Changes of cutaneous sensory thresholds induced by non-painful transcutaneous electrical nerve stimulation in normal subjects and in subjects with chronic pain.

Transcutaneous electrical nerve stimulation (TENS) of the nervi cutaneus surae medialis was applied to 59 healthy subjects and 30 patients suffering from chronic myofascial pain in one lower limb, with an intensity of current that induced a well tolerated tingling sensation. Each period of stimulation lasted 24 minutes. The thresholds of the tactile, tingling and painful sensations were tested at fixed intervals before, during and after stimulation. Trains of constant current square waves in the distribution area of the stimulated nerve (local thresholds) and in other areas (general thresholds) were used. In all subjects repeated changes of the current were necessary in order to maintain constant tingling during the first period of TENS (changing phase); after that few if any changes of the current were necessary (steady phase). There were changes in thresholds within the territory of the electrically stimulated nerve, and marked changes elsewhere and generally in the body. In healthy subjects local thresholds increased during both phases of TENS; general thresholds decreased during the changing phase and increased during the steady phase. After TENS, thresholds showed the same trend as during the steady phase. Trends of the sensory thresholds during and after TENS differed in different subjects according to their thresholds before TENS. Thresholds did not return to normal for more than 20 minutes after TENS. In the group of 30 patients there was a significant difference between thresholds on the two sides of the body. The difference between the two sides was reduced by TENS. Pain relief induced by TENS may be related to this fact.