The clinical application of Transcutaneous Electrical Nerve Stimulation (TENS), as we know it today, began in 1967 by American neurosurgeon Norman Shealy. Based on Melzak and Wall’s gate control theory of pain modulation that was introduced in 1965, Shealy began to develop a surgically implanted dorsal column stimulator to treat patients who were experiencing chronic recalcitrant pain.1,2 Shealy used portable TENS devices to predict if patients with chronic pain would be good candidates for a surgically implanted dorsal column stimulator to manage their pain. The trials demonstrated that a large percentage of patients reported effective pain relief with TENS and requested to continue using TENS rather than have a dorsal column stimulator surgically implanted into their spine.1 Since then, TENS has become a common therapeutic intervention to reduce both acute and chronic pain. Most recently, overthe-counter (OTC) TENS devices have been made available to the general public in the retail marketplace.
CONTEMPORARY USE OF TENS TENS is an inexpensive non-pharmacological method of treating acute and chronic pain without adverse side effects. For patients who are taking pain medication, the addition of properly dosed TENS to their pain management program can decrease their need for pain medication by 36% to 51%.3,4 The existing TENS clinical literature demonstrates conflicting evidence, which both supports and questions the effectiveness of TENS as a therapeutic intervention to treat pain. Despite overall mixed resultsin the pain management literature, “TENS has been shown to be effective for osteoarthritis, fibromyalgia, and neuropathic pain, all conditions with enhanced excitability and reduced inhibition.”5 Conflicting evidence within the existing TENS literature has led to more focused research and analysis to evaluate key factors regarding the dose response relationship and best practices for pain management. Clinically, the two most widely used TENS approaches are Sensory TENS (High Frequency TENS), and Motor TENS (Low Frequency TENS).
SENSORY TENS (HIGH FREQUENCY TENS) Sensory TENS is indicated for the treatment of acute pain and is characterized by the perception of a strong tingling sensation that is below the patient’s motor and pain threshold. The relatively gentle nature of Sensory TENS is less likely to violate tissue healing constraints in acute conditions. The primary objective of the intervention is to produce maximal depolarization of A-beta sensory nerve fibers which activate the spinal gating mechanism described by Melzack and Wall.2,4 A secondary endogenous opiate response induced by Sensory TENS has been long theorized. More recent research has validated this Sensory TENS induced endogenous opioid response for pain modulation.6 Sensory TENS (50-150 Hz) has been shown to activate delta-opioid receptors that supplement the spinal gating mechanism in modulating pain.6 It is important to note that research has also shown that caffeine can block the analgesic effect of Sensory TENS.7 Given that the half-life of caffeine is 4-6 hours, it is recommended that caffeine intake be limited and that it not be ingested in the 6 hours prior to TENS treatment.5 In order to maximally activate the wide diameter, heavily myelinated, and fast conducting A-beta sensory nerve fibers, a high pulse frequency and a short phase duration is used (Table 1). The intensity should be set to the desired physiologic response rather than a set number of milliamps. Intensity has been shown to be a critical parameter of TENS.5 In order to produce the strong, yet non-painful, sensory nerve depolarization characteristic of Sensory TENS, the stimulation can be turned up until the first signs of perceptible muscle twitching and then backed off slightly to the desired maximally strong tingling sensation. The quick onset of electroanalgesia produced by Sensory TENS makes it a valuable intervention to pair with other active interventions during a therapy session, or as part of a home treatment program. Clinically, enhanced pain reduction and improved function is noted when TENS is combined with movement.8 The duration of TENS treatment does not need to be limited to 10 to 20 minutes, which has become common in clinical practice. The importance of home devices should not be underestimated. It affords the patient the ability to administer the treatment more frequently and for longer treatment durations, which may be necessary to keep the patient’s pain under control. It also allows them to increase the intensity (mA) throughout the treatment as accommodation to the stimulation occurs. This upward titration of the intensity throughout the treatment has been shown to be a key factor in enhancing analgesia.9 The upward titration of Sensory TENS during the treatment session should proceed to maximum sensory nerve stimulation, without motor nerve activation, and should be just below the pain threshold. In instances where muscle recruitment is interfering with achieving maximum sensory nerve depolarization, the phase duration can be shortened to the lower end of the recommended range (eg, 20-50 µsec) to further isolate sensory nerve recruitment. When using alternating current waveforms (Interferential or Premodulated), in devices that allow for the manipulation of carrier frequency, choosing a carrier frequency between 5,000 and 10,000 Hz will result in a shorter phase duration which is believed to be more effective in preferentially targeting the recruitment of the A-beta sensory nerves (Table 2). MOTOR TENS (LOW FREQUENCY TENS) Motor TENS is indicated for the treatment of subacute and chronic pain. It is characterized by the perception of a strong, yet tolerable, rhythmical muscle contraction which the patient perceives as muscle twitching. The activation of A-delta and A-alpha fibers during this treatment activates the release of endogenous opioids from the brain (periaqueductal gray and ventromedial medulla) into the blood and cerebrospinal fluid. Since the opioids travel down the spinal cord to the dorsal horn, where they chemically block pain, this mechanism of pain relief is also referred to as the descending endogenous opioid model. The longer-acting pain relief associated with Motor TENS (1-10 HZ) is attributed to the specific activation of mu-opioid receptors.5 Motor TENS is more aggressive than Sensory TENS, and provides longer-lasting
pain relief than Sensory TENS.10 In order to activate the A-delta small diameter myelinated pain fibers, a low frequency between 1 to 10 pps (commonly 4 pps) and a phase duration of between 200 to 300 µsec is used. Intensity of stimulation has been shown to be an important factor in producing maximum analgesia. The intensity should be set to a strong visible, rhythmical, tremor-like muscle twitch. This strong rhythmical muscle twitch at a low frequency of stimulation produces longer-acting analgesia compared to Sensory TENS. However, it has greater potential to disrupt tissue healing when used in regions of acute conditions. Therefore, in acute pain and early stages of tissue healing, Sensory TENS is recommended.10-12 In instances where tissue healing has progressed, and pain is more chronic in nature, Motor TENS is indicated.11,13 As with Sensory TENS, the intensity (mA) should be titrated upward from time to time throughout the treatment session as habituation to the stimulus occurs. This makes a portable device a good choice because it is convenient and easily controlled by the patient. In the instance where a patient has developed a tolerance to