Technique and Setup

The procedure is performed in the clinic or examination room with the patient seated at the slit lamp. Topical anaesthetic drops (proparacaine 0.5% or tetracaine 1%) are instilled to numb the cornea. A small amount of sodium fluorescein dye is introduced. The examiner positions the biprism tip so it just touches the cornea at the apex (highest point), looking through the eyepiece of the slit lamp and adjusting the measurement dial until the mires (semicircular images) align correctly. The procedure takes less than one minute and is completely painless when anaesthetic is used properly.

Normal IOP Range

Statistically, normal IOP is 10–21 mmHg, with an average around 15 mmHg. However, there is significant variation:
– IOP below 10 mmHg may indicate hypotony (abnormally low pressure) and can cause vision-threatening complications.
– IOP of 21–30 mmHg is considered elevated and is associated with increased glaucoma risk, though not all individuals in this range develop glaucoma.
– IOP above 30 mmHg carries substantially increased glaucoma risk and typically warrants treatment.

Factors Affecting Tonometry Accuracy

Central Corneal Thickness (CCT)

CCT significantly affects IOP readings. The Imbert-Fick principle assumes a “standard” corneal thickness of approximately 555 μm. In patients with thicker corneas (>600 μm), Goldmann tonometry overestimates IOP; in thin corneas (<500 μm), it underestimates. CCT correction factors can be applied, though the exact magnitude of correction remains debated. Measuring CCT (using pachymetry) and adjusting tonometry readings accordingly is recommended in all glaucoma patients, particularly those with very thick or thin corneas.

Corneal Astigmatism

Significant astigmatism (>3 diopters) can affect accuracy. The more curved meridian applanates at lower force, potentially underestimating IOP. Rotating the biprism probe axis perpendicular to the steepest meridian or using corrected formulas can improve accuracy.

Corneal Oedema or Scarring

Corneal clouding or scarring (e.g., from prior surgery, infection, or congenital glaucoma with Haab’s striae) reduces visualization of the mires and can make readings unreliable or impossible.

Post-Refractive Surgery Eyes (LASIK, PRK)

After LASIK or PRK, the cornea is flattened and CCT is reduced, making standard Goldmann readings inaccurate. Alternative tonometry methods (iCare, Perkins, or specifically adjusted formulas) are often necessary in post-refractive surgery eyes.

Alternative Tonometry Methods

Non-Contact Tonometry (Air Puff)

A puff of air flattens the cornea; timing of applanation determines IOP. No contact with the eye, no anaesthetic required. Rapid — useful for large-volume screening. However, accuracy is significantly lower than Goldmann, especially with elevated IOP, corneal irregularities, or anxious patients who tense the eyelids.

iCare Rebound Tonometry

A tiny probe (roughly 0.2 mm diameter) touches the cornea and rebounds. The rebound profile calculates IOP. No anaesthetic required. Particularly useful in children, patients who cannot cooperate with slit lamp examination, and home monitoring. Accuracy is good but not equivalent to Goldmann. Works well in post-refractive surgery eyes.

Perkins Handheld Tonometer

A portable version of Goldmann applanation using the same principle. Useful for supine or recumbent patients, bedside measurements in hospital settings, and use in the operating theatre during eye surgery. Accuracy equivalent to Goldmann when used properly.

Pascal Dynamic Contour Tonometer (DCT)

A contoured probe matches the corneal shape; IOP is measured independent of corneal properties (theoretically independent of CCT). Measures ocular pulse amplitude in addition to IOP. Accuracy comparable to Goldmann; may be particularly useful in eyes with unusual corneal properties. Less widely available than Goldmann.

TonoPen

A portable electronic tonometer with a small probe. Useful in the operating theatre. Less accurate than Goldmann in routine clinical use but adequate for intraoperative monitoring.

Diurnal IOP Variation

IOP fluctuates throughout the day in a predictable pattern. In most individuals, IOP is highest in the early morning (between 7–9 am) and lowest in the late afternoon to early evening (2–4 pm). This diurnal variation is typically 4–6 mmHg but can exceed 10 mmHg in some individuals, particularly in those with glaucoma. For patients suspected of IOP spikes or whose pressure is difficult to control, phasing (measuring IOP at multiple times throughout the day) can help identify peak pressure times and guide treatment timing.

IOP in Glaucoma Diagnosis and Risk

IOP is the only modifiable risk factor for glaucoma. However, the relationship is not linear:
– Ocular hypertension (OHT): IOP persistently >21 mmHg without optic nerve damage or visual field loss. The 5-year risk of developing glaucoma in untreated OHT is approximately 2–3%.
– Normal-tension glaucoma (NTG): Glaucomatous optic neuropathy with established optic nerve damage and/or visual field loss, but with IOP that remains <21 mmHg on repeated measurements. Accounts for approximately 30% of POAG in Asian populations.
This means IOP must always be interpreted in the complete clinical context — optic disc appearance, RNFL on OCT, visual field, family history, age, and risk factors all contribute to glaucoma risk assessment and treatment decisions.