IOP Measurement: Pitfalls and How to Avoid Them

IOP measurement is the most repeated test in glaucoma care.

Small errors change decisions about diagnosis and treatment.

Knowing the pitfalls makes every reading more useful.

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Why IOP Measurement Matters

Pressure is the only modifiable risk factor for glaucoma.

Every treatment target depends on a reliable number.

A reading that is wrong by three millimeters can mislead care.

Therefore, treat IOP measurement as a skill, not a routine.

Goldmann Applanation as the Reference

Goldmann tonometry flattens a fixed area of the cornea and reads the force needed.

It remains the standard against which other devices are judged (see Goldmann applanation tonometry).

However, it has known sources of error.

So learn those errors before relying on it.

Calibration and Equipment

Check the calibration of the tonometer every month.

A tip that sits loosely or is dirty will give a false reading.

Disinfect prisms between patients.

Also replace damaged prisms.

Central Corneal Thickness and IOP Measurement

A thick cornea resists flattening and overestimates pressure.

A thin cornea underestimates pressure.

The Ocular Hypertension Treatment Study found thin corneas to be a risk factor for glaucoma.

Therefore, measure thickness in every glaucoma suspect (see ocular hypertension).

Correcting for Corneal Thickness

Correction formulas exist, but none is accurate for every eye.

Use thickness to adjust the risk, not to alter the number blindly.

A thin cornea raises the concern even when pressure is normal.

So interpret both values together.

Fluorescein Amount

Too much fluorescein makes thick mires and overestimates the pressure.

Too little makes thin, faint mires and underestimates it.

Aim for thin, bright semicircles.

Also dry the eye if the tear meniscus is large.

Astigmatism and IOP Measurement

Corneal astigmatism above three diopters distorts the reading.

Align the red mark on the prism with the least curved axis.

Alternatively, average readings from two perpendicular positions.

Therefore, check keratometry in high astigmatism.

Patient Factors

Squeezing the lids, holding breath or straining raises pressure.

A tight collar can do the same.

Ask patients to relax and breathe normally.

In addition, avoid pressing on the globe with a finger.

Corneal Disease and Surgery

Corneal edema raises measured pressure, while scarring and thick grafts distort it.

Refractive surgery thins the cornea and lowers readings.

Therefore, record the surgical history in the chart.

So expect underestimation after LASIK.

Diurnal Variation

Pressure varies by several millimeters within a day.

Peaks often occur in the early morning.

A single clinic reading can miss these peaks.

Also record the time of each visit.

Alternatives to Applanation

Rebound tonometers need no anesthetic and suit children and home use.

Dynamic contour tonometry is less affected by corneal thickness.

Non-contact devices are good for screening.

However, confirm abnormal readings with applanation.

Patient Positioning and Consistency

Measure pressure with the patient seated at the slit lamp.

Use the same technique and the same hand at each visit.

Document the device you used.

So changes in pressure reflect the eye, not the method.

Recording and Using the Results

Write the time, the device and the pressure.

Compare with previous values before deciding on treatment.

Look at the trend as well as the single number.

Therefore, review the chart at every visit.

IOP Measurement: Practical Summary

Check calibration, use a thin line of fluorescein and measure thickness.

If the readings are inconsistent, repeat them.

Because the stakes are high, trust the trend.

Finally, correlate pressure with the optic nerve and the visual field.

Pitfalls to Avoid

• Do not use a single reading to start or stop treatment
• Never forget the effect of central corneal thickness
• Avoid ignoring high astigmatism
• Do not skip tonometer cleaning

IOP Measurement in Children

Children rarely tolerate applanation tonometry.

Rebound devices are quick and need no drops.

If the child will not cooperate, an exam under anesthesia may be needed.

Because anesthetics change pressure, interpret those readings with care.

IOP Measurement at Home

Home rebound tonometers capture pressure peaks outside the clinic.

Patients who track values can show diurnal swings.

However, device accuracy varies, so confirm unusual values in clinic.

Therefore, use home readings as a trend only.

IOP Measurement After Corneal Surgery

Grafts and scars distort the reading.

When the cornea is irregular, use an alternative device.

Because no method is perfect, correlate pressure with the optic nerve.

Then record which device you used.

IOP Measurement Training

Teach trainees to look at the mires as well as the dial.

Supervised practice improves accuracy within weeks.

If two examiners disagree, repeat the measurement together.

In this way, errors become teaching points.

For the latest studies, see the PubMed results for IOP measurement.

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References

1. Whitacre MM, Stein R. Sources of error with use of Goldmann-type tonometers. Surv Ophthalmol. 1993;38:1-30.
2. Brandt JD, Beiser JA, Kass MA, Gordon MO. Central corneal thickness in the Ocular Hypertension Treatment Study (OHTS). Ophthalmology. 2001;108:1779-1788.
3. Gordon MO, Beiser JA, Brandt JD, et al. The Ocular Hypertension Treatment Study: baseline factors that predict the onset of primary open-angle glaucoma. Arch Ophthalmol. 2002;120:714-720.
4. Cook JA, Botello AP, Elders A, et al. Systematic review of the agreement of tonometers with Goldmann applanation tonometry. Ophthalmology. 2012;119:1552-1557.