Abstract
Purpose: :
Tear instability is considered a core mechanism of dry eye. Recently, visual disturbance has been added to the definition (DEWS report 2007) of dry eye, but the exact relationship between the two remains unclear. The purpose of this study was to develop a method to simultaneously measure tear instability, and the corresponding optical image quality and visual disturbance.
Methods: :
A COAS-HD Shack-Hartmann wavefront aberrometer was custom-fitted with a cobalt blue LED apparatus and internal yellow filter for fluorescein image capture, and an internal high-resolution video display for contrast stimulus presentation. Repeated measurements were acquired on each eye of ten dry eye (< 8 sec TBU time) subjects at baseline, and 10, 30, 60, 90, and 180 minutes following instillation of an artificial tear drop. Each measurement consisted of repeated optical and fluorescein image captures every 1.4 seconds for 18 seconds, while subjects monitored threshold contrast sensitivity (CS) by holding the eye open as long as possible. Two microliters of 2% fluorescein was instilled into each eye prior to a measurement series. Tear break-up was quantified using custom software which evaluated image pixel intensity. The RMS fit error (RMSfe) was calculated from each optical measurement. Eyes were classified into groups by a trained clinician observer based on the presence or absence of tear break-up (TBU) in front of the pupil (ave 3.1mm).
Results: :
During the 18 second trial, the group with minimal TBU in front of the pupil experienced an average±std change in TBU of 18.7±20.5%, a drop in CS of 2.6±3.0, and an increase in RMSfe of 129.3±291.0 microradians. The group with clinician visible TBU in front of the pupil experienced an average±std TBU of 31.5±12.9%, a drop in CS of 5.8±2.6, and an increase in RMSfe of 1085.8±453.8 microradians.
Conclusions: :
Short-term fluctuations in tear film quality over the pupil were associated with a measurable decrease in optical image and visual quality. These novel methods, which allow a simultaneous measurement of fluorescein tear break-up, optics and vision, suggest that visual disturbances in dry eye are most likely to occur when tear instability is centrally located over the pupil.
Keywords: optical properties • aberrations • contrast sensitivity