fied as 8-cm, which was considered strong, although not the strongest, probably in accordance to others [7] .

Among the mechanisms involved, TENS efficacy appears to be mediated by the release of mu- or delta-opioids [3] , and involve its ability to increase the vibration threshold probably due to distraction or antidromic block of large-diameter nerve fibres [10] . In addition, TENS analgesia appears to be caused mainly by differentially blocking the activation of large diameter primary afferents from deep somatic tissues, and not cutaneous afferents [11] . High frequency-TENS, as the device we used, was described capable of inducing analgesia, which was most likely related to increased serotonin release and also to block the adverse cardiovascular and respiratory changes induced by pain [12] . A superior mechanism of action was also identified through the periaqueductal gray that sends projections through the rostroventral medial medulla to the spinal cord to produce an opioid-mediated analgesia [13] .

In the actual study, all patients used the TENS device for three-days, twice daily, during 20-min, where patients from the TG applied 85 Hz frequency, wave duration of 75 µs and intensity of up to 30 mA. Tolerance to the TENS have been suggested before, although with much higher currents with pulse duration (110 μs), and pulse frequencies in healthy volunteers, during two consecutive days. However the authors found no ideal frequency versus time interaction and time-dependent for the outcome measures [14] . Other authors demonstrated that in rats, repeated administration of modulating frequency TENS lead to the development of opioid tolerance, which was delayed by approximately 5 days after low- or high-frequency TENS independently (3). Because we used the device for only three days with good results, we did not expect any tolerance effects development.

Recent trial revealed a better therapeutic effect obtained by combining traction with TENS in cervical spinal pain [15] . However neither neck manipulation, wearing neck collar, nor neck rehabilitation exercises were applied in the actual study during the one-month evaluation, in order to not interfere with the final results. Manipulation of miofascial trigger points were also not done as interactive neurostimulation therapy associated with myofascial trigger points in adults with mechanical neck pain was demonstrated effective [16] or at least similar when comparing manual therapy to TENS [17] . Interesting, both manipulation and mobilisation presented similar results for every outcome at immediate/short/intermediate-term follow-up after neck pain [18] . Again, no manual neck handling was included in the study protocol, in order to avoid bias.

Although the active TENS device was extremely effective in 20 of 22 patients, two patients applied the device with such an increased intensity that caused cervical muscles contraction and pain. Any TENS device is always supposed to be applied with enough intensity to obtain paresthesia only which recruits β-fibers [19] . When the amplitude is sufficient to Aα-fibers recrutation [19] the resulting muscle contraction would act as pain generator. These patients continued the evaluation up to one month and complained of worse pain, although the device was correctly applied on the second and third days. Another interesting aspect is the itching sensation after TENS, observed by three patients in the TG, as described by others [20] , although, controversially, TENS was also used to treat pruritus [21] , with unfortunately unknown mechanisms.

Regarding the capability of: 1) neck rotation, 2) neck lateral extension, 3) neck retroflexion, and 4) capacity of performing routine physical activities, all patients who correctly applied the TENS device at C7-T1 improved, in accordance to others [4] . Few patients with cervical pain may complain of dizziness, what was not our case because of the exclusion criteria. TENS was found to influence cardiovascular responses by alleviating sympathetic activity [11] [22] to enhance visuospatial abilities and postural control [23] , what would benefit patients. In our study, device was applied over C7-T1 dorsal spine, as the location of TENS electrodes is crucial for obtaining the strongest pain relief [22] .

6. Conclusion

In conclusion, the TENS device induced gradually pain relief after its first application, which persisted during the three-day treatment and also decreased the concurrent use of analgesic tablets. Its use resulted in effective treatment alternative for nociceptive somatic cervical/neck pain, with no serious adverse effects when correctly applied, and because its characteristics of disposable and light, it could be used by the patient while doing their daily activities.

Acknowledgements

The authors thank Medecell Brasil-Brazil for Tanyx and placebo donations. Other costs of the study were supported by the Center for Pain Management-HC-FMRP-USP. Teaching Hospital, School of Medicine of Ribeirão Preto, University of São Paulo, Brazil.

Cite this paper

Lauretti, G.R., An- tunes, M., Zuccolotto, V.M.R. and Franco, R.C. (2016) Improvement of Somatic Cervical Pain and Disability after the Application of a Portable TENS Device. J. Biomedical Science and Engineering, 9, 451-459. http://dx.doi.org/10.4236/jbise.2016.910041

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