Tablet-based 'ON/OFF' pathway test can distinguish between rod- and cone-dominated diseases

The photopic ON pathway defect is associated with nocturnal vision loss. However, the measurement of ON function to detect a rod-dominated disease (rods affected more than cones) has not been explored. We evaluated whether the psychophysical evaluation of ON/OFF pathways can be used to distinguish cone-dominated from rod-dominated diseases.

Thirty-seven patients with inherited retinal diseases were tested using the 'EyeSpeed' [iOS application] on an iPad. The test displayed a random number (1-3) of light or dark targets on a black-and-white noise background. Participants responded on a touch screen indicating the correct number of targets displayed. The outcome variables-reaction time, accuracy and performance index (speed [1/reaction time] * accuracy) to both light and dark targets were assessed for diagnostic ability using standard receiver-operating characteristic (ROC) analysis.

Mean ± standard deviation age and visual acuity for the cone- and rod-dominated groups were 25.15 ± 11.74 years, 0.80 ± 0.25 logMAR and 28.3 ± 14.29 years, 0.48 ± 0.26 logMAR, respectively. The median reaction time to light targets in rod-dominated disease [interquartile range] was 5.28 s [3.17], significantly greater than for patients with cone-dominated disease (2.07 s [0.93]; Mann-Whitney U test, p < 0.001). Amongst all of the outcome variables evaluated, the reaction time to light targets (criterion of ≥2.98 s) exhibited the highest area under the ROC curve (area = 0.89 ± 0.11; p < 0.001), with a sensitivity and specificity of 82.4% and 85% respectively.

Reaction time to light targets using the ON/OFF pathway paradigm is a valid marker to differentiate between rod- and cone-dominated retinal dystrophies. ON pathway function measured using a tablet-based test could act as a supplemental test in the diagnosis of challenging photoreceptor-specific inherited retinal diseases.