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Comment by Kaweh Mansouri on:

68907 Validity of the Monocular Trial of Intraocular Pressure-Lowering at Different Time Points in Patients Starting Topical Glaucoma Medication, King AJ; Rotchford AP, JAMA ophthalmology, 2016; 134: 742-747

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Current tonometry techniques provide single time-point measurements and fail to reflectthe true range of an individual's 24-h IOP behavior. This fact is of practical consequence as itimpinges on the ability to distinguish between spontaneous IOP changes and the effect of IOP-lowering medications.

The hypothesis that a medication's therapeutic effect can be distinguished from simple IOP fluctuationas the difference between IOP in the treated vs. the fellow eye has been discussed for decades. This so-called monocular trial remains contested and, in recent years, several studies have provided arguments against it.

For the monocular trial to be valid, two basic assumptions need to be fulfilled: 1) IOP patterns should be repeatable and 2) there should be a certain degree of between-eye symmetry. Realini et al.¹ found that diurnal IOP was not reproducible from one week to another. However, their study only used two 12-hour diurnal IOP assessment sessions, one week apart. Studies using continuous 24-h monitoring of IOP-related patterns with a contact lens sensor (CLS) have found good reproducibility from one week to another.² Regarding inter-eye symmetry, studies show that IOP may fluctuate moderately in parallel in healthy fellow eyes whereas IOP peaks may not appear at the same time for an individual subject. The concordance of diurnal IOP between fellow eyes with glaucoma was evaluated in one study which found a 68% to 90% probability for the absolute change in IOP between fellow eyes to be within 2 mmHg and 78% to 95% within 3 mmHg. Others, however, previously reported a weaker relationship, with correlations between fellow eyes ranging between 0.65 and 0.73 (mean r = 0.70) under various conditions. Asymmetry of IOP in fellow eyes, however, undermines the validity of inferring IOP changes in one eye based on measurements from the fellow eye. Therefore, and due to conflicting data on inter-eye symmetry,the monocular drug trial has largely fallen out of fashion in recent years. Our group, however, recently conducted a trial using bilateral simultaneous 24-h IOP-monitoring with the CLS and found food inter-eye correlations in glaucoma eyes (r = 0.76). (Unpublished data).

In the present study, King and Rotchford attempted to re-examine the validity of the monoculartrial by designing a prospective study with three diurnal IOP measurements and three pre-treatment and three post-treatment visits to overcome the well-known phenomenon of regressionto the mean. Their main results can be summarized as:

An important decrease in IOP as a result of regression to the mean was observed and ranged from 2.1 to 4.6 mmHg. They conclude that the normal practice of comparing one baseline visit (instead of three in this study) with one post-treatment visit can overestimate the treatment effect by as much as 36%.

Magnitude of IOP-reduction in fellow eye was similar to the trial eye, meaning that if an effect is observed in the first eye, physicians can expect a similar effect in the second eye.This is an interesting study and the authors are to be congratulated for addressing this complex question. Whether their results can be extrapolated to other medication classes (travoprost drops were used) and patient populations need to be evaluated. Ultimately, however, clinicians aspire to the routine use of 24-h IOP monitoring technologies in glaucoma patients, obviating shortcomings of current IOP assessment methods.

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