Sleep as a predictor of tDCS and language therapy outcomes

Study Objectives: To determine whether sleep at baseline (before therapy) predicted improvements in language following either language therapy alone or coupled with transcranial direct current stimulation (tDCS) in individuals with primary progressive aphasia (PPA). Methods: Twenty-three participants with PPA (mean age 68.13 ± 6.21) received written naming/spelling therapy coupled with either anodal tDCS over the left inferior frontal gyrus (IFG) or sham condition in a crossover, sham-controlled, double-blind design (ClinicalTrials.gov identifier: NCT02606422). The outcome measure was percent of letters spelled correctly for trained and untrained words retrieved in a naming/spelling task. Given its particular importance as a sleep parameter in older adults, we calculated sleep efficiency (total sleep time/time in bed x100) based on subjective responses on the Pittsburgh Sleep Quality Index (PSQI). We grouped individuals based on a median split: high versus low sleep efficiency. Results: Participants with high sleep efficiency benefited more from written naming/spelling therapy than participants with low sleep efficiency in learning therapy materials (trained words). There was no effect of sleep efficiency in generalization of therapy materials to untrained words. Among participants with high sleep efficiency, those who received tDCS benefitted more from therapy than those who received sham condition. There was no additional benefit from tDCS in participants with low sleep efficiency. Conclusion: Sleep efficiency modified the effects of language therapy and tDCS on language in participants with PPA. These results suggest sleep is a determinant of neuromodulation effects. Statement of Significance Sleep plays a pivotal role in cognitive functioning. Given this notion, and the prevalence of sleep disorders in neurodegenerative diseases, we investigated whether baseline sleep efficiency (before therapy) was a predictor of transcranial direct current stimulation (tDCS) therapy and language outcomes in primary progressive aphasia (PPA), a neurodegenerative disorder primarily affecting language. Our data reveals sleep efficiency as a positive predictor of both tDCS and language therapy outcomes (i.e. better sleep resulted in more significant tDCS effects and an increase language therapy outcomes up to two-month post-therapy). The present study fills a critical gap in the literature by investigating sleep as a modulator of therapeutic efficacy (with and without neuromodulation) in PPA.