Historical Perspective

Long before the development of modern electricity, “natural electricity” was used for its therapeutic properties of inducing analgesia. In the earliest written description of an electric fish, Aristotle remarked, “The torpedo is known to cause a numbness even in human beings.”¹ Perhaps the earliest record of local electrical analgesia was when Scribonius Largusm wrote in circa CE 46 that he advocated piscine “electrotherapy” for the relief of pain.¹ Centuries later, in the mid 1740s, the application of an electrical apparatus was first used in medicine by Kratzenstein.¹ In 1858, Francis received a patent for an electrical device that he claimed relieved dental pain; he boasted that he had performed 164 successful tooth extractions and that most had resulted in no pain.¹

Despite its long history, the use of electrical signals for clinical application has not been a widely accepted theory throughout the history of modern medicine. But since the 1960s, when Melzak and Wall pioneered the gate theory of pain, advancements in the field of electrical stimulation for pain management have come rapidly. Clinical and scientific analysis has led to a systematic collection of data that has defined the beneficial use of electrical stimulation in the clinical setting.

Electrical Stimulation in Modern Medicine

As a physical modality, electrical stimulation has been prescribed to strengthen muscles, promote healing, decrease urinary incontinence, and increase circulation, as well as for pain management.²

Electrical stimulation is a nonpharmacologic and, in most cases, a noninvasive pain-management method that has been promoted for its analgesic properties. It has demonstrated efficacy for a variety of pain conditions including chronic low back pain,³^–⁴ dysmenorrhea,⁵ hemiplegic shoulder pain,⁶ and arthritic pain.⁴⁰

In all three types of electrical stimulation that will be explored in this chapter, an electrical stimulation device is turned on and electrical current is sent through electrodes that are applied to the body causing a tingling sensation and/or muscle contractions in the underlying skin and muscle. This electrical signal disrupts the regular pain signals that are being sent from the affected area to its surrounding nerves. By interrupting this signal pathway, the recipient perceives less pain (see Fig. 46-2).

The most well known method of electrical stimulation, transcutaneous electrical stimulation (TENS) is a generic term used to describe a type of electrotherapy that applies low-voltage electrical pulses to the nervous system using surface electrodes placed on the skin around the affected area. Because of the variability of frequency, amplitude, voltage, pulse width, and/or pulse rate, TENS has stemmed into several different techniques. By using one or more of these variables, patients and practitioners can be allowed to design four different TENS protocols including conventional TENS, acupuncture mode TENS, burst TENS, and modulation TENS.

Neuromuscular Electrical Stimulation (NMES), sometimes known as Electrical Muscle Stimulation (EMS), is often referred to as functional electrical stimulation (FES) when it is used to activate paralyzed muscles in a precise sequence and magnitude so as to directly accomplish functional tasks.^6,⁸ Beyond its neurorehabilitation potential, NMES has shown efficacy in pain management. NMES uses high-intensity electrical stimulation to elicit intermittent contraction and relaxation of proximal muscle fibers and is widely prescribed following surgery and trauma.

Developed in the early 1950s, interferential current (IFC) therapy produces two alternating currents of slightly differing medium frequency using wave interference to create a resultant amplitude of therapeutic stimulation.⁹ Interferential current therapy is mainly used to relieve pain that is felt in deep tissues. Depending on the configuration of the electrodes applied to the skin, the effects can be localized or adjusted to be more general. Unlike other methods of low-frequency electrical stimulation, IFC encounters low electrical resistance and therefore can penetrate deeply without causing unnecessary discomfort.

Electrotherapeutic modalities are typically administered by physical therapists and physicians in a variety of clinical settings, including private practice, rehabilitation centers, and hospitals. Some units may be used in the home care setting following education by a clinician and those can be obtained directly by the patient or via the health care professional. Electrotherapeutic devices are regulated by the Food and Drug Administration (FDA) but generally require less evidentiary support than a new drug or surgical device for marketing approval.

This chapter will discuss in detail the uses, contraindications, mechanisms, techniques, and safety precautions involved with TENS, NMES, and IFC in pain management. We will also explore its evidence of efficacy and clinical acceptance.

Transcutaneous Electrical Nerve Stimulation

TENS is a method of pain relief in which a device transmits low-voltage electrical impulses through electrodes on the skin to an area of the body that is in pain.

Proposed Mechanisms of Pain Control

Research indicates that TENS can be a noninvasive, safe method for managing and reducing both acute and chronic pain. Although a uniform mechanism has not been established, there are a number of theories to explain the modulation of pain associated with TENS. Two main theories on pain transmission are the gate theory and the endorphin theory.¹⁰ The gate theory pioneered by Melzack and Wall has been attributed to motivating the development of electrotherapeutic equipment, and the endorphin theory has led to a decrease on the reliance of pain medication for treatment of postoperative pain. An additional theory includes the acupuncture theory, which hypothesizes that management of pain is related to energy lines and its associated acupuncture points.

Gate Theory

The gate theory asserts that stimulation of large, highly myelinated (afferent A-beta fibers) block the transmission of pain signals by relatively small, nociceptive fibers with little or no myelination (A-delta and C fibers) at the level of the spinal cord. Small, unmyelinated C fibers are responsible for chronic, throbbing pain, whereas the thicker A-delta fibers with slight myelination make them more conducive to transmitting faster more intense pain information. It is postulated that electrical stimulation decreases the perception of pain by increasing the activation of A-beta fibers and therefore flooding the pain signal pathway and ‘closing the gate’ of transmission in the spinal cord.^11,¹² These target cells are located in the substantia gelatinosa (Rexed’s laminae I, II, and IV) of the dorsal horn (Fig. 46-1).^11,¹³

Figure 46-1 Simplistic view of the gate control theory. Pain signals are blocked at the spinal cord level before they can be transmitted by the thalamus and perceived by the individual.

Also known as the opiate-mediated theory, the endorphin theory is based on the discovery of the presence of natural opiates in the body (Fig. 46-2).^14,¹⁵ Acting as the body’s own natural pain relievers, they are produced in the spinal cord and pituitary gland as enkephalins and beta-endorphins, respectively. These endogenous opiates are effective at decreasing the perception of pain and, in turn, mimicking the action of narcotic drugs. Basic science studies show that high and low-frequency TENS produce their effects by activation of opioid receptors in the central nervous system. High-frequency TENS and low-frequency TENS activate delta-opioid and mu-opioid receptors, respectively, both in the spinal cord and supraspinally (in the medulla). Studies suggest that TENS also stimulates the body’s production of endogenous opiates that interact with specific receptor cites in the central and peripheral nervous systems, thereby blocking the perception of pain.

Figure 46-2 The placebo effect: pain inhibitory network owing to placebo. A descending pain-blocking network that involves the rostral anterior cingulate cortex, the orbitofrontal cortex, the periaqueductal gray, and the pons/medulla are activated. It is believed that endogenous opioids also act along this same pathway.

Cheng and Pomerantz demonstrated that pain relief produced at 4 Hz of stimulation (low frequency) was blocked by the opiate antagonist, naloxone, whereas pain relief induced at 200 Hz was not blocked by naloxone. When administered with a strong, subnoxious intensity at an adequate frequency, TENS has been demonstrated to decrease reliance on pain medication for postoperative pain.¹⁶^–²²

Acupuncture Theory

A lesser-known theory that has been presented as a possible explanation for the ability of TENS to be effective in pain management is the acupuncture theory. Some believe that the use of TENS opens and stimulates common acupuncture points along the same meridians or energy lines used in traditional acupuncture. Acupuncture points can lie on, be adjacent to, or be distant from the site of pain. When applied to these points, TENS is believed to modify the flow of energy or chi resulting in a decrease in pain.^23,²⁴ Some theorists propose that hyperirritable spots in the skeletal muscle that are associated with palpable nodules in taut bands of muscle fibers, known as trigger points, can be stimulated by TENS to successfully decrease pain. It is believed that trigger points cause tissue ischemia and that the application of TENS causes vasodilation to occur, which modifies the ischemic area, thereby decreasing pain.²⁵^–²⁸

Practical Application

Electrode Placement for TENS

Electrodes may be placed over peripheral nerves, nerve roots, and acupuncture points, as well as proximal to, distal to, over, and (more controversially) contralateral to the areas of pain. The usual practice is to apply the electrodes to where the pain is felt. It can be applied with a dermatome, sclerotome, or myotome where the pain is felt, or even over a trigger or acupuncture point.

Clinical Use and Application

There are two basic types of units available. Marketed to consumers, the simplified version offers LCD screens and digital control with a range of preset programmers. The more complex units are the professional versions that do not have presets but allow frequency and pulse width settings to be varied to suit individual patient needs.

There are four main types of stimulation (Table 46-1) namely: conventional mode TENS, acupuncture mode TENS, burst or pulse mode TENS, and modulation mode TENS.

Table 46-1 Comparison of Various TENS Modes