Hieroglyphs dating as early as 3100 BC depicted electrogenic catfish as a method practiced by Ancient Egyptians to potentially treat ailments. In the first century AD, Roman Scribonius Largus used electricity from a large Mediterranean electric ray, Torpedo marmorata, to treat headaches and gout pain. Greek physician Galen also wrote that this fish may have been the best remedy known for epilepsy at the time. Electrostatic generators increased the use of electricity in medicine in the early 1900s, but lack of portability and increased focus on pharmacologic therapy lead to decreased use for pain management [25]. In 1965, Melzack and Wall published a paper summarizing literature on pain pathways, theorizing a gate-control system for pain perception (see Fig. 14.6) mediated by both peripheral and central input that can be modulated for analgesia by decreasing small fiber nerve input (Aδ) or activation of large fiber input (Aβ) [26]. Wall and Sweet went on to test this theory in humans and found that chronic pain could be modulated using high-frequency percutaneous electrical stimulation to activate large-diameter peripheral afferents (Aβ) to produce focal analgesia [27]. Central analgesic pathway in rats was demonstrated by Reynolds et al. with direct focal electrical stimulation of the periaqueductal gray region of the midbrain [28]. In 1967, neurosurgeon Dr Norman Shealy performed one of the first dorsal columns stimulation procedures in a human case report, showing this was a potential analgesic benefit, human application, and safety [29]. In 1974, the first portable TENS machine was patented in the United States by Don Maurer, one of the founders of the neurological division of Medtronic [30]. TENS was used as a predictive test in patients to determine the potential for success of dorsal column stimulation implants; however, many of these patients found relief with the TENS modality without necessity of an implant [31].

Animal studies initially showed TENS analgesia to be mediated through both peripheral and central mechanisms. Centrally, the dorsal column of the spinal cord and brainstem utilizes opioid, serotonin, and muscarinic receptors to mediate TENS analgesia, while peripherally opioid and α-2 noradrenergic receptors are involved [32]. The central mechanism of TENS activation was shown in animal studies through reduction of hyperalgesia with the application of TENS contralateral to the site of injury [33, 34]. TENS-induced small-diameter afferent (Aδ) activation causes long-term depression of central nociceptive cell activity for up to 2 h mediated by the activation of the midbrain periaqueductal gray and rostral ventromedial medulla (i.e., descending inhibitory pathways) and inhibition of descending pain facilitatory pathways [35].

Peripherally, the blockade of cutaneous afferents with EMLA cream (lidocaine and prilocaine emulsion) during TENS application had no effect on analgesia after induction of painful stimuli into the knee joint of rats, showing the importance of deep-tissue afferents at the site of TENS application (see Fig. 14.7). However, when local anesthetic was injected into the knee joint during TENS application, there was a complete blockade of the analgesic effects of TENS, showing TENS analgesia was mediated by large-diameter primary afferents from deep somatic tissues [36]. Human studies on pressure pain threshold at the dorsal interosseous muscle using an electronic algometer found that maximal TENS-induced hypoalgesia is obtained when high-intensity, “strong but comfortable” current is applied regardless of frequency [37].

Patient experience and satisfaction suggest that TENS is useful as an analgesic modality as solitary treatment for mild to moderate pain or in combination with pharmacotherapy for moderate to severe pain. Systematic reviews of randomized controlled trials (RCT) on the clinical application of TENS as an analgesic modality for various types of pain have been inconclusive. This section summarizes modern literature on TENS as an analgesic intervention.

An RCT of 100 women undergoing major gynecological, lower abdominal surgery with a standardized general anesthetic technique showed that TENS decreased postoperative opioid analgesic requirements and opioid-related side effects when utilized as an adjunct to patient-controlled analgesia (PCA) after lower abdominal surgery. The use of TENS at mixed (2- and 100-Hz) frequencies of stimulation produced a slightly greater opioid-sparing effect than either low- (2-Hz) or high (100 Hz)-frequencies alone [38].

Despite an earlier systematic review that showed no significant benefit from TENS on postoperative pain, a subsequent meta-analysis of 21 RCTs published in 2003, which included 11 trials and a total of 964 patients, found a mean reduction in analgesic consumption of 35.5 % (range 14–51 %) better than placebo. Of note, inclusion criteria for TENS stimulus parameters for this meta-analysis were TENS titration to optimal treatment, defined as “strong, sub noxious electrical stimulation with adequate frequency at the site of pain.” The median frequency of TENS usage from this study was 85 Hz. Patients in this meta-analysis were assumed to have free access to additional analgesics for supplementation to achieve a tolerable level of pain intensity [39].

TENS electrodes are applied to the lower back over T10–L1 dermatomes 1.5–3 cm lateral to the spinous process bilaterally for the first stage of labor with a second set of electrodes placed over the S2–S4 dermatomes for the second stages of labor. Despite studies that have shown maternal satisfaction [40, 41] with TENS versus placebo, a systematic review of seven RCTs with a total sample size of 1168 on TENS for labor pain did not support the theory that TENS significantly provides more analgesia than placebo (sham TENS). It is worth noting that TENS was noted to show no mention of significant effect on fetal monitoring during this study [42].

In a case series on patients with primary dysmenorrhea, 102 nulliparous women previously being treated with pharmacologic agents were additionally treated with an experimental TENS device (Freelady, Life Care, Tiberias, Israel) to the lower abdomen: 56.9 % reported marked pain relief, 30.4 % reported moderate pain relief with the addition of TENS intervention, 58 % of patients reported self-discontinuation of other analgesic interventions, and 31 % reported decreased use of other analgesics. This study had no placebo control and was not randomized [43].

A Cochrane meta-analysis published in 2002 included 7 RCTs involving 164 women with primary dysmenorrhea high-frequency/conventional TENS, 50–120 Hz, was shown to be more effective for pain relief than placebo TENS (odds ratio (OR): 7.2; 95 % confidence interval (CI): 3.1–16.5) whereas low-frequency TENS (1–4 Hz), also referred to as acupuncture-like TENS, was found to be no more effective in reducing pain than placebo TENS (OR: 1.48; 95 % CI: 0.43–5.08) [44].

A Cochrane meta-analysis published in 2015 included 6 of 19 analyzed RCTs (1346 patients) on the effects of TENS versus placebo (sham TENS without current) as a sole treatment for acute pain. The types of acute pain included in this Cochrane review were procedural pain, for example, cervical laser treatment, venipuncture, screening flexible sigmoidoscopy, and nonprocedural pain, for example, postpartum uterine contractions, and rib fractures. The analysis provides tentative evidence that TENS reduces pain intensity greater with placebo (no current) TENS when administered as a stand-alone treatment for acute pain in adults. The authors noted that the quality of evidence was weak due to a high risk of bias associated with inadequate sample sizes and unsuccessful blinding; however, TENS should still be considered as a treatment option solely or in combination with other treatments for acute pain [45].

Nonrandomized controlled clinical trials have found benefit for many types of chronic pain but systematic reviews remain inconclusive for chronic pain. Meta-analyses for chronic musculoskeletal pain suggested that TENS is superior to sham TENS for pain and stiffness and patients may need to administer TENS throughout the day to achieve best effects [46]. A Cochrane meta-analysis on RCTs with consideration of TENS use for chronic neuropathic pain is pending investigation and additional studies for adequate powering [47].

A Cochrane qualitative synthesis of four high-quality RCT studies with 585 patients concluded that the evidence from the small number of available placebo-controlled trials does not support the use of TENS in the routine management of chronic low back pain. Consistent evidence in two trials (410 patients) showed that TENS did not improve back-specific functional status. There was moderate evidence that work status and the use of medical services did not change with TENS treatment [48].

A Cochrane review of three RCTs (88 participants) showed no significant differences between TENS and placebo in women with chronic pain secondary to breast cancer or bone pain related to different types of cancer treatment. In the other RCT, there were no significant differences between acupuncture-type TENS and sham in palliative-care patients. The review concluded that there remains insufficient evidence to judge whether TENS should be used in adults with cancer-related pain and that additional large multicenter RCTs are necessary to assess the value of TENS for cancer-related pain [49].

In a retrospective cohort of 100 patients at a urogenital pain clinic at The Institute of Urology and Nephrology in London, TENS was found to have benefit in some patients with interstitial cystitis, testicular pain, and dysmenorrhea. [50] No RCTs or reviews were found on literature review.

Origins of Yoga techniques date back to stone carvings found at archaeological sites in the Indus Valley dating back more than 4000 years, predating written history [52]. Since this time, the tradition of Yoga practices has evolved as they have passed from teacher to student through direct instruction and demonstration. The Vedas is the sacred scripture of Brahmanism dating to the twelfth to tenth centuries BCE that is the basis of modern-day Hinduism, which may contain the oldest documented Yoga teachings [53]. Later, around 500 BCE the Bhagavad‐Gita was a scripture dedicated to yoga and has served as a philosophical inspiration for Hindu religion. Buddhism and Hinduism incorporate aspects of yoga and meditation as a means to quiet the mind and create a sense of space in the body, striving to achieve enlightenment through transcendence of the limitations of the mind [54].

Patanjali, considered “the father of yoga,” compiled oral traditions into “The Yoga Sūtras” around 400 CE creating the collective foundation of what is now Classical Yoga or “Ashtanga” (eight-limb) Yoga, providing guidance on how to master the mind to achieve spiritual growth [51]. According to the American Yoga Association, Yoga was first practiced in the USA in the 1800s by a select few but was not commonly practiced until the 1960s when interest grew for Eastern philosophy and culture. Americans travelled to India to learn Yoga and invited yogis to the USA such as B.K.S. Iyengar and Pattabhi Jois to teach yoga workshops [55]. Today, over 100 different schools of yoga are now practiced worldwide. Most of these schools fall under the “Hatha Yoga” umbrella, which emphasizes Asana [posture] and Pranayama [breath control] techniques sometimes practiced without the broader philosophic focus of the other more spiritual limbs of yoga [51]. Yoga is now commonly performed in western culture without religious intention [56]. Instead, yoga is often practiced as a preventative health measure with benefits such as reducing stress and improving overall physical fitness, strength, and flexibility, but it can also have deleterious health effects if practiced improperly. Table 14.2 summarizes some yoga positions that have shown benefits for urogenital pain conditions. While some schools of yoga incorporate rapidly transitioning, self-guided poses, Iyengar Yoga is a particular school of yoga that may be better suited for chronic pain patients, offering customized poses and use of props such as blankets, belts, blocks, and chairs to assist students in optimizing benefits while preventing injury. Iyengar Yoga emphasizes individualized attention and emphasis on the alignment and precision of postures for particular pain pathologies. One example of Iyengar poses recommended for urogenital pain is shown below in Fig. 14.8, which depicts extended triangle pose, suggested to tone abdominal and pelvic organs and help relieve menstrual pain. This form of yoga also has the most robust certification process in the USA, requiring a minimum of 3 years of continuous training before one can begin the mentoring and teaching education process required to apply for certification to teach Iyengar Yoga [57].