Professors Liu and Weinreb at the University of California, San Diego have reported the results of an analysis of fellow-eye symmetry of 24-hour IOP curves in three groups: healthy younger people, healthy older people, and untreated glaucomatous older people. In general, the two healthy groups exhibited reasonable symmetry of mean 24-hour IOP, timing of peak IOP, and amplitude of IOP, while the glaucoma group exhibited substantially less symmetry of these parameters. Asymmetry of IOP in fellow glaucomatous eyes limits the validity of inferring IOP behavior in one eye based on observations in the fellow eye.
This result critically undermines the validity of the monocular therapeutic drug trialThere are a number of important clinical implications of this finding. Firstly, this result critically undermines the validity of the monocular therapeutic drug trial, in which one eye is treated and the fellow eye left untreated in an attempt to distinguish between therapeutic and spontaneous IOP change. The monocular trial requires symmetric IOP variation between fellow eyes in order for the untreated fellow eye to serve as a control for the treated eye. The monocular drug trial has largely fallen out of fashion, although efforts have been made to restore its utility by considering diurnal or even 24-hour IOP curves before and after treatment. The current study demonstrates that even these modifications of the monocular drug trial are unlikely to improve its performance. Secondly, and more timely and relevant, is the significance of this finding in the era of continuous 24-hour IOP monitoring. Triggerfish contact-lens based 24-hour IOP assessment is technically noninvasive but is far from user-friendly: an uncomfortable contact lens, an unsightly antenna taped to the face around the eye, a cord attaching it to a recorder worn on a belt or in a pocket and fraught with risk for entanglement during sleep. There will be great temptation to conduct this test unilaterally and make the assumption that the output represents IOP behavior bilaterally. Based on the results of this study, such a temptation should be resisted.