What tests are used to diagnose glaucoma during an eye exam? 

Glaucoma is a complex condition that cannot be identified through a single measurement or observation. Because the disease involves the progressive decay of the optic nerve, often driven by internal pressure, clinicians must perform a battery of specific tests to build a comprehensive picture of an individual’s eye health. In the United Kingdom, a standard glaucoma assessment involves a combination of structural inspections and functional evaluations to detect damage at the microscopic level. These tests are designed to identify the disease in its earliest stages, often years before a patient notices any change in their visual field. By combining data from multiple diagnostic tools, optometrists can distinguish between normal age-related changes and the pathological signs of active glaucoma. 

What We’ll Discuss in This Article 

• The role of tonometry in measuring intraocular pressure. 

• Ophthalmoscopy and the physical inspection of the optic nerve head. 

• Visual field testing to identify patterns of peripheral vision loss. 

• The importance of gonioscopy in assessing the eye’s drainage angle. 

• How pachymetry measures corneal thickness to refine pressure readings. 

• The use of high-resolution OCT scans for microscopic nerve analysis. 

• Answers to common questions regarding the comfort and necessity of these tests. 

Tonometry: Measuring Intraocular Pressure 

Tonometry is the clinical procedure used to determine the internal pressure of the eye, known as intraocular pressure (IOP). This is often the first test performed during a screening, as elevated pressure is the primary modifiable risk factor for glaucoma. In many high street practices, this is initially conducted using a non-contact tonometer, which sends a small puff of air against the cornea to measure its resistance. While the air-puff test is a useful screening tool, it is often supplemented with more precise methods if the results are borderline or high. 

The gold standard for pressure measurement in the UK is Goldmann applanation tonometry. During this procedure, the clinician applies a small amount of yellow dye and anaesthetic drops to the eye before gently touching the surface of the cornea with a small, blue-lighted probe. This method is highly accurate and provides the definitive pressure reading used to guide treatment decisions. According to clinical standards from the College of Optometrists, consistent monitoring of IOP is essential, as pressure can fluctuate throughout the day, requiring multiple readings over time to establish a reliable average. 

Ophthalmoscopy: Inspecting the Optic Nerve 

Ophthalmoscopy, also known as fundoscopy, allows the clinician to look directly at the back of the eye to assess the structural health of the optic nerve. Using a high-powered lens and a bright light, the optometrist examines the optic disc, which is the point where the nerve fibres exit the eye to travel to the brain. They are specifically looking for “cupping,” a process where the central indentation of the nerve head enlarges as the surrounding nerve fibres die away. A larger “cup-to-disc ratio” is a hallmark physical sign of glaucomatous damage. 

In addition to cupping, the clinician looks for other subtle markers, such as tiny splinter haemorrhages on the disc or changes in the way the blood vessels enter the eye. Modern UK practices often complement this physical check with digital retinal photography, which creates a permanent high-resolution image of the nerve. Research by the Association of Optometrists indicates that these photographic records are invaluable for longitudinal monitoring, as they allow clinicians to spot minute changes in the nerve’s appearance over several years that might otherwise be missed. 

Perimetry: Visual Field Testing 

While tonometry and ophthalmoscopy look at the structure of the eye, perimetry or visual field-testing measures how well the eye is functioning. Because glaucoma typically destroys peripheral vision first, a standard letter chart test is often insufficient for detection. During perimetry, the patient sits in front of a bowl-shaped machine and clicks a button every time they perceive a small flash of light in their peripheral vision. This process creates a detailed map of the patient’s visual field, identifying any areas of reduced sensitivity or complete “blind spots.” 

The patterns of loss found in perimetry are highly characteristic of glaucoma. For example, damage often appears as an arc-shaped defect that follows the pathway of the nerve fibres. According to the British Journal of Ophthalmology, these maps are essential for determining the functional impact of the disease on a patient’s daily life. Because the brain is adept at masking early vision loss, perimetry is the only way to “see” what the patient is missing. Regular visual field tests are conducted to ensure that the current treatment is successfully halting the expansion of these blind spots. 

Gonioscopy: Assessing the Drainage Angle 

Gonioscopy is a specialised test used to determine whether the drainage angle—the area where fluid exits the eye—is “open” or “closed.” This distinction is critical because it dictates whether a patient has open-angle glaucoma or the more urgent angle-closure variety. During the test, a special mirrored lens is placed directly onto the surface of the eye (after numbing drops are applied). This allows the specialist to look into the “corner” of the eye to see if the drainage structures are visible or if they are being blocked by the iris. 

In the UK, gonioscopy is a fundamental part of a comprehensive glaucoma assessment, especially for patients who are long-sighted or have an unusual eye anatomy. If the angle is found to be dangerously narrow, the patient may be at risk for a sudden, painful pressure spike known as an acute glaucoma attack. Identifying a narrow angle allows clinicians to perform preventative laser treatments, such as a peripheral iridotomy, to ensure the fluid can always exit the eye safely. This test is a primary tool for preventing the emergency form of the disease. 

Pachymetry: Measuring Corneal Thickness 

Pachymetry is a simple, quick test that measures the thickness of the cornea, the clear front window of the eye. This measurement is vital because corneal thickness can significantly influence the accuracy of eye pressure readings. A naturally thick cornea may resist the tonometer more, leading to a falsely high-pressure reading. Conversely, a thin cornea may be more easily compressed, leading to a falsely low reading that could mask active glaucoma. 

Knowing the corneal thickness allows the clinician to “correct” or interpret the pressure readings more accurately. For example, if a patient has a thin cornea and a “normal” pressure of 18 mmHg, their true internal pressure might actually be 21 mmHg, which could be high enough to cause damage. Specialist guidance from Cardiff University notes that pachymetry is also a risk factor in its own right; individuals with thinner corneas are statistically more likely to experience progressive nerve damage and require more intensive monitoring. 

Optical Coherence Tomography (OCT) 

Optical Coherence Tomography (OCT) has become a revolutionary tool in UK eye care, providing a microscopic cross-section of the retina and the optic nerve. Using light waves rather than radiation, the OCT scan measures the thickness of the retinal nerve fibre layer (RNFL) with sub-micron precision. This allows clinicians to detect the thinning of the nerve years before it would be visible through traditional ophthalmoscopy or appear as a blind spot on a visual field test. 

By creating a three-dimensional map of the optic nerve head, OCT provides a definitive “baseline” for each patient. According to clinical data from Moorfields Eye Hospital, the ability to track these minute structural changes over time is the most effective way to manage chronic glaucoma. Many modern high street opticians now offer OCT as an “enhanced” part of their eye exam. For patients with a family history or other risk factors, this advanced imaging offers the highest level of diagnostic certainty and the best chance for early intervention. 

Test Name	What it Measures	Clinical Importance
Tonometry	Intraocular pressure	Identifies the primary risk factor for damage
Ophthalmoscopy	Physical nerve structure	Looks for cupping and visible fibre loss
Perimetry	Peripheral visual field	Maps functional blind spots the brain hides
Gonioscopy	Drainage angle status	Distinguishes between open and closed angle risk
Pachymetry	Corneal thickness	Refines the accuracy of pressure measurements
OCT Scan	Nerve fibre layer thickness	Detects microscopic damage years in advance

Conclusion 

Diagnosing glaucoma requires a multi-faceted approach involving at least five distinct clinical tests to assess both the structure and function of the eye. From measuring the internal pressure to mapping the peripheral vision and taking microscopic scans of the optic nerve, each test provides a piece of the diagnostic puzzle. In the UK, these assessments are standard practice for identifying the disease in its earliest, most treatable stages. If you experience severe, sudden eye pain, redness, or a rapid loss of vision, call 999 immediately. 

Authority Snapshot 

This article provides a clinical overview of the diagnostic pathway for glaucoma in the UK, detailing the specialised tests used to identify the condition. The information is based on established medical standards provided by the Association of Optometrists and the College of Optometrists. Dr. Rebecca Fernandez, a UK-trained physician, has reviewed this content to ensure it accurately reflects the clinical technology and diagnostic procedures currently utilised within the UK healthcare system.