Nine studies (n=565) were included in the review. Seven studies used a randomised controlled trial design (one was crossover study) and two studies used a 2x2 factorial design. Sample sizes ranged from 15 to 155. Three studies were graded as evidence class I, five as class II and one as class III.
Chronic low back pain (five studies): Two class II studies showed that TENS was beneficial in terms of pain and quality of life in comparison with sham TENS. However, two class I studies and another class II study did not show any difference in comparison with sham TENS or alternative TENS regimens.
Painful diabetic neuropathy (four studies): Two class II studies reported that TENS improved pain in comparison with control. One class I and one class III study showed that a larger proportion of participants felt a benefit with high frequency muscle stimulation compared with TENS.