5.1 Transcutaneous Electrical Nerve Stimulation (TENS) and Pain Relief
Key Takeaways
For a defined response and electrode setup, rheobase is the limiting threshold current at long pulse duration. Chronaxie is the threshold duration at twice the rheobase.
Conventional sensory TENS aims for strong, comfortable tingling without visible muscle contraction. Pulse settings and amplitude are adjusted to the observed response.
Low-rate motor-level TENS can produce rhythmic twitches, and burst TENS groups pulses into packets. Spinal and descending mechanisms can overlap, and carryover varies.
A charge-balanced biphasic pulse has equal and opposite phase charge. Check electrode contact and skin even with a charge-balanced waveform.
Use indication-specific evidence and follow the stimulator instructions for placement, sensation, pregnancy and implanted electronic devices.
Transcutaneous Electrical Nerve Stimulation (TENS) and Pain Relief
TENS delivers electrical pulses through surface electrodes to modulate pain. Conventional sensory TENS is adjusted to a strong but comfortable tingling without a visible contraction. Check whether temporary pain relief helps with a specific activity, such as gripping a tool during rehabilitation. The goal is symptom management.
Excitation and the strength–duration curve
A pulse creates an extracellular field that changes the voltage across nearby cell membranes. An excitable axon can fire when the local depolarization reaches threshold. Threshold depends on factors including axon properties, its position in the field, electrode placement, pulse duration and amplitude. In a typical sensory TENS setup, comfortable tingling can be obtained below the threshold for a visible motor response. A short pulse does not guarantee recruitment of a particular fiber class or prevent discomfort at high amplitude.
For a specified electrode setup and response, the strength–duration curve shows the minimum current needed at each pulse duration. Longer pulses generally need less current to reach the same threshold. The rheobase is the limiting threshold for a very long pulse. The chronaxie is the pulse duration at twice the rheobase. For example, if the rheobase for the specified response is 2 mA and the threshold at 4 mA occurs at 200 µs, the chronaxie is 200 µs. Use the response and electrode setup for which the curve was measured.
Waveform and contact
In a symmetric biphasic pulse the two phases are mirror images. In a balanced asymmetric biphasic pulse they can have different shapes and widths, but their signed current–time areas sum to zero. In an unbalanced waveform the net charge per pulse differs from zero. Charge balance reduces net charge delivery and does not eliminate skin irritation or burns. An adhesive reaction, damaged pad or poor contact can still cause a problem.
For the same current, the average current density is greater under a smaller contact area. For example, 12 mA distributed uniformly over 12 cm² averages 1 mA/cm², versus 2 mA/cm² over 6 cm². This calculation describes average density in a simplified field. It does not determine a safe setting or the local peak at a lifted electrode edge. Use the supplied or approved electrodes, apply them fully to suitable intact skin and inspect the site before and after treatment.
Modes and expected response
| Mode | Pulse pattern | Intended response in this setup |
|---|---|---|
| Conventional, high-rate sensory TENS | Regular relatively high pulse rate, for example 100 Hz | Strong, comfortable tingling without visible twitch. Adjust from low amplitude to the desired sensation. |
| Low-rate, acupuncture-like TENS | A few pulses per second, for example 4 Hz. Wider pulses may help elicit a motor response at a lower current. | In a motor-level application, comfortable rhythmic twitches where contraction is appropriate. |
| Burst TENS | Packets of more closely spaced pulses repeated at a slower burst rate | Distinguish pulses per second within a packet from bursts per second. Adjust intensity for the intended sensory or motor response. |
As an example of a studied burst pattern, an experiment in healthy volunteers used 100 pulses per second within a burst, four bursts per second and a 200 µs pulse duration. It measured immediate pressure-pain responses. The settings illustrate the waveform and do not define a clinical protocol.
Melzack and Wall's gate-control model proposes that large-diameter cutaneous afferent input can reduce transmission of nociceptive signals in the dorsal horn. This is one explanation for sensory TENS. Descending inhibitory circuits and endogenous opioids also have experimental support. In a rat model of joint inflammation, spinal opioid receptor blockade affected low- and high-frequency TENS differently. In a human experimental-pain study, high-dose naloxone also reduced high-frequency TENS analgesia. These results support overlapping mechanisms and cannot predict how long an individual's pain relief will last.
Assess pain and task performance during or shortly after stimulation, when short-term effects have been measured. Condition and outcome matter. NICE NG226 recommends against offering TENS to manage osteoarthritis, and NG193 recommends against offering it for chronic primary pain in the populations it covers. The 2022 APTA lateral elbow pain guideline allows selected TENS applications for short-term pain relief. These recommendations address different populations and outcomes.
Electrode placement and safety
Follow the device instructions for electrode placement. Avoid the anterior neck or carotid sinus and a current path through the head or across the chest. Some devices, including the Intelect Mobile 2, also prohibit application on the chest. For a local application, a pair can bracket the symptomatic site on intact skin. Placement in the symptomatic segment or along a superficial nerve may also be considered when appropriate. Choose the position by the perceived field and treatment goal, without assuming that crossing two channels focuses a current at a deep lesion.
| Screening issue | Decision |
|---|---|
| Implanted cardiac rhythm device or other electronic implant | Read the stimulator instructions first. For example, the Intelect Mobile 2 labeling excludes patients with relevant implanted cardiac devices. If the intended equipment permits an evaluation, obtain device-specialist input before starting. EHRA recommends a plan that limits interference and monitoring at first use for pacemaker-dependent patients and patients with an ICD. |
| Pregnancy | Follow the device and obstetric protocol. The named Intelect Mobile 2 electrotherapy instructions exclude use during pregnancy. An obstetric protocol for a suitable device is a separate setting. |
| Suspected active thrombosis, infection or malignancy in the application area | Defer stimulation and address the underlying problem. Suspected acute DVT warrants prompt assessment. |
| Reduced sensation, unreliable feedback, or broken or irritated skin | Avoid routine sensory TENS where the patient cannot judge intensity or the pads cannot be used safely. The named device prohibits electrotherapy over skin lacking normal sensation. Stop for painful stimulation, burning or skin injury. |
Clinical case
A worker with lateral elbow tendinopathy has intact sensation and skin. Pain with gripping makes it difficult to complete wrist extensor exercises. After screening, the clinician and patient choose a brief trial of burst TENS around the painful region as an adjunct to graded loading. They compare the same grip task before and immediately after stimulation and inspect the electrode sites. If gripping pain falls on reassessment and the task is easier, the patient can judge whether that temporary change is worth continuing. If the task is unchanged or the skin becomes irritated, adjust or discontinue the modality and continue the loading plan.
The limiting threshold for a very long pulse is 2 mA. At 4 mA, the response first occurs at 200 microseconds. Which quantity is 200 microseconds?
Chronaxie
Rheobase
Peak current
Absolute refractory period
A screened patient is using conventional sensory TENS as a short-term adjunct. Which response indicates the intended intensity?
Painful rhythmic contraction
Strong comfortable tingling without visible twitch
A sustained contraction with joint movement
No perceptible sensation despite intact sensation
A human experimental-pain study found that a high dose of naloxone reduced high-frequency TENS analgesia. What is supported by that observation?
High-frequency TENS only acts through direct C-fiber conduction block
Low-rate TENS reliably provides five hours of relief
Endogenous opioid mechanisms can contribute even to high-frequency TENS analgesia
The response identifies the exact opioid receptor in every patient
Which proposed electrode placement must be rejected under the Intelect Mobile 2 electrotherapy instructions?
A pair bracketing lateral elbow pain on intact skin in a screened adult
A pair on the same forearm with normal sensation and full contact
A pair near a symptomatic thigh region on suitable intact skin after clinical screening
An electrode over the anterior neck at the carotid sinus
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