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Editors Selection IGR 9-4

Risk Factors: Limbal Changes during Face-down Sleep

Comment by Gustavo de Moraes on:

66544 Measured Changes in Limbal Strain During Simulated Sleep in Face Down Position Using an Instrumented Contact Lens in Healthy Adults and Adults With Glaucoma, Flatau A; Solano F; Idrees S et al., JAMA ophthalmology, 2016; 0: 1-8

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Flatau et al. described an interventional trial in which they measured changes in the corneo-scleral junction (herewith called limbal strain changes) in control and glaucoma eyes as a result of mechanical forces applied to the eye when the face of participants rested against a pillow (face-down (FD) position). They used a contact-lens sensor (Triggerfish, Sensimed) that has been previously validated as a tool to indirectly measure volumetric changes in the anterior chamber known to be correlated with intraocular pressure (IOP). The authors also applied a mathematical model derived from experimental studies to determine the relationship between strain change and IOP given pre-defined baseline IOP values. They found that contact with a pillow in FD position during simulated sleep produced a sustained strain increase in glaucoma eyes but not in controls. Of note, the mean FD change in glaucoma eyes was equivalent to strain increase associated with a mean sustained IOP elevation of 2.5 mmHg. More interestingly, a sub-analysis of glaucoma patients with at least five visual field tests prior to the experiment revealed that these strain changes were more meaningful in eyes with previous progression.

This is an original study on a clinically-relevant topic and may result in practical applications. Assuming that limbal strain changes indeed reflect IOP elevation (which is supported by the existing literature),^1-3 glaucoma patients may experience detrimental IOP elevation during sleep. Given the amount of time people spend sleeping during their lives, such IOP elevation could lead to a substantial amount of energy applied to the axons in optic nerve head during and individual's lifetime.

Another important conclusion is that the study supports the premise that the biomechanical properties of the eye differ between glaucoma and healthy subjects. This could help explain how different tissues respond to mechanical stress from IOP and why some patients are more susceptible to IOP changes and progressive glaucomatous damage.

The study methodology is clearly described and allow replication of the study. Moreover, the authors should be congratulated for the thorough discussion section, in which they provide supporting literature to explain their findings and speculate - with reasonable basis - on potential clinical implications. In particular, the role of the facial bone structure and the depth of the orbit warrants investigation as to how they could intensify or minimize the present findings. As the authors suggested, preventing mechanisms (such as protective eye shield) should be tested and warrant further investigation.

The study's main limitation is that we cannot assume that the changes reported during the experiment are sustained during the average eight-hour sleep period. The eye is not a closed system. It is possible, for instance, that after some time the eye's outflow facility undergoes changes and the true IOP could return to normal. Notwithstanding this possibility, studies showing visual field asymmetry associated with preferred sleep position^4,5 support the authors' hypothesis. Future studies ought to investigate whether interventions with a protective eye shield during sleep can benefit glaucoma patients.

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