Understanding Glaucoma: Diagnosis, OCT Imaging, and What Your Results Mean

A glaucoma diagnosis — or the news that you are "glaucoma suspect" — raises many questions. What exactly is happening to my eyes? What do the test results mean? What will my vision be like in ten years? This guide answers those questions in plain language, drawing on current evidence and clinical practice. It is intended to complement the personalised explanation you receive from Dr. Sundaram, not to replace it.

What Is Glaucoma?

Glaucoma is a group of conditions characterised by progressive damage to the optic nerve — the bundle of nerve fibres that carries visual information from the eye to the brain. As optic nerve fibres are lost, the corresponding areas of the visual field go dark. The damage is irreversible, and glaucoma is the leading cause of irreversible blindness worldwide. In Canada, it affects an estimated 250,000 people, with many more undiagnosed because the early stages are usually symptom-free.

One of the most important things to understand about glaucoma is that it is strongly — but not exclusively — associated with elevated intraocular pressure (IOP). Elevated IOP is the most modifiable risk factor, and reducing it is the cornerstone of all current treatments. However, elevated IOP alone does not define glaucoma — and normal IOP does not rule it out.

Types of Glaucoma

Primary Open-Angle Glaucoma (POAG)

The most common form of glaucoma in populations of European and African descent. In POAG, the drainage angle of the eye (where the cornea meets the iris) appears open and structurally normal, but the eye's natural drainage mechanism is impaired at a microscopic level. IOP rises gradually over time, and optic nerve damage develops slowly — often over years or decades before vision loss is noticed.

Normal Tension Glaucoma (NTG)

In Normal Tension Glaucoma, characteristic optic nerve damage and visual field loss occur despite IOP that is consistently within the statistically normal range (below 21 mmHg). NTG is particularly prevalent in populations of East Asian descent. The exact mechanism is less well understood — vascular factors, optic nerve susceptibility, and systemic blood pressure fluctuations are thought to contribute. NTG underscores why diagnosis cannot rely on IOP measurement alone.

Angle-Closure Glaucoma

In angle-closure glaucoma, the drainage angle between the cornea and iris is physically narrow or closed. This can occur gradually (chronic angle closure) or acutely (acute angle closure crisis). Acute angle closure is a medical emergency — presenting with sudden, severe eye pain, headache, nausea, blurred vision, and coloured halos around lights — and requires immediate treatment. Chronic angle closure is more insidious and may go unnoticed until significant damage has occurred.

Secondary Glaucomas

These are forms of glaucoma caused by an identifiable underlying condition: pseudoexfoliation syndrome (abnormal material accumulating in the drainage angle), pigment dispersion syndrome, prior eye surgery, trauma, uveitis, or prolonged steroid use. Identifying the underlying cause informs treatment selection.

How Glaucoma Is Diagnosed

No single test diagnoses glaucoma. A comprehensive assessment combines multiple data sources:

Intraocular Pressure (IOP) Measurement

IOP is measured in millimetres of mercury (mmHg). The classic "air puff test" at optometry appointments gives a rough estimate; the more accurate Goldmann applanation tonometry — performed at a slit lamp — is used in ophthalmology practice. A statistically normal IOP is below 21 mmHg, but as discussed above, this threshold neither confirms nor excludes glaucoma in isolation. IOP measurement is important as a baseline and for treatment monitoring, but it is one piece of a larger diagnostic picture.

Gonioscopy

Gonioscopy involves placing a special contact lens on the eye and using mirrored prisms to examine the drainage angle — the anatomical space where the iris meets the cornea. This allows the ophthalmologist to classify the angle as open, narrow, or closed, which directly determines the type of glaucoma and the appropriate treatment approach. Patients with narrow angles may benefit from preventive laser iridotomy to prevent angle-closure crisis.

Visual Field Testing (Perimetry)

Visual field testing — also called perimetry — maps the full extent of your peripheral and central vision. The most common form used in glaucoma monitoring is automated static threshold perimetry, in which you respond to lights of varying intensity appearing at different locations in your visual field. The results are plotted as a map showing areas of reduced sensitivity.

Visual field changes in glaucoma follow characteristic patterns that correspond to the anatomy of the optic nerve fibres: arcuate defects (arc-shaped areas of loss), nasal steps, and paracentral scotomas. Identifying these patterns helps distinguish glaucomatous loss from other causes of visual field change. Visual field tests can be somewhat variable due to patient fatigue and attention — a single test result is rarely interpreted in isolation; trends over multiple tests are what matter most.

OCT of the Optic Nerve

Optical Coherence Tomography (OCT) is the most important objective structural test in modern glaucoma practice. OCT uses light waves to produce high-resolution cross-sectional images of the optic nerve and retina, allowing measurement of specific nerve tissue thicknesses with reproducible precision.

Understanding Your OCT Results

Your OCT printout can look overwhelming at first glance. Here is what the key measurements mean:

Retinal Nerve Fibre Layer (RNFL) Thickness

The RNFL is the innermost layer of the retina, composed of the axons (projections) of retinal ganglion cells. In glaucoma, these fibres are progressively lost. The OCT measures RNFL thickness in microns around the circumference of the optic disc and compares your measurements to an age-matched normative database.

The colour-coded output uses a standard traffic light system:

• Green (within normal limits): Your measurement falls within the range seen in the normative database (above the 5th percentile). This does not guarantee the absence of glaucoma — very large optic nerves in otherwise healthy eyes can have thin RNFL within "normal" limits — but it is a reassuring finding.
• Yellow (borderline): Your measurement falls between the 1st and 5th percentile of the normative database. This may represent early loss or simply natural variation. Borderline results require correlation with other clinical findings and longitudinal monitoring.
• Red (outside normal limits): Your measurement falls below the 1st percentile. This suggests significant RNFL thinning and, in the context of other glaucomatous features, is consistent with glaucomatous damage.

Ganglion Cell Complex (GCC)

Macular-based OCT analysis measures the combined thickness of the ganglion cell layer and inner plexiform layer — the cell bodies and connections of the same neurons whose fibres make up the RNFL. GCC analysis adds diagnostic information particularly useful for detecting early central glaucomatous damage that may be missed by RNFL measurement alone.

Optic Disc Parameters

OCT also measures the optic disc itself: disc area, cup area, rim area, and cup-to-disc ratio (the proportion of the disc occupied by the central "cup" compared to the "rim" of neural tissue). A larger cup-to-disc ratio is associated with glaucoma, but there is significant natural variation in cup size — a large cup in a healthy eye (called physiological cupping) can look alarming on paper but may be entirely normal for that individual. This is why OCT disc parameters must always be interpreted alongside the clinical examination, RNFL thickness, and visual field data.

Glaucoma Treatment Overview

The goal of all glaucoma treatment is to lower intraocular pressure to a level at which optic nerve damage is arrested or slowed to an acceptable rate. Three main treatment modalities are used:

Eye Drops (Medical Therapy)

Eye drops remain the most commonly used first-line treatment. The main drug classes include:

• Prostaglandin analogues (e.g., latanoprost, travoprost, bimatoprost): once-daily drops that increase aqueous outflow; highly effective with 25–35% IOP reduction on average; very well tolerated systemically
• Beta-blockers (e.g., timolol): reduce aqueous production; twice-daily; require consideration in patients with asthma or heart block
• Carbonic anhydrase inhibitors (e.g., dorzolamide, brinzolamide): reduce aqueous production; used as adjuncts or in combination drops
• Alpha-agonists (e.g., brimonidine): dual mechanism; used as adjuncts

Adherence to prescribed drops is the most important determinant of treatment effectiveness. If you are finding your drop schedule difficult — due to side effects, cost, or practical challenges — discuss this openly with Dr. Sundaram. There are often alternatives.

Selective Laser Trabeculoplasty (SLT)

SLT is an office-based laser procedure performed at the slit lamp, in which short pulses of low-energy laser are applied to the trabecular meshwork (the eye's drainage tissue). This stimulates a biological response that improves aqueous drainage and lowers IOP by approximately 20–30% on average.

The landmark LiGHT trial (Gazzard et al., The Lancet, 2019) provided strong Level I evidence that SLT is at least as effective as prostaglandin eye drops as initial treatment for open-angle glaucoma and ocular hypertension — with no systemic side effects and a significant proportion of patients maintaining adequate IOP control for years after a single treatment. SLT is fully covered by BC MSP and is available at Dr. Sundaram's clinic.

SLT can be repeated if the initial effect wears off over time. It does not preclude the use of drops or surgery if needed later. Many patients find it a convenient and well-tolerated option, particularly those who struggle with drop adherence.

Surgical Options

For patients with advanced glaucoma, IOP that remains inadequately controlled despite maximum tolerated medical and laser therapy, or those who require very low target pressures, surgical intervention may be recommended:

• Trabeculectomy: The traditional glaucoma filtering surgery, in which a small opening is created in the eye wall under the conjunctiva to allow fluid to drain. Highly effective at achieving low IOPs but requires close post-operative monitoring and has a more substantial recovery period.
• MIGS (Minimally Invasive Glaucoma Surgery): A growing category of micro-scale surgical procedures that are typically performed at the time of cataract surgery, including iStent injection, Hydrus microstent, and others. MIGS procedures carry lower risk than trabeculectomy but generally provide more modest IOP reduction; they are best suited to mild to moderate glaucoma combined with cataract surgery.

The Importance of Regular Monitoring

Glaucoma is a chronic condition. Even with stable treatment, the risk of gradual progression over time requires ongoing surveillance. Regular monitoring visits — typically every 3 to 12 months depending on disease severity and stability — include OCT, visual field testing, and IOP measurement. This longitudinal data allows Dr. Sundaram to detect any progression trend and adjust treatment before meaningful vision loss occurs.

Understanding that these visits are not just "checking your pressure" — but are tracking the structural and functional health of your optic nerve over time — may help frame their importance. A single test result matters less than the trend over years.

Why Specialist Expertise Matters

Dr. Sundaram completed fellowship training specifically in glaucoma and cataract surgery, in addition to her general ophthalmology residency. Fellowship training involves intensive subspecialty practice under mentorship at academic centres, developing expertise in complex diagnostic interpretation, surgical technique, and the management of difficult cases that extend beyond routine practice.

For patients with complex glaucoma — advanced disease, unusual presentations, the co-existence of cataract requiring surgical planning, or normal tension glaucoma with normal IOP — subspecialist expertise in interpreting the full clinical picture is particularly valuable.