Objective: In this meta-analysis, we assess whether short-afferent inhibition (SAI) is impaired in Parkinson’s disease (PD) patients as well as identify clinical variables associated to SAI magnitude in PD. We further assess the potential value of SAI as a marker of cognitive impairment in PD, and its association with four specific cognitive domains.

Background: Research over the past 20 years has shed light on the involvement of the cholinergic system in PD. SAI is characterized by the suppression of the transcranial magnetic stimulation motor evoked potential by the cortical arrival of a somatosensory afferent volley. SAI is widely considered a non-invasive measure of sensori-motor integration, and has emerged as a putative neurophysiological marker of cholinergic function in the human brain.

Methods: Pubmed and PsyINFO were consulted to identify all studies that compared SAI between PD patients and healthy controls. Separated meta-analyses comparing 1) PD patients vs. healthy controls 2) patients with cognitive impairment vs patients with preserved cognitive functions were conducted, using the random effects model. Analysis of heterogenity followed by moderator analysis for dichotomous moderator and meta-regression were performed.

Results: A moderate-to-severe reduction in SAI was observed in PD patients (Figure 1). Furthermore, through moderator analysis, the impairment of SAI was shown to be associated with disease duration and therapeutic state. Patients under dopaminergic agents (“on” state) displayed worse SAI than those after dopaminergic agent withdrawal (“off”). Our meta-analysis also revealed that patients with cognitive impairment displayed significantly lower levels of SAI than those without cognitive impairment (Figure 2a). Cognitive domains associated to worse SAI include visuospatial/visuoperceptive and executive functions (Figure 2b).

Conclusions: This study highlights the importance of assessing SAI as a neurophysiological marker associated to cognitive deficits in PD.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/meta-analysis-of-short-afferent-inhibition-and-cognitive-impairment-in-parkinsons-disease/