A Comparison of the Medmont Dark Adapted Chromatic Perimeter (DAC) with the Full-Field Stimulus Threshold (FST) in Subjects with Retinitis Pigmentosa (RP)

Abstract

Purpose : Many retinal degenerative diseases initially involve the rod photoreceptors, making rod-mediated vision an attractive outcome measure for treatment trials. However, previously established rod measurements yielded either a single summed electrical response from all rods (full-field ERG) or a single psychophysical threshold response from, presumably, the most sensitive area of the retina (Full-Field Stimulus Threshold, FST - 1. Roman A, et al. Physiol. Meas. 2007; 2. Klein M., Birch D.G. Doc. Ophthalmol. 2009; 3. Messias K, et al. Doc. Ophthalmol. 2013). The Dark-Adapted Chromatic (DAC) perimeter (Medmont Int PTY LTD; Nunawading, Australia) is a new LED driven, static visual field covering a wide area over a large dynamic range (75 dB). Here we compared DAC measurements to the FST.

Methods : Thirteen subjects with RP were dilated in the eye with worse acuity and dark-adapted for 45 minutes. Seven normal subjects served as controls. Subjects were tested first with the DAC and then with the FST. The DAC presented 505 nm stimuli (0 dB = 0.4 log cd.s/m²; range bright to dim: 0 dB – 75 dB) over an area of 144 degrees horizontally and 72 degrees vertically. Total threshold was the sum of the inverse sensitivity measures from 164 locations. Minimum threshold was the lowest threshold of a single spot. The FST was performed as described previously² (0 dB = -1.0 log cd.s/m²; range dim to bright: -75 dB to 30 dB) yielding a single threshold. The DAC and FST results are presented as the log threshold elevation from the normal average (DAC current, FST published²).

Results : Total and minimal threshold elevations were strongly correlated with each other (r=0.93; p<0.001) and with FST elevations (r=0.89, p<0.001, r=0.90, p<0.001, respectively). However, the Bland-Altman test revealed a 0.56 log unit bias toward smaller FST elevations when compared to the minimum threshold elevation.

Conclusions : The DAC is a useful tool for testing location-based rod-mediated vision, however, the more sensitive FST may be more useful when testing very advanced subjects. The fact that both the DAC minimum and total threshold elevation correlated well with the FST elevation suggest that the FST may not be determined by the most sensitive spot in the retina as previously assumed, but could be driven by summation similar to the ERG.

This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.