Intraocular pressure (IOP) remains a major risk factor for glaucoma and, thus far, is the only parameter that can be manipulated via topical medications, laser therapy, and/or surgery to decelerate development of and progression into severe stages of the disease. Achievement of target IOP is the key factor in the decision-making process when selecting the appropriate treatment for glaucoma patients.

Diurnal variations in IOP occur in healthy subjects and are more pronounced in patients who have glaucoma.¹ A single IOP measurement using Goldmann applanation tonometry (GAT), the gold standard technique, during office hours doesn’t always reveal the complete picture of a patient’s IOP patterns and daily fluctuations, a comprehensive picture that is required for accurate diagnosis and evaluation.² Fluctuations occur due to body position, patient-specific responses to topical medications, physical activity, and nocturnal elevation.^3,4

One study in glaucoma patients suggested that about half of the peak IOP values are recorded outside office hours, resulting in change of glaucoma management in almost 80% of the study patients.⁵ Importantly, more than 75% of single IOP measurements taken between 7:00 a.m. to 9:00 p.m. tend to miss the highest point of a diurnal curve.² Studies have suggested that a higher peak pressure may be a potential independent risk factor for glaucomatous progression,⁶ with many highlighting the clinical utility of 24-hour IOP monitoring for better understanding of the efficacy and safety of available treatments.⁷

Conventional methods of assessing patients’ diurnal IOP curves involved hospitalization in a sleep laboratory, which make these tests cumbersome and costly.⁸ Consequently, self-tonometry has been an emerging field with a number of devices being tested for the evaluation of diurnal IOP curves in patients.⁹

At-home Monitoring

Icare tonometers use the principles of rebound tonometry described by Dekking and Coster in 1967.¹⁰ It is a handheld device consisting of a metallic motion probe situated in a coil system. At a speed of approximately 0.2 m/s in a magnetic field, the probe accelerates towards the cornea and rebounds away from it after impact. The IOP is then computed based on the deceleration of the probe after recording six reliable measurements.^11,12

IOP measurements taken using Icare tonometers have shown a strong correlation with GAT outcomes with the advantage of not requiring topical anesthetic.^12,13 However, it has been shown to overestimate IOP values when compared with GAT in patients with thicker central corneas.¹⁴ The angle at which the probe impacts the cornea has been reported to influence IOP measurements showing over and under estimation for central and peripheral measurements, respectively when compared with GAT.¹⁵

Recently, a new version of the Icare tonometer — Icare HOME — became available commercially. It was preceded by the Icare ONE device, which employed rebound tonometry for self measurement of IOP. The system has been reported to be a safe and reliable method in clinical setting.¹⁶

The Icare HOME is a handheld device and measures 11x3x8 cm at a weight of 150 grams. Similar to Icare ONE, it utilizes the rebound method to estimate IOP after six reliable measurements and does not require topical anesthetic prior to measurement. The patient loads the probe and sets the probe-cornea distance. The device promises better operability compared with the Icare ONE through a positioning assistant via a fixation ring and clinometer.¹⁷ As an additional feature, the machine alarms the patient if the optimal distance from the probe to the eye is not achieved, or if the patient’s hair or hands obstruct the path of the probe.¹⁸

The IOP measurements are stored directly in the device’s built-in memory component along with the date, time, quality, and laterality.⁹ This data can then be accessed on a personal computer using the device’s specialized software.

Application and Accuracy

Different generations of the Icare device have been evaluated to determine the accuracy of measurements compared with GAT as well as device application in various types of glaucoma. In a study of 18 consecutive patients with treated normal tension glaucoma, the Icare ONE device identified significantly higher peak nighttime IOP values, measured by the patients at home, when compared with daytime in-clinic measurements. As a result, one-third of these patients had their glaucoma management changed due to the peak IOP values detected by the Icare ONE device at nighttime.¹⁹

Recently, the Icare ONE was used to record the diurnal IOP profiles in patients with primary open-angle glaucoma (POAG) and primary angle-closure glaucoma (PACG) at home. Diurnal IOP fluctuations were found to be greater in patients with POAG than in PACG, and significantly greater midnight IOP was measured in PACG patients.²⁰ This device has also been reported to be feasible to measure IOP in the pediatric population.²¹

The Icare HOME device was used both by 130 patients with suspected or confirmed glaucoma as well as an ophthalmologist, and results were compared with GAT. The mean difference between the Icare HOME measurements taken by the patients and ophthalmologist was 0.21 mmHg, which was not significantly different (p=0.068). However, Icare HOME measurements overestimated the IOP by 0.7 mmHg when completed by patients (p<0.001) and by 1.0 mmHg when completed by ophthalmologist (p<0.001) when compared with GAT.²²

A 2016 study of 154 patients by Termuhlen et al. found that, although Icare HOME measurements underestimated IOP when compared with GAT, the two modalities correlated very well.²³ This study also found significant differences between Icare HOME and GAT IOP results in patients with thicker central corneal thickness (CCT). However, when compared with the Icare ONE, the Icare HOME device scored significantly higher with respect to patient satisfaction due to the fixation light and inclination sensor.

With regard to the use of Icare tonometry in short- and/or long-term IOP evaluation, one recent study assessed IOP fluctuations in 92 eyes of healthy patients at home over 3 consecutive days using the Icare HOME. Approximately 70% of measurements completed by the Icare HOME were within 3 mmHg of GAT measurements, with 10% to 16% of the cohort showing IOP peaks outside of office hours.²⁴

Given the very recent approval of the new generation of Icare devices, there will be a need for additional long-term studies to investigate their reliability and clinical usefulness of IOP measurements taken by patients independently.

The Future of Glaucoma Care Delivery

In keeping with the current era of advanced bioinformatics paving the future of healthcare, it is critical for healthcare providers to consider and utilize supplementary scientific data when making decisions and providing options for their patients. Self-measurements using the Icare ONE and HOME rebound tonometers provide great promise to feasibly and accurately identify diurnal IOP profiles, which, ultimately, allows customization of glaucoma care. Although challenges with such devices exist, including patient compliance, cost, and device maintenance, given their potential value in glaucoma care, it can be expected that soon we may be able to widely adapt this technology to better understand the patient presented to us. GP

References

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Harrish Nithianandan, HBSc, is part of the faculty of medicine at the University of Ottawa, Ontario, Canada.

Dr. Shahidi is head of clinical research and innovation at Prism Eye Institute, Mississauga, Ontario, Canada.

Dr. Ahmed practices at Prism Eye Institute, is a member of Trillium Health Partners, Mississauga, Ontario, Canada, and is a member of the Department of Ophthalmology and Vision Sciences at the University of Toronto.

* The authors do not have any relevant financial relationships to disclose.