in eyes with a shallow anterior chamber — often smaller, farsighted (hypermetropic) eyes, more common with increasing age as the lens thickens.
- Closure can be chronic (gradual, partial, often unnoticed) or acute (sudden, complete — an emergency).
- The pupil plays a role: when it dilates (e.g., in dim light), the iris bunches up and can plug the angle.
Key Differences at a Glance
| Feature | Open-angle glaucoma | Angle-closure glaucoma |
|---|---|---|
| Drainage angle | Open, normal-looking | Narrowed or physically blocked by iris |
| Site of blockage | Trabecular meshwork itself (resistance to outflow) | Iris covering the trabecular meshwork |
| Onset | Gradual, over years | Acute attack can be sudden (minutes to hours) |
| Symptoms | Usually none until late disease | Acute: severe pain, red eye, blurred vision, halos, nausea |
| Eye shape association | No strong anatomical predisposition | Shallow anterior chamber; hypermetropia; older age |
| Emergency? | Not an emergency — chronic management | Acute angle closure is an ocular emergency |
| First-line laser option | Selective laser trabeculoplasty (targets meshwork) | Laser peripheral iridotomy (creates alternate drainage path) |
Risk Factors
Open-angle glaucoma
- Elevated intraocular pressure (the strongest modifiable risk factor)
- Increasing age
- Family history of glaucoma (a strong risk factor — screen relatives)
- African or Hispanic ancestry (higher prevalence and earlier onset in many studies)
- Thin central cornea (a thinner cornea is associated with higher risk of progression)
- Myopia
Angle-closure glaucoma
- Shallow anterior chamber (short axial length, hypermetropia)
- Increasing age (the lens thickens and pushes the iris forward)
- Female sex and East/South Asian ancestry (higher prevalence of narrow angles)
- Family history of angle closure
- Pupil dilation in susceptible eyes (dim light, certain medications)
Clinical Presentation
Open-angle glaucoma — the “silent” disease
- Typically asymptomatic until advanced stages. Patients do not feel raised pressure.
- Vision loss begins in the mid-periphery of the visual field — patients often don’t notice until the field is markedly constricted (“tunnel vision” is a late stage).
- Central visual acuity is preserved until very late, which is why patients feel their vision is “fine.”
- Detection is usually incidental — found during routine eye examination, which is why regular screening matters.
Angle-closure — chronic vs acute
- Chronic angle closure may be as silent as open-angle disease, with gradual pressure rises.
- Acute angle closure is unmistakable and dramatic:
- Sudden, severe eye pain, often with headache
- Markedly red eye
- Blurred vision with coloured halos around lights
- Nausea and vomiting (the pain can be severe enough to mimic an abdominal emergency)
- The eye feels rock-hard to palpation
- Mid-dilated, non-reactive pupil; cloudy (oedematous) cornea
- Acute angle closure is an emergency: every hour of very high pressure risks permanent optic nerve damage. It must be recognised and referred immediately.
Key Examination Findings
A complete glaucoma assessment examines five domains:
- Intraocular pressure (IOP). Measured by tonometry (commonly Goldmann applanation). A single normal reading does not exclude glaucoma — pressure fluctuates, and some patients develop nerve damage at statistically “normal” pressures.
- Optic disc evaluation. The optic nerve head is examined (directly and with slit-lamp biomicroscopy) for characteristic changes: increased cup-to-disc ratio, thinning of the neuroretinal rim, disc hemorrhages, and asymmetry between the two eyes. Serial disc photography or OCT imaging documents change over time.
- Visual field testing. Automated perimetry maps the patient’s field of vision and detects the characteristic patterns of glaucomatous loss (e.g., arcuate defects, nasal steps). This is how functional damage is tracked.
- Gonioscopy. The definitive test that distinguishes open-angle from angle-closure. A special lens is placed on the eye to directly visualise the drainage angle and determine whether it is open, narrow, or closed, and whether the iris is obstructing it. Every glaucoma workup should include gonioscopy.
- Corneal thickness (pachymetry). Central corneal thickness influences tonometry readings and is an independent risk factor.
Management Overview (Conceptual)
Treatment aims to lower intraocular pressure to a level that halts or slows optic nerve damage. The “target pressure” is individualised. Management is stepwise:
Medical (eye drops)
- First-line for most open-angle glaucoma. Drug classes work by either reducing aqueous production or increasing outflow. Exact agents and dosing are clinical decisions — no dosages are given here.
Laser
- Selective laser trabeculoplasty (SLT): applied to the trabecular meshwork in open-angle glaucoma to improve outflow; increasingly a first-line option.
- Laser peripheral iridotomy (LPI): creates a small hole in the peripheral iris, giving aqueous an alternate route past a pupil block; the definitive preventive and acute treatment for angle closure.
Surgical
- Reserved for cases where medical and laser therapy fail to control pressure, or where the disease is advanced at presentation.
- Trabeculectomy creates a new drainage channel (a guarded fistula) under the conjunctiva.
- Glaucoma drainage devices (tubes) shunt fluid to a reservoir plate.
- Minimally invasive glaucoma surgery (MIGS): lower-risk options for mild-to-moderate disease, often combined with cataract surgery.
Acute angle closure — emergency management (conceptual)
- Immediate pressure-lowering measures are started urgently, followed by definitive laser iridotomy once the cornea clears — usually in the fellow eye too, which shares the same risk.
- Any patient with suspected acute angle closure must be referred the same day, without delay.
The Optometrist’s Role
Optometrists are often the first to detect glaucoma: measuring IOP, assessing the optic disc, performing visual fields, and doing or arranging gonioscopy. Patients with suspicious discs, elevated pressure, or narrow angles need a structured referral pathway — and asymptomatic patients need to understand why lifelong follow-up and consistent drop use are non-negotiable.
Key Takeaways
- Glaucoma is irreversible optic nerve damage, usually driven by raised intraocular pressure from impaired aqueous drainage.
- IOP is a balance of aqueous production (ciliary body) and drainage (trabecular meshwork at the drainage angle).
- Open-angle: the angle looks open but the meshwork resists outflow; silent, gradual, the commonest form.
- Angle-closure: the iris physically blocks the angle; can present as a sudden, painful emergency (pain, red eye, halos, nausea, rock-hard eye).
- Gonioscopy is the examination that distinguishes the two — it directly visualises the drainage angle.
- Assessment covers five domains: IOP, optic disc, visual fields, gonioscopy, and corneal thickness.
- Management is stepwise — medical, then laser (SLT for open-angle, iridotomy for angle closure), then surgery — all aimed at lowering pressure.
- Acute angle closure needs same-day emergency referral; iridotomy is typically done in both eyes.
This material is for educational purposes only and does not constitute medical advice. Clinical decisions should always involve qualified supervision.