Investigation of the effect of low frequency transcutaneous electrical nerve stimulation on central pain modulation by functional magnetic resonance imaging

OBJECTIVE AND HYPOTHESIS: The history of pain is as old as human history and its treatment is a struggle since prehistoric times. The use of neurostimulation techniques for the control of pain started in ancient ages. Nowadays one of the most frequently used neurostimulation techniques is transcutaneous electrical nerve stimulation (TENS). Transcutaneous electrical nerve stimulation is a low frequency analgesic current that is used in many acute and chronic painful states. There are different subtypes of TENS according to intensity and the frequency of the current. Altough there are many studies reporting analgesic effect of TENS, most of them have poor level of evidence. However recent meta-analyses and reviews have started to report evidence for local efficacy of TENS. The aim of this study is to investigate whether low frequency TENS has objective effect on central pain modulation that can be documented by functional magnetic resonance imaging (fMRI) on shoulder pain model.MATERIAL AND METHOD: Twenty patients who applied Dokuz Eylül University Department of Physical Medicine and Rehabilitation with symptom of shoulder pain and diagnosed as shoulder subacromial impingement syndrome according to history and physical examination were enrolled in the study. None of the patients had physical treatment before. Patients were randomized in two groups. All the patients had fMRI while the affected shoulder was positioned in the painful internal rotation position. Then 250 µs, 3 Hz low frequency TENS was applied to the patients in the first group for 30 minutes. For the second group 30 minutes sham TENS, where the electrodes were positioned and current was closed, was applied. Second fMRI was performed in the same position immediately after the low frequency TENS or sham TENS application. The data of the fMRI images were analysed by radiologists using statistical parametrical mapping (SPM). Ten central regions of interest that were reported to role in pain perception in literature were chosen and analysed bilaterally. Perceived pain intensity during fMRI was evaluated with ten milimeter visual analog scale (VAS).RESULTS: In the low frequency TENS group there was statistically significant decrease in VAS values. The intergroup analysis of Z values which represent theintensity of activation in the region of interest on fMRI images, revealed statistically significant decrease in the activity of contralateral primary sensory cortex, bilateral caudal region of anterior cingulate cortex and ipsilateral supplementary motor area. There was statistically significant correlation between the change of VAS value and change of activity in contralateral thalamus, prefrontal cortex and ipsilateral posterior paryetal cortex. In the sham TENS group there was no significant change in VAS value and activity of regions of interest.CONCLUSION: In this study one session low frequency TENS application caused statistically significant decrease in VAS values for pain in the patients having shoulder pain. Low frequency TENS application caused decrease in the activity of contralateral primary sensory cortex which is known to role in discriminative aspect of pain perception and this decrease was shown with fMRI. This finding led us to consider an objective documanteble analgesic effect of TENS on cerebral level. The bilateral decrease in the activity of caudal region of anterior cingulate cortex which has discriminative and affective role in pain perception, and decrease in the activity of ipsilateral supplementary motor area which represents motor response to pain led us consider low frequency TENS affected discriminative, affective and motor aspects of central pain perception together with decrease in perceived pain.