At a glance......
TENS; Transcutaneous Electrical Nerve Stimulation/ is an inexpensive nonpharmacological intervention used in the treatment of acute and chronic pain conditions. These small battery-powered devices deliver alternating current via cutaneous electrodes positioned near the painful area. The parameters of pulse frequency and pulse intensity are adjustable and linked to TENS efficacy. This article will provide a critical review of the latest basic science and clinical evidence for TENS. We will summarize mechanisms of action, factors that influence TENS efficacy, and describe and critique the use of TENS for pain control in a variety of patient populations.
Mechanisms of TENS Reduction on Analgesia
TENS activates a complex neuronal network to result in a reduction in pain. At frequencies and intensities used clinically, TENS activates large diameter afferent fibers [rx,rx]. This afferent input is sent to the central nervous system to activate descending inhibitory systems to reduce hyperalgesia. Specifically, blockade of neuronal activity in the periaqueductal gray (PAG), rostral ventromedial medulla (RVM) and spinal cord inhibit the analgesic effects of TENS showing that TENS analgesia is maintained through these pathways [rx–rx]. In parallel, studies in people with fibromyalgia show that TENS can restore central pain modulation, a measure of central inhibition [rx]. Therefore, TENS reduces hyperalgesia through both peripheral and central mechanisms.
Neurotransmitters & Receptors That Mediate TENS Analgesia
HF TENS increases the concentration of β-endorphins in the bloodstream and cerebrospinal fluid, and methionine-enkephalin in the cerebrospinal fluid, in human subjects [rx,rx]. The analgesia produced a reduction in hyperalgesia by HF TENS is prevented by blockade of opioid receptors in the RVM or spinal cord, or synaptic transmission in the ventrolateral PAG [rx–rx,rx]. This opioid-mediated analgesia produced by HF TENS has been confirmed in human subjects [rx]. Furthermore, the reduction in hyperalgesia produced by HF TENS is prevented by blockade of muscarinic receptors (M1 and M3) and GABAA receptors in the spinal cord [rx,rx]. However, blockade of serotonin or noradrenergic receptors in the spinal cord has no effect on the reversal of hyperalgesia produced by HF TENS [rx]. Thus, HF TENS produces analgesia by activating endogenous inhibitory mechanisms in the central nervous system involving opioid GABA, and muscarinic receptors.
Peripheral Mechanisms of TENS
Both HF and LF TENS have effects at the site of stimulation. HF TENS reduces substance P, which is increased in dorsal root ganglia neurons in animals after tissue injury [rx]. Blockade of peripheral opioid receptors prevents the analgesia produced by LF, but not HF TENS [rx, rx]. Thus, TENS may also alter the excitability of peripheral nociceptors to reduce afferent input to the central nervous system.
In α-2a adrenergic knockout mice, analgesia by LF and HF TENS does not occur [rx]. Blockade of peripheral, but not spinal or supraspinal, α-2 receptors prevents the analgesia produced by TENS [rx] suggesting a role for peripheral α-2a-adrenergic receptors in analgesia produced by TENS. Further, the reduction in cold allodynia by LF TENS is reduced by administration of systemic phentolamine to block α-adrenergic receptors [rx]. This adrenergic effect may alter the autonomic system. There are increases in blood flow with LF TENS at intensities that produce motor contractions; greater than 25% above motor threshold [rx–rx]. Thus, some of the analgesic effects of TENS are mediated through peripheral adrenergic receptors.
Types of TENS
- Continuous or conventional TENS (High frequency and low intensity).
- Pulsed or burst TENS (Low frequency and low intensity).
- Acupuncture-like TENS (Low frequency and high intensity).
|Physiological Intention||Clinical Technique|
|Conventional TENS||To selectively activate large diameter non-noxious afferents to elicit segmental analgesia||Low-intensity / high-frequency TENS at the site of pain to produce “strong but comfortable TENS paraesthesia”. Administer whenever in pain|
|Acupuncture-like TENS||To produce muscle twitches to activate small diameter motor afferents to elicit extrasegmental analgesia||High-intensity / low-frequency TENS over muscles, acupuncture points or trigger points to produce “strong but comfortable muscle contractions”. Administer for 15–30 minutes at a time|
|Intense TENS||To activate small diameter noxious afferents to elicit peripheral nerve blockade and extrasegmental analgesia||High-intensity/high-frequency TENS over nerves arising from painful site to produce “maximumly tolerable (painful) TENS paraesthesia”. Administer for a few minutes at a time|
- Conventional TENS (low-intensity, high-frequency)
- Acupuncture-like TENS (high-intensity, low-frequency)
- Intense TENS (high-intensity, high-frequency)
Location of Electrodes
Electrodes must be positioned on healthy sensate skin which should be checked prior to application. Accurate placement of pads can be time-consuming. Electrodes are positioned on relevant dermatomes so that paraesthesia can be directed into the painful area. Exceptions include:
- Hyperaesthesia and mechanical allodynia because TENS may aggravate pain
- Hypoaesthesia because TENS is not effective in nonsense skin and may cause skin irritation due to accidental use of excessively high intensities
- The absence of a body part (i.e. phantom pain), damaged or frail skin (e.g. wounds, eczema)
In these situations, electrodes are positioned along the main nerves proximal to the site of pain in the first instance. Electrodes can also be positioned paravertebrally at spinal segments related to the pain or at contralateral ‘mirror’ sites. Dual channel TENS devices with 4 electrodes are used for large or multiple pains.
Inappropriate electrode positions include:
- Anterior neck because carotid sinus stimulation may cause a hypotensive response and laryngeal nerve stimulation may cause a laryngeal spasm
- Over the eyes because it may increase intraocular pressure
- Through the chest (anterior and posterior positions) because stimulation may affect the electrical conductivity of the heart and interfere with intercostal muscle activity
- Internally except using devices designed for dental, vaginal and anal stimulation.
The position of the red and black lead usually has little effect in clinical practice as biphasic waveforms with zero net current flow are often used in devices.
Indications of TENS
- Musculoskeletal pain – Acute postoperative pain and acute post-traumatic pain – A study showed that TENS used in the postoperative cardiac patient significantly reduced both spontaneous and cough induced pain. It improved pulmonary mechanics and lead to demonstrable increases in thoracic and girdle muscle function.
- Chronic pain – e.g. (rheumatoid arthritis, osteoarthritis, etc.) a recent Cochrane review article suggests an overall positive analgesic outcome in favor of TENS. However, due to poor and inconsistent reporting of methods and results, no meta-analysis was able to be done.
- Chronic low back pain – Two Class I studies demonstrating that TENS is ineffective
- Painful diabetic neuropathy – Two Class II studies demonstrating that TENS is most likely effective
- Neuropathic pain – A recent article suggests that conventional TENS may be better placebo (level C); however’ is most likely not as good as other types of electrical stimulation including acupuncture-like stimulation (level B).
- Visceral pain and dysmenorrhea – A small study of women showed improvement from baseline primary dysmenorrhea pain shortly after the application of TENS.
- Relaxing muscles spasms – From sciatic nerve pain to impinged nerves in the shoulder, e-stim can help relax and reduce muscle spasms.
- Preventing or slowing tissue atrophy due to disuse – TENS is able to produce contractions that mimic the body’s movements, this helps prevent atrophy such as stroke patients recovering the use of their limb(s).
- Increasing blood circulation – By activating the muscles, e-stim enhances blood flow and can be used in post-recovery for athletes and the typical patient population.
- Re-educating muscles, such as in situations where a muscle injury limited its use – TENS can be used to initiate muscle recruitment, for example, research has shown that the use of e-stim helps patients regain strength following total knee arthroplasty.
- Maintaining or increasing the range of motion – Electrotherapy can be used to aid in increasing a patient’s range of motion. Research has shown that TENS is helpful in improving motor recovery and range of motion in patients with hemiplegia.
Contra-Indications of TENS
- Patients with a Pacemaker should not be routinely treated with TENS though under carefully controlled conditions it can be safely applied. It is suggested that routine application of TENS for a patient with a pacemaker or any other implanted electronic device should be considered a contraindication.
- Skin irritation
- Unpleasant sensation
- Influence on the cardiac pacemaker
- Stimulation of sensitive carotid sinus nerves
- Altered sensation
- Spasm of laryngeal and pharyngeal muscles
- Removal of protective influence of pain
- Neurotic addiction to stimulation
- Destruction of operators of dangerous machines
- Patients who have an allergic response to the electrodes, gel or tape
- Electrode placement over dermatological lesions e.g. dermatitis, eczema
- Application over the anterior aspect of the neck or carotid sinus
- If there is abnormal skin sensation, the electrodes should preferably be positioned elsewhere to ensure effective stimulation
- Electrodes should not be placed over the eyes
- Patients who have epilepsy should be treated at the discretion of the therapist in consultation with the appropriate medical practitioner as there have been anecdotal reports of adverse outcomes, most especially (but not exclusively) associated with treatments to the neck and upper thoracic areas
- Avoid active epiphyseal regions in children (though there is no direct evidence of adverse effect)
- The use of abdominal electrodes during labor may interfere with fetal monitoring equipment and is, therefore, best avoided.