Intraocular pressure (IOP) is the greatest modifiable risk factor for the onset and progression of glaucoma. However, in standard glaucoma care, IOP is measured only 3–4 times a year and is limited to clinic hours 3).
Even in eyes without glaucoma, IOP can fluctuate by 4–5 mmHg over the course of a day, and the fluctuation is even greater in glaucomatous eyes. At night, a reproducible IOP elevation related to the supine position occurs, but this is not captured by clinic measurements taken in the seated position. IOP peaks outside clinic hours may contribute to glaucoma progression, and there is a growing need to understand the 24-hour IOP profile.
Home tonometry is a method in which patients measure their own IOP in their daily living environment. Currently available devices include the iCare HOME rebound tonometer and the Triggerfish contact lens sensor, each providing information on IOP fluctuations through different approaches.
IOP fluctuates throughout the day, and peaks may occur at night or in the early morning. Clinic measurements taken 3–4 times a year capture only a small part of this circadian rhythm. Multiple studies have shown that IOP peaks can occur outside clinic hours, so relying solely on clinic measurements may be inadequate for evaluating treatment efficacy and prognosis. Treatment decisions should be based on multiple IOP measurements and should not depend on a single reading.
Rebound tonometry is a method that accelerates a small probe toward the cornea and calculates IOP by measuring the deceleration (rebound acceleration) after impact 1). The higher the IOP, the shorter the contact time between the probe and the cornea, and the greater the rebound acceleration.
The iCare is the most common rebound tonometer, using a 40 mm metal probe with a 1.7 mm diameter plastic tip 1). Because the probe is small and the contact time is short, anesthetic eye drops are not required 1)3). Comparative studies with GAT have shown agreement generally within ±5 mmHg.
The iCare often gives slightly higher readings than GAT 3). It is useful in patients with corneal disease or surface irregularities because of the small contact area 1). It is also easy to use for measuring IOP in children 3).
Device Features
Approval: Approved by the FDA in 2017 for self-measurement of intraocular pressure.
Principle: Operates on the same rebound tonometry principle as the clinical iCare. The average of six consecutive measurements is recorded as one IOP value.
Design: Compact size, light ring for probe centering, adjustable forehead and cheek rests. A green light indicates when the device is level and ready to measure.
Data Recording: Each measurement is stored with time, date, and eye laterality.
Clinical Utility and Limitations
Utility: Captures diurnal fluctuations outside office hours, providing a more complete picture of the patient’s circadian rhythm. No need for topical anesthesia, suitable for home use.
Limitations: 16–25% of patients find training completion difficult. Conflicting reports show slightly lower or higher values compared to GAT. Relies on self-measurement, so nocturnal IOP spikes during sleep cannot be captured.
Accuracy: Agreement with GAT is generally within 5 mmHg, but differences may occur in individual patients.
Approved by the FDA in 2016, the Triggerfish CLS (Sensimed) is a device that continuously measures changes in corneoscleral shape as a surrogate for IOP over 24 hours using a strain gauge and microchip embedded in a disposable silicone contact lens2).
| Parameter | iCare HOME | Triggerfish CLS |
|---|---|---|
| Measurement Principle | Rebound tonometry | Corneoscleral deformation |
| Unit | mmHg | mVeq (not convertible) |
| Nighttime measurement | Not possible (self-operation) | Possible (continuous wear) |
It takes 300 readings over 30 seconds every 5 minutes, collecting 86,400 data points in 24 hours2). However, since the measured values are output in millivolt equivalents (mVeq), direct conversion to mmHg is not possible2).
Minor adverse events (transient blurred vision, conjunctival hyperemia, punctate superficial keratitis) have been reported but resolve upon discontinuation of wear. Reproducibility is rated fair to good, and while some reports indicate usefulness in detecting nighttime intraocular pressure peaks, other results show it is insufficient for detecting intraocular pressure reduction by prostaglandin-related drugs.
Direct comparison is not possible. Triggerfish CLS measures the degree of corneoscleral deformation in millivolt equivalents (mVeq), not absolute intraocular pressure (mmHg). Due to the viscoelastic properties of the eye and the nonlinear relationship between volume and pressure, conversion from mVeq to mmHg is not feasible. However, the pattern of the 24-hour profile (peak timing and relative fluctuation range) provides clinically useful information and can be used to determine the timing of clinic appointments.
EyeMate (Implandata Ophthalmic Products) is a permanently implantable intraocular pressure sensor inserted into the ciliary sulcus during cataract surgery. It consists of eight pressure- and temperature-sensitive capacitors and is charged and transmits data via electromagnetic coupling through an external handheld device.
Patients can measure intraocular pressure on demand at any time, enabling long-term continuous monitoring. In the ARGOS-1 trial, no serious adverse events were observed after implantation, but some degree of pupil distortion was noted in all patients. It has obtained CE marking in Europe but is not yet approved by the US FDA.
It reveals intraocular pressure peaks and diurnal variation patterns outside office hours, enabling a more comprehensive assessment of treatment efficacy. Particularly in cases where progression continues despite treatment, the presence of out-of-office IOP spikes may be suggested. Additionally, 24-hour profile patterns may serve as predictors of the speed of disease progression. However, at present, the clinical significance of diurnal variation is not fully established.
