Transcutaneous Electrical Nerve Stimulation: New Applications of an Established Treatment

81 THE PUBLICATION of the gate control theory in 1965,1 was quickly followed by clinical reports describing the use of cutaneously applied electrical stimulation to produce pain relief. Consistent with the gate theory, early research supported that other forms of peripheral stimulation such as vibration were very effective in reducing the perception of pain.2–4 The fact that counterstimulation techniques produced analgesia is not surprising because humans have used a variety of physical and sensory modalities to relieve pain for thousands of years. Modalities ranging from massage and heat treatment to more severe counterirritation measures such as cauterization, blood letting, acupressure, and acupuncture are common across a diverse array of cultures. The subsequent focus of commercial development on peripheral electrical stimuli in the form of transcutaneous electrical nerve stimulation (TENS) was largely related to the technological limitations and the inefficiencies of other forms of mechanical counterstimulation. Transcutaneous electrical stimulators are simple to build, highly portable, and relatively energy-efficient. Mechanical vibratory stimulators are not, therefore, the commercial focus on electrical stimulation devices. Perhaps the best way of initiating the discussion of the clinical significance of cutaneously applied electrical stimulation, henceforth generically called TENS, is to acknowledge that certain aspects of the effectiveness of this treatment are at best ill defined. Briefly, there is a solid body of literature based on wellcontrolled laboratory studies that TENS produces analgesia in both animals and in humans.5,6 These studies focus on short-term analgesia in both experimental and clinical pain states. There is also substantive but not unanimous literature that indicates that TENS produces significant analgesia in the postoperative and other acute pain states as measured by both subjective patient report and objective measures such as postoperative opiate use7 or tests of pulmonary function.8,9 However, the literature describing TENS treatment for chronic pain conditions is much more complex. Although clinical studies number in the hundreds, many suffer from significant methodological weaknesses.10 These weaknesses include failure to control for the significant treatment effect of TENS, to define clear outcome measures, to monitor subject adherence to the study protocol, to define a homogeneous study group, or utilize standard clinical TENS treatment parameters. In other instances, TENS has been used as the primary or only treatment modality to effect change in groups of subjects with entrenched chronic pain. In some of the groups of subjects studied even proven pharmacological analgesic drugs show only marginal benefit when measured over the long term. Nevertheless, even with the acknowledged limitations, it appears that for many patients with chronic pain, TENS offers a longterm treatment that provides some dimension of sustained analgesic effects. There are several specific points fundamental to the present article that require a brief re-