10. Transcutaneous Electrical Neuromuscular Stimulation
Electroanalgesia has been used since 63 AD, when ancient Roman Scribonius Largus would stand on electric fish (eels presumably) to relieve physical pain. The development of the modern version of the transcutaneous electrical neuromuscular stimulation (TENS) unit generally is attributed to C. Norman Shealy. The efficacy of TENS, like that of many of the other devices, is debated and still controversial. However, the most powerful evidence to date supporting the validity of TENS is that of functional MRI. These studies have for the most part supported the idea that high-frequency TENS decreases pain-related cortical activation in patients with carpal tunnel syndrome. Low-frequency TENS also decreases or modulates the pain-induced cortical activation of shoulder impingement syndrome. Studies using evoked potentials also supported TENS efficacy by showing that TENS stimulation suppressed A-delta fiber nociceptive processing (gate control). At this point, I should think it’s safe to move forward in an affirmative manner regarding the efficacy of TENS (Table 1).
Strength of Treatment
The use of TENS continues to be a popular choice as front-line therapy, as adjunctive therapy, and when all else fails. There is a favorable benefit/risk ratio, if for no other reason than the risk to the patient is minimal to nil. The provider can control the key parameters of frequency, intensity, and pulse width. A TENS session is unlikely to cause adverse effects, and the strength of the current is controlled by the patient at all times. This form of treatment works well when the goal is to interrupt a pain/spasm cycle, which in itself might be a worthwhile endeavor and an important factor in preventing the hardwiring (centralization) of pain in the CNS.
Ease of Treatment
The TENS treatment protocol is simple and forms the basis for application of the other electrotherapy devices, which tend to have a more elaborate set up than a TENS device. The basic analogue units (non digital) continue to be very useful for home care regimens, especially for patients who find complicated instructions and memory tasks challenging.
TENS treatments are relatively comfortable, and the devices can be worn during the course of a regular workday, with many patients being unaware that the units are even turned on most of the day. Units should not be worn during sleep. When patients are selected properly, the greatest operational drawback of TENS application is that of accommodation or tolerance, whereby a few minutes into the stimulation, the intensity of the current will need to be increased. It is not uncommon to have patients “max out the intensity” of a TENS unit because of this phenomenon. Units, such as the Codetron TENS device, have an anti-accommodation feature to counteract this limitation.
The TENS machine is a pain-masking device and does not typically have much contribution beyond this objective. However, having said that, I don’t want to minimize the value of pure non-pharmacologic analgesia. After all, opioid analgesics and lesser potency analogues exist simply for pain relief. Sometimes, pain relief is all that is desired, and when that’s the case, conservative approaches should always be attempted first. TENS is the standard electrotherapy choice for electroanalgesia.
There have been numerous systematic reviews performed using various databases and the response results have been mixed. Arguably, the more important research was cited previously, including functional MRI and brain evoked potential studies that support the use of TENS application for suppressing cortical A delta activity in the parts of the brain involved in pain processing.