Transcutaneous electrical nerve stimulation (TENS) is clinically used by a variety of healthcare professionals for the reduction of pain. Pain is typically classified as either acute or chronic. Chronic pain persists for weeks or months and is usually associated with an underlying condition, such as post-operative pain, cancer pain, and low back pain,etc. 

Several recent studies have suggested that how to evaluate clinical effectiveness of TENS with different experimental designs. Approximate 1000 articles on TENS studies have already published. Scientists describe theories that support the use of TENS based on the release of endogenous opioids and the gate control theory. Low frequencies, usually below 10 Hz, activate µ-opioid receptors according to the endogenous opioid system；however, high frequencies, above 50 Hz, activate δ-opioid receptors based on the gate control system. The main tools available for measuring pain fall into two categories, namely unidimensional scales and multidimensional scales. Unidimensional tools include numeric rating scales, visual analogue scale and face rating scale. Multidimensional tools include brief pain inventory and McGill pain questionnaire.

In this study, we review the clinical literature on TENS effectiveness, experimental designs, number of the cases , TENS with varying frequency, classification of diseases, and the assessment of pain. We construct a literature-curated TENS and pain research database to make it easy to do systematic reviews and also demonstrate visualization of the results.