Intraocular pressure (IOP) is the only directly modifiable risk factor for glaucoma and is the target of all therapeutic interventions. IOP measurement forms the basis for glaucoma detection, progression assessment, and evaluation of treatment efficacy.
However, the reliability of IOP measurements has various limitations. Inter-examiner and intra-examiner variability, principle-based constraints of measurement devices, influence of corneal parameters, and physiological fluctuations of IOP itself affect measurement accuracy. Clinicians must recognize these limitations and comprehensively evaluate multiple measurements and clinical findings.
| Measurement Method | Reliability | Characteristics |
|---|---|---|
| Goldmann Applanation | Gold Standard | Affected by central corneal thickness |
| Non-contact | Moderate | Underestimates high IOP |
| iCare rebound | Good | No need for topical anesthesia |
The Goldmann applanation tonometer is clinically the most accurate and is standardly used in glaucoma care 1). It measures the force required to flatten a corneal area of 3.06 mm in diameter and calculates intraocular pressure based on the Imbert-Fick law. However, because it is affected by central corneal thickness (CCT), corneal parameters must be considered when interpreting the measurements.
Intraocular pressure is not constant; it shows diurnal variation. About two-thirds of glaucoma patients have their highest intraocular pressure outside regular clinic hours, especially at night or during sleep. Fluctuations of 10 mmHg or more over 24 hours can occur, so a single measurement during a routine outpatient visit may not accurately capture intraocular pressure variability.
Seasonal variation has also been reported, with average intraocular pressure in winter being significantly higher than in summer. Even in normal-tension glaucoma, a clear seasonal pattern has been confirmed over 20 years of follow-up.
Diabetes: Diabetic patients with high HbA1c have higher intraocular pressure than healthy individuals
Steroids: Long-term use of corticosteroids increases intraocular pressure. 2.8% of users develop glaucoma
Medications: Even a single dose of antidepressants (e.g., fluoxetine) can increase intraocular pressure
Smoking: Current and past smoking history is associated with increased intraocular pressure.
Posture change: Intraocular pressure is higher in the supine position than in the sitting position. Elevated intraocular pressure during sleep is easily overlooked.
Goldmann Applanation Tonometer
Principle: Based on the Imbert-Fick law, intraocular pressure is calculated from the force required to flatten the cornea to a diameter of 3.06 mm.
Accuracy: It is the gold standard in glaucoma care 1). It has the highest clinical accuracy.
Limitations: Designed with a central corneal thickness of 520 μm as a reference, measurement errors occur due to corneal thickness. Intra-observer reliability: 1.5±1.96 mmHg; inter-observer reliability: 1.79±2.41 mmHg.
Perkins Applanation Tonometer
Principle: A handheld tonometer using the same applanation prism as the Goldmann tonometer 1).
Advantages: Highly portable and can be used without a slit lamp. Also allows measurement in the supine position.
Limitations: The measurement principle is the same as Goldmann, and it is similarly affected by central corneal thickness.
Even with two experienced glaucoma specialists performing consecutive measurements, a difference of 2 mmHg or more occurs in 17% of eyes, and among technicians, discrepancies are seen in 25%. Individual intraocular pressure measurements can vary by 3 mmHg or more due to instrument reliability alone.
Intraocular pressure is measured by flattening the cornea with a puff of air 1). The measurement procedure is simple, but it is easily affected by pulse waves, and three or more repeated measurements are necessary 1). It tends to overestimate in low-pressure ranges and underestimate in high-pressure ranges.
Ocular Response Analyzer (ORA) is a non-contact tonometer that can simultaneously measure corneal hysteresis (CH) and intraocular pressure. It calculates IOP corrected for the influence of central corneal thickness by measuring CH. It tends to overestimate IOP compared to Goldmann applanation tonometry, and the degree of overestimation increases with higher IOP.
A portable tonometer that can measure in both sitting and supine positions, measuring IOP with a pressure transducer at the tip 1). It is affected by central corneal thickness, with reported errors of 0.29 mmHg per 10 μm in men and 0.12 mmHg per 10 μm in women.
It may miss one-third of eyes with elevated IOP, and compatibility with GAT is limited. However, some reports indicate it is more reliable than GAT in post-LASIK eyes. In pressure ranges above 16 mmHg, it is less reliable than the Perkins tonometer.
A portable tonometer that can measure without topical anesthesia 1). It shoots a small probe onto the cornea and estimates IOP from the rebound speed. It correlates well with Goldmann applanation tonometry but tends to show slightly higher values 1).
Agreement with GAT within 5 mmHg is 73%, and it shows values on average 3.35 mmHg higher compared to the Perkins tonometer. The iCare IC200 can measure even when facing downward, making it suitable for supine or nighttime measurements. Its application for home IOP monitoring is also being studied.
It is said to minimize the influence of corneal properties (central corneal thickness, corneal curvature). While GAT shows a significant correlation with central corneal thickness, DCT shows no correlation, offering the advantage of being less affected by corneal thickness.
After LASIK, the cornea is thinner, so Goldmann applanation tonometry carries a risk of underestimating IOP. Studies report that Tonopen is more reliable than GAT after LASIK. Dynamic contour tonometry (DCT) is also less affected by central corneal thickness, making it useful for IOP measurement after corneal surgery. In clinical practice, it is desirable to combine multiple measurement methods for evaluation.
Central corneal thickness (CCT) is one of the factors that most significantly affects the accuracy of applanation tonometry. Since the Goldmann applanation tonometer is designed based on an average central corneal thickness (520 μm), it overestimates IOP in thicker corneas and underestimates it in thinner corneas.
A difference of 50 μm from the normal value can cause a measurement error of 2.5 to 3.5 mmHg, but there is no universally accepted formula for “correcting” IOP based on central corneal thickness.
In the Ocular Hypertension Treatment Study (OHTS), among patients with baseline mean intraocular pressure >25.75 mmHg, the 5-year risk of glaucomatous damage was 36% for those with thin to average central corneal thickness (555 μm), compared to 13% for those with central corneal thickness of 565–588 μm.
| Device | Intra-device variability | Inter-device difference |
|---|---|---|
| Scheimpflug method | 5–15 μm | Up to 120 μm |
| Ultrasound method | 5–15 μm | Up to 120 μm |
| OCT | Minimal | — |
The variability within the same device is 5–15 μm, but the difference between devices can reach up to 120 μm. OCT measurements have the least variability. Caution is needed when comparing central corneal thickness values between different devices.
In physiologically thick corneas, intraocular pressure is overestimated, while in pathologically thick corneas (edema), the actual intraocular pressure is underestimated. Contact lens-induced corneal edema causes slight underestimation with DCT and overestimation with GAT. Corneal astigmatism also affects GAT measurements; in eyes with moderate or greater astigmatism, measurements in two directions 90 degrees apart are recommended.
It has also been pointed out that the association between central corneal thickness and glaucoma may be due to collider bias rather than a biological relationship, and there is a view that central corneal thickness alone should not be used as a risk factor.
The Goldmann applanation tonometer is designed based on an average central corneal thickness (approximately 520 μm). When the cornea is thin, the measured pressure is lower than the actual intraocular pressure (underestimation), and when thick, it is measured higher (overestimation). A difference of 50 μm from the normal value can cause an error of 2.5 to 3.5 mmHg. However, no correction formula has been established, and central corneal thickness should only be used as a reference for interpreting intraocular pressure values 1).
Conventional intraocular pressure measurement has been limited to single measurements during outpatient visits, but the development of continuous monitoring devices that capture 24-hour IOP fluctuations is progressing.
The SENSIMED Triggerfish® is a contact lens-type sensor that records relative changes in intraocular pressure from changes in corneal shape. Its correlation with the Tonopen XL is weak, and it is said to provide only relative changes, not absolute IOP.
Eyemate is an intraocular microsensor, and the mean difference from GAT measurements is −0.2 mmHg, with 100% of measurements within ±5 mmHg of GAT, indicating high reliability.
The iCare HOME is a home rebound tonometer; survey results show that 73.7% of patients found it easy to use, and 100% found it useful. It contributes to understanding IOP fluctuations outside office hours.
The iCare HOME (home rebound tonometer) is a device that allows patients to measure their own intraocular pressure. It does not require anesthetic eye drops and is relatively easy to operate, making it useful for understanding 24-hour intraocular pressure fluctuation patterns, including outside of clinic hours. However, the agreement with GAT is about 73% within 5 mmHg, so the reliability of absolute values is limited. It is recommended to use it in combination with outpatient measurements.
- 日本緑内障学会. 緑内障診療ガイドライン（第5版）. 日眼会誌. 2022;126(2):85-177.
