Cerebellar Transcranial Magnetic Stimulation in Psychiatric Disorders: A Systematic Review

Background The cerebellum is an important area of the brain implicated in severe mental disorders. It can be therapeutically modulated in a neuroscience-informed manner through transcranial magnetic stimulation (TMS). In this review, our objective was to identify and summarize the findings of studies which have utilized cerebellar TMS as an intervention in psychiatric disorders and primarily reported clinical outcomes. Methods Case reports, open-label studies, and randomized controlled trials (RCTs) which recruited patients with a psychiatric disorder diagnosed as per standard diagnostic criteria who received cerebellar repetitive TMS (rTMS) as an intervention with a primary clinical outcome were included in the review. Neurological studies, animal studies, studies done on healthy participants, investigative TMS studies that study a neurophysiological/neuropsychological outcome, and studies that do not primarily stimulate the cerebellum were excluded. MEDLINE, PsychINFO, and EMBASE were searched till August 2023. The risk of bias was assessed qualitatively, and details of individual studies were tabulated. Results Eight studies comprising 215 participants were included in the review out of which 2 were case reports, 1 was an open-label study, and 5 were double-blind sham-controlled RCTs. All the studies recruited participants with schizophrenia, specifically targeting negative symptoms. The open-label trial and case reports demonstrate an improvement in negative symptoms which are not replicated consistently in the double-blind RCTs. Amongst the double-blind RCTs, 2 are positive, one is inconclusive, and 2 are negative. Discussion Cerebellar TMS as an intervention for psychiatric disorders is studied primarily in schizophrenia, mostly for negative symptoms. The current evidence is mixed and inconclusive. This is because of the limited number of studies with small sample sizes and significant heterogeneity in the conduct of rTMS trials for negative symptoms of schizophrenia. Future studies may focus on (a) optimizing stimulation parameters and localization techniques and (b) exploring alternative clinical targets (e.g., auditory hallucinations and depressive symptoms) for cerebellar neuromodulation. Other This systematic review was funded by the Department of Health Research (DHR) fellowship program for NRI/PIO/OCI, Reference ID: 2021–1004 and Narmadanagar Rural Development Society (NARDES), the CSR arm of GNFC, Gujarat, Reference: NARDES/PH/2023/1205. The systematic review proposal was registered in PROSPERO, International prospective register of systematic reviews (CRD42023459268).