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Editors Selection IGR 11-3
Intraocular Pressure: 24-hour IOP measurements
Comment by George Lambrou on:
51806 Analysis of continuous 24-hour intraocular pressure patterns in glaucoma, Mansouri K; Liu JH; Weinreb RN et al., Investigative Ophthalmology and Visual Science, 2012; 53: 8050-8056
The Goldmann applanation tonometer is a wonderful instrument: elegant and ubiquitous, it has permitted the diagnosis of millions of patients with glaucoma and the development of effective medication to prevent or delay vision loss in many among them. Yet its use is restricted to the ocular examination room, on a sitting subject, which renders night IOP-recording impracticable and casts doubts on the reliability of measurements performed on patients aroused from sleep and rising from a supine to a sitting position. This, in turn, has resulted in significant controversies on the role of circadian IOP fluctuations on disease development and progression, and many of us have dreamt of a reliable 24-hour 'IOP-Holter' to better assess our patients and optimize their treatment.
Can it be that this dream is now coming true? Two years after the first clinical reports of 24-hour IOP monitoring in glaucoma, Mansouri and co-workers are going one step further, describing a mathematical modeling of 24-hour IOP curves obtained from 40 glaucoma patients or suspects using a telemetry device based on a Contact Lens Sensor which can be used in an ambulatory environment including undisturbed sleep. Ten-Hz measurements were averaged over 30-second recording periods repeated every five minutes, resulting in 288 data points over 24 hours. Each patient underwent two 24-hour recording sessions, one week apart. Complete data could be obtained from 35 out of the 40 enrolled patients, 23 of whom were under IOPlowering medications.
The circadian IOP-patterns were modeled using 'cosinor rhythmometry' and specific parameters were estimated for each recorded session: the amplitude, mean and acrophase (time of peak value). In most patients (between 23 and 26 in each session) the acrophase (IOP peak) occurred during sleep (group acrophase was between 1:45 and 2:00 am). A small number of patients (two to three in each session) showed a diurnal peak value, while for the remaining patients it was not possible to extract a reliable peak time, because of 'flat' or nonmodelable curves. This is comparable to previously published data (Mansouri and Sharawi, 2012), where 69% of the patients examined with the same device had their peak IOPs during the night.
The methodology used in this study is still novel and has some drawbacks: it provides data that, while strongly correlated to IOP values as previously shown, are difficult to calibrate not only among eyes but also between sessions and must therefore be expressed in arbitrary units. The reproducibility was also somewhat inconclusive, as in 17% of the patients the acrophases (diurnal/nocturnal) did not match between sessions and it was unclear whether this reflected true IOP-pattern variability or device-related issues. The approach, however, seems highly feasible technically (this is the second published patient cohort) and has some obvious strengths, including the possibility to
Continuous, 24-hour IOP-monitoring is an immense opportunity to better diagnose glaucoma and individualize treatment
monitor patients in their natural habitat with unrestricted activities and undisturbed sleep, as well as the non-reliance on activity diaries (sleep times can be determined through characteristic signs on the IOP curve). When the technique is further validated, it will be an extremely useful tool to understand the prognostic significance of circadian IOP variations and an immense opportunity to better diagnose glaucoma and to optimize its management, one step nearer to individualized treatment.
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