Is critical flicker‐fusion frequency a valid measure of visual fatigue? A post‐hoc analysis of a double‐masked randomised controlled trial

PURPOSE: Critical flicker‐fusion frequency (CFF) has been used in clinical studies as a measure of visual fatigue. We examine the correlation between CFF and subjective reports of visual fatigue in a group of symptomatic computer users, to consider whether CFF may be used as a surrogate measure of visual fatigue symptoms. METHODS: We analysed data from a previous randomised controlled trial. One hundred and twenty adults, diagnosed with computer vision syndrome, had CFF and visual fatigue symptoms quantified before and after a visually demanding 2‐h computer task. Symptoms were assessed using a questionnaire with nine subcomponents that summed to a total score of 900. CFF was measured using a two‐interval forced‐choice method, with the flicker rate altered by a computer‐controlled staircase procedure. For our primary analysis, we determined Spearman correlation coefficients between post‐task symptom scores and CFF, and between change from baseline symptom scores and CFF. We also used a bootstrap procedure to consider whether symptom score subcomponents were significantly (Bonferroni‐corrected) different from overall scores with regard to their correlations with CFF. RESULTS: Although visual fatigue symptom scores altered significantly post‐task (mean change: 92 units; 95% confidence interval [CI]: 11 to 122), CFF did not (mean change −0.7 Hz; 95% CI: −1.7 to 0.3). There was no significant correlation between overall symptom scores and CFF, either for the post‐task (r = −0.13; 95% CI: −0.31 to 0.05) or the change from baseline (r = −0.18; 95% CI: −0.35 to 0.01) analysis. Subcomponents of the symptom questionnaire did not show a significant correlation with CFF, either for the post‐task or the change from baseline analysis. CONCLUSIONS: We find that CFF is not a useful surrogate for symptoms of visual fatigue, given its low correlation with scores on a visual fatigue symptom questionnaire.