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Original Investigation |

Measured Changes in Limbal Strain During Simulated Sleep in Face Down Position Using an Instrumented Contact Lens in Healthy Adults and Adults With Glaucoma

Alison Flatau, PhD1,2; Francisco Solano, MD3; Sana Idrees, MD3; Joan L. Jefferys, ScM3; Peter Volpe, MS1; Christopher Damion1; Harry A. Quigley, MD3
[+] Author Affiliations
1Department of Aerospace Engineering, University of Maryland, College Park
2The Fischell Department of Bioengineering, University of Maryland, College Park
3Glaucoma Center of Excellence, Wilmer Eye Institute, The Johns Hopkins University School of Medicine, Baltimore, Maryland
JAMA Ophthalmol. 2016;134(4):375-382. doi:10.1001/jamaophthalmol.2015.5667.
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Published online

Importance  Eyes of patients with glaucoma may be damaged during sleep.

Objective  To measure strains in glaucoma eyes and control eyes produced by mechanical force or deformation of the eye from contact when one side of the face rests against a pillow.

Design, Setting, and Participants  This study took place in a clinic-based setting among 22 patients with glaucoma and 11 age-matched controls. The research was conducted at Wilmer Eye Institute between February 4, 2014, and December 2, 2014. Data analysis was done from June 3, 2014, to June 30, 2015.

Main Outcomes and Measures  We used a contact lens sensor (CLS) to measure change in limbal strain associated with placing one side of the face down (FD) on a pillow in simulated sleep. Baseline intraocular pressure (IOP) was measured with a tonometer. The CLS data were collected every 5 minutes during intervals of up to 60 minutes in various positions, including sitting, lateral decubitus, FD (with the CLS-instrumented eye toward the pillow), and supine. Measured changes in limbal strain were related to estimated changes in IOP and to modeled strain produced by changes in IOP.

Results  Among 22 patients with glaucoma and 11 controls, 17 were female. The mean age for the glaucoma group was 62.6 years, while the mean age for the control group was 61.4 years (P = .68). Baseline IOP was also similar for the 2 groups. The mean IOP sitting at the start was 13.7 mm Hg for the glaucoma group and 13.8 mm Hg for the control group (P = .73), and the mean IOP lying at the start was 17.5 mm Hg for the glaucoma group and 16.0 mm Hg for the control group (P = .88). By multivariable linear regression, FD position was associated with an increase in limbal strain in glaucoma eyes (mean [SE], 44.1 [20.4] mV Eq; P = .03) but not in control eyes (mean [SE], 13.6 [13.9] mV Eq, P = .33). While FD, the increased CLS values in patients with glaucoma did not decrease over time (slope, 0.275 mV Eq/min; P = .53 by univariable linear regression). Magnitudes of measured changes in limbal strain were greater in glaucoma eyes with past visual field worsening (P = .006 by multivariable linear modeling). The mean limbal strain increase among patients with glaucoma in FD position was equivalent to strain expected for a mean (SE) IOP increase of 2.5 (1.1) mm Hg from a baseline IOP of 14.2 mm Hg.

Conclusions and Relevance  Contact with a pillow in FD position during simulated sleep produced a sustained strain increase in glaucoma eyes, particularly those eyes with past progressive visual field loss. The mean FD change in glaucoma eyes was equivalent to strain increase associated with a mean (SE) sustained IOP elevation of 2.5 (1.1) mm Hg. Further experiments are planned to determine if facial features or a protective eye shield prevents sleep position–induced limbal strains during a mean 8-hour sleep period.

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Figure 1.
Examples of Contact Lens Sensor (CLS) Values vs Time During Testing

The CLS data from 2 patients with glaucoma are shown. The blue line represents SENSIMED AG algorithm median values, while the black line represents the mean values. Solid circles and triangles show the corresponding 30-second interval median and mean CLS values. The colored shading indicates patient body position. The intraocular pressure levels near the top of each graph are tonometer data from the start and end of the protocol in sitting and lying positions.

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Figure 2.
Change in Contact Lens Sensor (CLS) Output by Position for All Study Participants and by Patient Group

The mean CLS values are estimated from generalized estimating equation models for the 5 body positions tested, including lateral decubitus (lying-at-start [LS]), FD1, supine (relax-1 [R1]), FD2, and supine (relax-2 [R2]) for durations indicated in the Methods section. The mean (SE) values are shown. The trend lines were generated by the generalized estimating equation linear regression model when the 5 body positions are sequentially assigned values 1 through 5. FD indicates face down.

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Figure 3.
Contact Lens Sensor (CLS) Values During FD1 in Patients With Glaucoma

The linear regression line slope from the generalized estimating equation model is 0.275 mV Eq/min (P = .53). The values plotted represent change in CLS from the mean CLS lying-at-start (LS) to standardize across participants. FD indicates face down.

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Figure 4.
Estimation Nomogram Relating Modeled and Measured Changes in Limbal Strain and in Intraocular Pressure (IOP)

Solid lines show the modeled limbal strain vs change in IOP for known initial values of IOP. The right axis shows contact lens sensor (CLS) values in patients with glaucoma when moving to and from face down (FD) position (the mean ±1 SE values are shown as dashed and dotted orange lines, respectively). The right axis shows that the mean (SE) CLS value of 40.9 (18.0) mV Eq in glaucoma eyes intersects the initial IOP equals 14.2 mm Hg line at the blue diamonds, where the mean (SE) IOP increase is 2.5 (1.1) mm Hg, which in turn corresponds to the mean (SE) modeled strain increase (on the left axis) of 283 (116) microstrain.

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