The Effects of Transcranial Magnetic Stimulation on Depression and the Associated Olfactory Dysfunction

Abstract

Introduction: Depression negatively impacts olfactory functioning and as such, olfactory dysfunction has been found in individuals with depression. Expansions of this research have demonstrated an improvement in olfactory functioning after successful pharmaceutical and psychotherapy treatments for depression. However, it is unknown if intermittent theta burst stimulation (iTBS), a form of transcranial magnetic stimulation (TMS) has a similar effect. Objectives: The current study examined the olfactory functioning and depression severity in depressed individuals before and after iTBS, comparing them to non-depressed controls. We hypothesized that depressed individuals would have significantly poorer olfactory functioning compared to controls before treatment, but little to no difference between after successful treatment. We also predicted that depression severity and olfactory dysfunction were negatively correlated. Methods: We recruited 20 depressed individuals receiving iTBS and 9 non-depressed controls. The olfactory function of depressed patients was tested before and after treatment using Sniffin’ Sticks Extended Test (examining olfactory threshold, discrimination, and identification). Depression severity was also evaluated. Controls were tested in the same manner with a 5-6 week waiting period in lieu of iTBS. Results: Both the comparison of olfaction before and after treatment in depressed patients and with controls found no significant differences. An effect of age (p<0.05) and baseline depression severity (p<0.05) were found in the depressed group, with younger and less severely depressed individuals having significantly better olfactory functioning than their counterparts both before and after treatment. There were a number of significant correlations between depression severity and olfactory discrimination, identification, and total olfaction score at both time points. Discussion: While no significant difference was found between the depressed and control groups or within the depressed group, the impact of depression severity provided support for the assumption of a reciprocal relationship. The significant difference between younger and older depressed participants highlights the limitation of age present in the study that may have contributed to the lack of significant differences. Overall, the relationship between depression and olfaction has the potential to be an important part of early diagnosis, examination of treatment progression, and on quality of life of depressed individuals.