A contact lens sensor may aid in 24-hour IOP monitoring and help measure the true diurnal and nocturnal IOP.
Accurate assessment of IOP is vital in the effective management of glaucoma. However, although IOP is known to be a highly dynamic parameter, it is usually only assessed during clinical appointments using static tonometry techniques. As a result, the true diurnal and nocturnal IOP remains unknown.
A 24-hour IOP contact lens sensor (CLS) has been developed to address this issue, however the practical use of the collected data now requires validation. As the CLS output is provided in relative units (corresponding to electrical units of voltage) and tonometry is provided in absolute mmHg units, a direct comparison between the two cannot be performed.
In addition, it is not possible to use tonometry simultaneously on the CLS-wearing eye. In an attempt to counteract this problem, researchers recently conducted a study, published in PLoS One, using the fellow eye as a comparator.
The overall purpose of the study was to assess the performance of the CLS for 24-hour monitoring of IOP-related short-term patterns and to compare with IOP measurements obtained by pneumatonometry.
The prospective trial correlated CLS output with heart rate (the systemic parameter) and with the IOP measurements obtained via the pneumatonometer.
Next: Breaking down the study
Two parameters were identified, representing both short duration (seconds) and longer duration (hours) IOP-related patterns. The first, ocular pulse frequency (OPF) corresponds to IOP variations due to systole and diastole during the cardiac cycle. The second, the wake/sleep slope (W/S), is derived via the CLS and serves to quantify the characteristic IOP rise that occurs when subjects go from the wake/sitting to the sleep/supine state.
The study was conducted over 24 hours in sleep laboratory.
One randomly assigned eye was fitted with a CLS (Triggerfish, Sensimed, Switzerland) in 31 volunteers and 2 glaucoma patients. The CLS measures spontaneous dimensional changes of the eye at the corneoscleral junction to record the IOP-related profile. In the contralateral eye, IOP measurements were taken using a pneumatonometer every two hours with study participants in the habitual body positions. Heart rate (HR) was measured 3 times during the night for a 6 minute, every 2 hours.
Performance of CLS was defined in two ways: 1) by recording the known pattern of IOP increase going from awake (sitting position) to sleep (recumbent), defined as the wake/sleep (W/S) slope, and 2) accuracy of the ocular pulse frequency (OPF) concurrent to that of the HR interval. Strength of association between overall CLS and pneumatonometer curves was assessed using coefficients of determination (R2).
The W/S slope was significantly positive in both eyes of each study participant (CLS, 57.0 ± 40.5 mVeq/h, p<0.001 and 1.6 ± 0.9 mmHg/h, p<0.05 in the contralateral eye). Study graders agreed on evaluability for OPF in 83.9% of the 87 evaluated CLS plots concurrent to the HR interval. Accuracy of the CLS to detect the OPF was 86.5%. Coefficient of correlation between CLS and pneumatonometer for the mean 24-h curve was R2 = 0.914.
Next: Study limitations
The study was subject to a few limitations:
· Simultaneous OPF and HR measurements could not be obtained for periods of 30 seconds. It is possible that a different accuracy of OPF would have been identified if longer periods of simultaneous measurements were available.
· Although awakening of participants for pneumatonometry and HR measurements was found not to have a significant effect on the correlation with OPF assessment, there is a possibility that awakening subjects for HR measurements produced OPF changes.
· A significant reduction of CCT (-12.3 μm in CLS vs. -0.8 μm in fellow eye) in the CLS eye, but not in the fellow eye, was observed. The extent to which this change may have affected CLS measurements is unknown.
· Due to the difficulty of recruiting glaucoma patients with less than 3 mmHg asymmetry between fellow eyes and the need for timely enrolment, only 2 glaucoma patients could be included in this study. However, in this study, no significant difference between the healthy eyes and glaucoma eyes for the two major outcome measures were expected.
The study demonstrated that CLS outputs reflect known changes in IOP that are in agreement with pneumatonometry measurements of the contralateral eye. This agreement was demonstrated both for the characteristic IOP rise when individuals transition from the wake to the sleep state (W/S slope) as well as for the overall 24-h curve.
Next: Further understanding the results
Except for one participant, all consistently demonstrated a positive W/S slope, despite the IOP-related pattern being unique for each participant. In line with previous investigations, the results in the current study showed the mean W/S slope of the nocturnal CLS pattern was statistically significantly positive in 97% of participants. Furthermore, CLS detects short-term changes of IOP related to the cardiac cycle with good accuracy.
The current CLS software, however, is not well suited for detection of ocular pulsations, which are graphically identifiable as regular low-amplitude oscillations.
Ocular pulsations are mostly visible in the sleep period when eyes are mostly closed and eye movements are reduced. Characteristics of ocular pulsations, such as OPF and ocular pulse amplitude (OPA) have been suggested as potentially useful clinical parameters.
Providing 24-hour data on timing, frequency, and amplitude of ocular pulsations may provide information about the complex relationships between ocular blood flow, IOP, and glaucoma damage.
Overall, the data provided by this study offers additional support for the use of the CLS as a 24-h assessment tool for IOP-related patterns and highlights that measurements derived by the CLS may be of practical use for detection of sleep-induced IOP changes. The CLS also is able to detect ocular pulsations with good accuracy in a majority of eyes.