Theta burst stimulation modulates cerebellar-cortical connectivity in patients with progressive supranuclear palsy

Progressive Supranuclear Palsy (PSP) is an atypical degenerative Parkinsonism characterized by postural instability, supranuclear gaze palsy and frontal deficits. Recent imaging studies revealed that the volume of cerebellar peduncles and midbrain were reduced in PSP. Transcranial magnetic stimulation (TMS) studies demonstrated a cerebellar involvement in PSP showing an impairment of functional connectivity between the cerebellar hemisphere (Cb) and the contralateral primary motor cortex (M1) (cerebellar brain inhibition-CBI).

To investigate the plasticity of the cerebello-thalamo-cortical circuits in ten PSP patients after two-week course of cerebellar intermittent theta burst stimulation (iTBS), a form of repetitive TMS.

Before and after the iTBS sessions we measured functional connectivity between Cb and contralateral M1 (CBI), short intracortical inhibition (SICI) and intracortical facilitation (ICF) and short latency afferent inhibition (SLAI) in contralateral M1. We also performed resting state functional magnetic resonance (rs-fMRI) and we administered clinical rating scale (PSP-RS).

At baseline PSP patients had decreased efficiency of CBI, SICI and SLAI in comparison to PD patients and healthy subjects. Cerebellar iTBS increased the deficient functional cerebellar-motor connectivity as assessed by CBI. No effect was seen for SICI/ICF and SLAI circuits. Following iTBS there was an increased signal in the head of the caudate nucleus bilaterally as shown by rs-fMRI. Moreover, PSP-RS showed an improvement of dysarthria in all patients.

iTBS enhanced functional connectivity between the cerebellar hemisphere, the caudate nucleus and the cortex, that was paralleled by some clinical improvement. Future randomized, sham-stimulation controlled studies are warranted to support the clinical efficacy of this technique.