Cross-modal Neuromodulation to Improve Balance and Cognition in Older Adults With Glaucoma: A Randomized Trial of Cerebellar Transcranial Electrical Stimulation

Previous research on tES has primarily focused on the effects of different brain cortex stimulation on balance and visual cognitive functions in healthy adults. However, there is a scarcity of research examining the combined effects of these techniques on the cerebellum in visually impaired adults, especially those with glaucoma. Given the cerebellum's crucial role in maintaining balance and cognitive function in glaucoma patients, as evidenced by the latest research, it is essential to determine whether non-invasive cerebellar stimulation can simultaneously improve balance and visual cognitive functions in these patients. Meanwhile, most previous studies only focused on the short-term effects of stimulation, while potential long-term benefits remained unexplored. Our study aims to investigate the long-term effects of two types of tES stimulation on the cerebellum in adults with glaucoma. This research not only aims to fill existing gaps in the literature but also has the potential to develop new intervention strategies to improve functioning in older adults, ultimately enhancing their quality of life. This study employs a within-subject, sham-controlled, cross-over design. The participants will receive constant tDCS, tACS, and sham stimulation in counterbalanced order. The participants will have three sessions (each session will include five consecutive training sessions) with outcome measurements at six time points. These assessments consist of a baseline test (T1), a test after completing the first session (T2), a test after the first interval week (T3), a test after completing the second session (T4), a test after the second interval week (T5), and a test after completing the third session (T6). The total time will be five weeks. The outcome measurements will include dynamic balance tests: narrow path walking test and miniBEST test; static balance test: Sensory Organization Test; and two visual cognitive function tests: Reaction Time Test and the Useful Field of View test. For statistical analysis, IBM SPSS Statistics version 23.0 will be utilized. The Shapiro-Wilk test will be employed to assess the normality of the data distribution. For skewed numerical data, the appropriate transformations will be used to make the data meet the normality assumption. The differences in baseline performance among tACS, tDCS, and sham stimulation were analyzed using repeated-measures ANOVA. To compare the differences between changes in primary and secondary outcomes among baseline and post-intervention time points, the repeated-measures ANOVA will be performed with interventions (tACS, tDCS, and sham) as a within-group factor. Changes in the values will be calculated by subtracting baseline values from the post-intervention values. Paired t-statistics will be obtained to compare the changes in outcome data (subtract baseline from post-intervention) between the interventions (tACS versus tDCS, tACS versus sham, and tDCS versus sham). Post-hoc pairwise comparisons will be performed using Bonferroni corrections. The level of statistical significance is set at p \< 0.05.