Author Response

[Editor's note: Both the letter to the editor by Bergeron-Vezina and Leonard and the response by Sluka and colleagues are commenting on the page proof version of the article that was published ahead of print on July 11, 2013.]

We thank Bergeron-Vezina and Leonard for their thoughtful comments¹ on our article.² They have brought up important additional and potentially confounding factors that can affect the efficacy of transcutaneous electrical nerve stimulation (TENS). Interactions with drugs such as caffeine—or, as we point out, with opioids—is an area that needs additional study. For example, noradrenergic agonists, such as clonidine, are synergistic with opioid agonists,³ and preclinical studies show synergism with TENS.⁴ On the other hand, low-frequency TENS also uses serotonin to produce its analgesic effects,⁵ and use of low-frequency TENS in patients who are on reuptake inhibitors might improve analgesic effectiveness. Thus, understanding the known mechanisms of action of TENS will help to determine whether some drugs interfere with, or whether others can enhance, the efficacy of TENS.

Electrode placement is indeed important and is greatly understudied. There are many ways to place electrodes, including over nerves, in dermatomes, at acupoint sites, and surrounding the area of pain. Basic science studies show the greatest effect of TENS on reducing the activity of spinally located pain transmission neurons when the electrodes are placed within the receptive field.⁶ Preclinical studies also show that placement of electrodes over areas of pain, regardless of whether they were the originating site or a referred site, is equally effective.⁷ For human control participants who are healthy, the greatest analgesia typically occurs within the paresthesia site.⁸ Together, these data suggest and further support Bergeron-Vezina and Leonard's suggestion that the greatest effects of TENS occur when the sensation of paresthesia covers the painful region.

We appreciate the comments about translation of these findings to physical therapist practice and agree that for TENS to be adequately and effectively used in the clinic, numerous factors outlined by us and by Bergeron-Vezina and Leonard should be considered.

• © 2013 American Physical Therapy Association