Developing optimal test characteristics for measuring dark adaptation in people with intermediate age-related macular degeneration (iAMD)

Session Details

Session Title: Free Paper Session 22: AMD V

Session Date/Time: Sunday 10/09/2017 | 08:00-09:30

Paper Time: 08:00

Venue: Room 115

First Author: : L.Edwards UK

Co Author(s): :    D. Taylor   D. Crabb   A. Binns                       

Abstract Details

Purpose:

Dark adaptation is a candidate functional biomarker for AMD. AdaptDx (Maculogix, Pennsylvania, USA) measures the time taken for threshold to reach a criterion level (the rod intercept time; RIT) after a substantial photopigment bleach. The sensitivity of the test to AMD has been established, however it has also been shown that the RIT in iAMD can be >45 minutes. The aim was to compare the RIT elicited under different test conditions in people with iAMD in order to determine the optimal clinical protocol for longitudinal evaluation, which would provide a valid RIT within a clinically viable timeframe (<20 mins).

Setting:

This exploratory cross-sectional study was carried out at City, University of London. Participants with iAMD in the test eye (large drusen >125 µm and/or any AMD pigmentary abnormalities) were recruited through a local database of volunteers with AMD, and through advertisement in the Macular Society members’ magazine.

Methods:

Eligible participants had iAMD in at least one eye, in the absence of other retinal pathology, and a visual acuity of 0.3 logMAR or better in the test eye. The test eye was either the eye with iAMD, or was randomly selected in the case of bilateral disease. All participants attended for 2 visits, over which they performed 5 tests (in randomised order). The photopigment bleach was produced by a brief flash (505nm) subtending 4 degrees, centred at either 5 or 12 degrees on the inferior vertical meridian. The effective bleaching levels employed were 65%, 70% and 76% (5 degrees) and 70% and 76% (12 degrees). The test stimulus comprised a 2 degrees diameter, 500nm circular target. Threshold was estimated using a 3-down/1-up modified staircase estimate procedure, beginning 15 seconds after the bleaching flash and continuing at 30-second intervals for 30 minutes or until the rod intercept was reached. A 30 minute washout period was allowed between bleach conditions. Optical coherence tomography and fundus photography were performed at the end of the visit to allow fundus grading.

Results:

Twelve individuals with iAMD were recruited. The mean age of participants was 70.5 years (±9.1), mean visual acuity was 0.17 logMAR (±0.21). One participant was excluded due to failure to provide reliable data (fixation error exceeded 33% for all tests). As expected, the RIT increased with increasing bleach intensity at both locations. For 6 participants, it was not possible to calculate the RIT for the 65% bleach at 5 degrees due to insufficient reduction in threshold post bleach. RIT exceeded 20 mins for 2 participants each at the 70% and 76% bleach (5 degrees) and for no participants at the 12 degree location at either bleach intensity. Participants tended to show a more rapid recovery at the 12 degree location than the 5 degree location for a comparable bleach level.

Conclusions:

This exploratory study has indicated that reliable data may be obtained from participants at bleach intensities of 70% or higher. Whilst the same number of participants exceeded the 20 minute desirable test limit for the 70% and the 76% bleach levels, the mean RIT was lower for the 70% bleach at both locations, indicating that this would be a more expedient clinical test parameter for evaluation in a prospective study. The finding that recovery times were faster at the 12 degree location is unsurprising for this group as it falls outside the region of maximal abnormality in AMD, and may suggest a more desirable test location with respect to testing time. A further study will be conducted to determine whether the delay in adaptation at the more peripheral location is sufficient to discriminate between patients with iAMD, early AMD and macular normal controls.

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