Objectives: Noninvasive brain stimulation (NIBS) interventions have demonstrated promising results in the clinical treatment of pain, according to several preliminary trials, although the results have been mixed. The limitations of clinical research on NIBS are the insufficient understanding of its mechanisms of action, a lack of adequate safety data, and several disparities with regard to stimulation parameters, which have hindered the generalizability of such studies. Thus, experimental animal research that allows the use of more invasive interventions and creates additional control of independent variables and confounders is desirable. To this end, we systematically reviewed animal studies investigating the analgesic effects of NIBS. In addition, we also explored the investigation of NIBS in animal models of stroke as to compare these findings with NIBS animal pain research.
Methods: Of 1916 articles that were found initially, we identified 15 studies (stroke and pain studies) per our eligibility criteria that used NIBS methods, such as transcranial direct current stimulation, paired associative stimulation, transcranial magnetic stimulation, and transcranial electrostimulation. We extracted the main outcomes on stroke and pain, as well as the methods and electrical parameters of each technique.
Results: NIBS techniques are effective in alleviating pain. Similar beneficial clinical effects are observed in stroke. The main insights from these animal studies are the following: 1) combination of NIBS with analgesic drugs has a synergistic effect; 2) effects are dependent on the parameters of stimulation, and in fact, not necessarily the strongest stimulation parameter (i.e., the largest intensity of stimulation) is associated with the largest benefit; 3) pain studies show an overall good quality as indexed by Animals in Research: Reporting In Vivo Experiments guidelines of the reporting of animal experiments, but insufficient with regard to the reporting of safety data for brain stimulation; 4) these studies suggest that NIBS techniques have a primary effect on synaptic plasticity, but they also suggest other mechanisms of action such as via neurovascular modulation.
Conclusions: We found a limited number of animal studies for both pain and stroke NIBS experimental research. There is a lack of safety data in animal studies in these two topics and results from these studies have not been yet fully tested and translated to human research. We discuss the challenges and limitations of translating experimental animal research on NIBS into clinical studies.