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Measuring Rod and Cone Dynamics in Age-Related Maculopathy

¹From the Centre for Eye Research Australia, Department of Ophthalmology, The University of Melbourne, East Melbourne, Victoria, Australia; the ²School of Optometry and the ³Institute of Health and Biomedical Innovation, Queensland University of Technology, Brisbane, Australia; and the ⁴Department of Optometry and Vision Sciences, The University of Melbourne, Parkville, Victoria, Australia.

PURPOSE. A cathode-ray-tube (CRT) monitor–based technique was used to isolate clinically significant components of dark adaptation. The utility of the technique in identifying adaptation abnormalities in eyes with age-related maculopathy (ARM) is described.

METHODS. A CRT dark adaptometer was developed to assess cone and rod recovery after photopigment bleach. The following measures were obtained: cone recovery rate (R_c; in decades per minute) and absolute threshold (Tf_c; log candelas per square meter), rod recovery rate (R_r; decades per minute), and rod–cone transition (rod–cone break [RCB], in minutes). These components were isolated by appropriately selecting stimulus size, stimulus location, pigment bleach, and test duration and by coupling the CRT with judiciously selected neutral-density (ND) filters. The protocol was developed by using 5 young observers and was tested on 27 subjects with ARM in the study eye and 22 age-matched control subjects.

RESULTS. The parameters necessary for effective isolation of cone and early phase rod dark adaptation were a 2.6 ND filter (for a standard CRT monitor, 0.08–80 cd · m^–2 luminance output); a 4° foveated, 200-ms, achromatic spot; 30% pigment bleaching; and a 30-minute test duration. These settings returned obvious rod and cone recovery curves in control and ARM eyes that were compatible with conventional test methods and identified 93% of participants with ARM as having delayed dynamics in at least one of the parameters. Cone recovery dynamics were significantly slower in the ARM group when compared with age-matched control subjects (R_c, 0.99 ± 0.35 vs. 2.63 ± 0.61 decades · min^–1, P < 0.0001). Three of the 27 eyes with ARM did not achieve RCB during the allowed duration (30 minutes). The remaining eyes with ARM (n = 24) exhibited a significant delay in rod recovery (R_r, ARM, 0.16 ± 0.03 vs. controls, 0.22 ± 0.02 decades · min^–1, P < 0.0001) and the average time to RCB (±SD) in the ARM group was significantly longer than in the control subjects (19.12 ± 5.17 minutes vs. 10.40 ± 2.49 minutes, P < 0.0001).

CONCLUSIONS. The CRT dark-adaptation technique described in this article is an effective test for identifying abnormalities in cone and rod recovery. Slowed cone and rod recovery and a delayed RCB were evident in the eyes with ARM. The test method is potentially useful for clinical intervention trials in which ARM progression is monitored.