Assumptions of the Monocular Trial
There are several underlying assumptions of the monocular trial which must be validated prior to accepting or rejecting this method as a useful tool in clinical practice.3,4
Assumption 1. IOP response to topical hypotensive medications (therapeutic effects) is similar in both eyes.
This assumption was recently supported by a retrospective study by Realini et al (Ophthalmology. 2005;112:599-602) who report a strong correlation (r=0.84) between eyes undergoing bilateral simultaneous treatment with topical hypotensive medications. The 43 patients with bilateral glaucoma or ocular hypertension had an IOP-lowering response of 5.0 mm Hg (26.9%) and 4.3 mm Hg (23.7%) in the right and left eyes respectively, suggesting that medication-lowering effects are similar between eyes.
Data from the OHTS also support this assumption. There was a strong correlation of IOP between both eyes at a baseline visit prior to treatment (r=0.78) (unpublished data). The correlation between eyes remained strong (r=0.78) after bilateral treatment with at least a 20% reduction in IOP 5 months later. These remarkably similar correlations pre-and post-treatment suggest a similar IOP-lowering response of both eyes to the medications, thus further supporting this assumption.
Assumption 2. Nontherapeutic effects on IOP are similar in both eyes.
Nontherapeutic influences on the monocular trial can be both intrinsic and extrinsic. Examples of intrinsic effects of the eye include diurnal fluctuations of IOP during separate visits and asymmetric diurnal fluctuations of IOP between eyes during the same visit. Extrinsic factors influencing IOP may include measurement error (observer and instrument), body position, fluid intake, caffeine or alcohol intake, Valsalva, and other factors.
The monocular trial was designed to control for nontherapeutic influences by using the untreated eye as a control. The assumption that nontherapeutic influences are symmetrical between the two eyes is supported by an earlier study which found fairly symmetric IOPs between eyes over a 24-hour period in normal, ocular hypertensive, and glaucoma patients (Am J Ophthalmol. 1975;79:557-56). More recently, Sit et al showed a lack of a significant difference in mean, peak, or trough IOPs in fellow eyes in the sitting and supine positions (Ophthalmology. 2006;113:425-430).
The argument against this assumption arises from an earlier study which suggests dissimilar shapes of diurnal curves for fellow eyes (Acta Ophthalmol. 1964;78:1-131). Dissimilar diurnal curves between fellow eyes was found in 36% of glaucoma and 6% of normal patients in a study by Wilensky et al (Ophthalmology. 1993;100:940-944). In a more recent study, spontaneous IOP fluctuations between eyes were found to occur in 16% of visits (1 in 6 visits) for glaucoma patients (Ophthalmology. 2002;109:1367-1371). Supporting and dissenting studies make it unclear whether this assumption is true.
Assumption 3. Treatment in one eye will not affect IOP in the untreated eye.
This assumption was proven false after Zimmerman and Kaufman introduced the concept that unilaterally instilled topical beta-blockers lower IOP in the fellow eye through systemic absorption (Arch Ophthalmol. 1977;95:601-4). This contralateral IOP-lowering effect of topical beta-blockers was found to be 1.5 mm Hg or 5.8% of the treated IOP response in the OHTS (Am J Ophthalmol. 2000;130:441-453). The magnitude of the effect was correlated with the magnitude of IOP reduction in the treated eye as well as the baseline IOP in the untreated eye.
In an attempt to minimize contralateral effects, prostaglandins have been studied for their assumed minimal crossover effect. Realini et al analyzed a subset of patients on latanoprost and still found a poor correlation (r=0.16) in IOP-lowering response between eyes (Ophthalmology. 2004;111:421-6). This poor correlation therefore is likely due to nontherapeutic influences. The OHTS is currently analyzing trials using prostanglandin analogues.
Is the Monocular Trial Useful In Clinical Practice?
Predicting the accuracy of the monocular trial may not be as straightforward as once believed. There is a strong correlation between fellow eyes in their response to IOP-lowering medications: if one eye responds, the other eye will likely respond in kind. The problem lies in determining whether the IOP response of the first eye after treatment is attributable to therapeutic or nontherapeutic factors.
Recommendations have been made to measure several pre- and post-treatment IOP readings to better understand an individual’s IOP range and diurnal variability.3,12,13 Although ideal in theory, this may not be practical. Additional readings would require multiple diurnal curves resulting in increased time and economic burden, compromise to follow-up, decreased patient compliance, and prolonged delay until treatment. Additional methods to reduce nontherapeutic influences include measuring IOP at several times to reduce measurement error, ensuring proper patient positioning, and scheduling follow-up visits at a consistent time of day.
Future studies and modifications of this therapeutic trial are likely necessary to increase the accuracy of our treatment decisions. Analysis using data from eyes with similar baseline IOPs or comparisons of diurnal variation between eyes in the untreated control group in the OHTS may provide further insight. When using the monocular trial in clinical practice, one should keep in mind the nontherapeutic influences which may affect the results. Future studies may clarify the validity of the monocular trial.