Transcutaneous Electrical Nerve Stimulation

Anatomy and Physiology

TENS works through several peripheral and central mechanisms to reduce hyperalgesia. At the peripheral level, TENS activates either large-diameter, low-threshold, non-noxious afferents (A-β) or small-diameter, high-threshold peripheral afferents (A-δ) to unlock the cascade of postulated mechanisms. This targeted stimulation reduces pain by mitigating nociceptor activity and curtailing unwanted sensations.[5][6] Molecular studies indicate that the activity and expression of key pain-related ion channels, including voltage-gated sodium channels in the peripheral nerves, are modulated by TENS, inhibiting nociceptors' neurotransmission.[7]

Afferent input leads to the activation of descending inhibitory pathways within the periaqueductal gray (PAG) area, rostral ventromedial medulla (RVM), and spinal cord. TENS curbs the activity in the Rexed laminae I to V of the dorsal horn.[7] Communication with these areas occurs with different neurotransmitters and receptors. Concentrations of β-endorphins and methionine-enkephalin, which both interact with opioid receptors, have been shown to increase after high-frequency TENS. Activation of opioid receptors reduces glutamate and substance P release in the spinal cord, thus reducing excitability and potentially decreasing the risk of central sensitization. GABA-A and muscarinic receptors for high-frequency TENS are implicated in mediating CNS effects. Low-frequency TENS utilizes all these receptors in the CNS in addition to serotonin.[8]

Differences in TENS frequency used during treatment are important considerations as they affect which receptor mediates the analgesic effects. Specifically, low-frequency treatment is mediated by the μ-receptor, and the δ-opioid receptor mediates high-frequency treatment. This is relevant for patients using opioid medications for pain relief as the efficacy of low-frequency treatment may be blunted in this population.[9] 

Peripheral mechanisms are also crucial to the efficacy of TENS. Interestingly, blockage of peripheral α-2A adrenergic receptors, not central α-receptors, prevents TENS analgesia.[8]  

Indications

TENS has been used for the following conditions.

• Postoperative pain
• Acute non-postoperative pain
• Low back pain
• Osteoarthritis pain
• Diabetic peripheral neuropathy
• Fibromyalgia
• Neuropathic pain, eg, spinal cord injury [10]

Its versatility makes TENS a formidable ally in pain management, catering to acute and chronic cases. However, a schism within the scientific community persists regarding its efficacy for specific pain types or conditions fueled by methodological concerns.[11] This is due to the lack of high-quality studies or disagreement between existing high-quality studies.[10] 

Three distinct kinds of TENS emerge, each with its conceptual indications:

1. Intense TENS: Functioning as a "counter-irritant," this type zeroes in on small-diameter, high-threshold cutaneous afferents (A-δ), deploying high frequencies and intensities for brief intervals.[5]
2. Acupuncture-like TENS (AL-TENS): Geared for hyperstimulation, AL-TENS steps in for patients unresponsive to conventional TENS. It employs titration to low frequency, higher intensity, and extended pulse width, targeting small-diameter, high-threshold peripheral afferents (A-δ).[12]
3. Conventional TENS: This high-frequency, low-intensity, and short pulse duration approach selectively stimulates large-diameter, low-threshold, non-noxious afferents (A-β). This variation finds its niche in treating dermatomal pain distributions.

Literature evaluating the use of these different modalities in various kinds of pain (eg, acute, non-postoperative, chronic, etc) demonstrates that TENS is effective at treating various types of pain. Often, there was no significant difference in the amount of analgesia between modalities, with some exceptions for specific pathologies such as rheumatoid arthritis.[10]

Although high-frequency TENS has shown to be more beneficial than low-frequency TENS, the number of high-quality RCTs are insufficient to recommend the application of TENS for chronic pelvic pain or dysmenorrhea.[13]

Affecting 10% to 24% of the general population, chronic neck pain has not been overtly responsive to TENS therapy.[14]

Contraindications

Correctly administering TENS demands understanding its contraindications. Contraindications include pregnancy, epilepsy, and the presence of a pacemaker.[6] However, TENS use is possible in these patient populations if not applied to the abdomen, head, or chest.[2] Precision in electrode pad placement is paramount, and users must avoid areas prone to skin irritation.[15] Electrodes should not be placed over active, treatable tumors or close to transdermal drug delivery systems.[2]

Equipment

The manufacturer's instruction booklet should be consulted before TENS equipment use. Today's TENS units, compact and lightweight, offer a seamless integration into daily life. However, caution dictates that sessions exclude operating heavy machinery, driving, or aquatic endeavors. Electrode pad placement, adept power management, and precise setting titration are pillars for practical TENS application.[11] Pads are generally placed on relevant dermatomes, but there are a few scenarios where this is contraindicated, including sites over the neck, face, and chest, as these sites contain electrically sensitive structures. It is recommended that most patients increase the intensity to the maximum strength while still being comfortable to derive the most benefit from treatments.[2]

Technique or Treatment

It is imperative to read the manufacturer's instruction booklet, as each unit might have slight variances in recommendations. TENS units are now small and lightweight. Patients can place them in their pocket, on a desk, or hold them. Treatment sessions occur at the patient's convenience except while operating heavy machinery, driving, bathing, showering, or swimming. The unit should be powered off before placing the pads. Electrode pad placement is at least 1 inch apart in the appropriate dermatomal region of the targeted pain. The skin underlying the pad placement must have an intact sensation. After powering the unit, the setting titration follows the manufacturer's recommendations. Red and black lead placement have little impact on clinical outcomes. 

Complications

Despite its safety profile, there are cautions associated with TENS use.[16] Due to potential complications, individuals with pacemakers, epilepsy, or those in the throes of pregnancy are universally advised against TENS use. Theoretical risks extend to transdermal drug delivery systems and dermatologic concerns, urging a cautious approach.[15] Steering clear of TENS during activities like driving or operating heavy machinery mitigates potential complications.

Clinical Significance

A consensus on the efficacy of TENS in pain management has not been achieved, but the tapestry of positive outcomes is evident with prudent usage. Short-term triumphs demand strategic breaks and dynamic electrode pad placement. TENS seamlessly integrates into diverse clinical settings, necessitating formal pain assessments and crystal-clear instructions before initiation.

TENS is not a cure for pain conditions or syndromes and should only be offered as a tool for symptomatic pain relief.[13]

The positive impact that TENS has on an individual might diminish over time. Habituation might even lead to a worsening of pain. Recommendations are that patients take breaks from treatment, modulate between high frequency and low frequency within treatment sessions, increase the intensity of TENS daily, and vary their placement of electrode pads over time.[10]

Other methods of preventing analgesic tolerance include pharmacologic blockade of NMDA-glutamate receptors (such as ketamine or dextromethorphan) or CCK receptors in the spinal cord.[8]

Regarding acute postoperative pain, efficacy is contingent upon investigators utilizing adequate TENS parameters and specific pain reduction outcomes. Adequate TENS parameters are defined by sufficient frequency and proper intensity. Adequate frequency is defined as 1 to 8 Hz for LF-TENS and 25 to 150 Hz for HF TENS, while adequate intensity is defined as solid sub-noxious, maximum tolerable, or >15 mA. If these parameters are strictly adhered to, then a significant reduction in postoperative opiate use (36%) is observed. Additionally, TENS is more effective in treating the pain associated with postoperative movements, such as walking or incentive spirometer use, but not in reducing pain at rest. Investigators have proposed that future trials should distinctly analyze movement pain from resting to assess the efficacy of TENS in treating acute postoperative pain.[8]

Enhancing Healthcare Team Outcomes

TENS emerges as a linchpin in various clinical domains, from hospice and palliative care to orthopedic surgery. However, before embarking on TENS treatment, a formal pain assessment led by seasoned professionals becomes paramount.[17] Clear and understandable instructions pave the way, with monitored sessions guided by trained providers ensuring optimal understanding and utilization of TENS for pain management.[6] The collaborative synergy of nurses, allied health professionals, and clinicians is pivotal, guaranteeing a comprehensive grasp of TENS's potential in pain management.