Application of low frequency and medium frequency currents in the management of acute and chronic pain-A narrative review
Correspondence Address: Source of Support: None, Conflict of Interest: None DOI: 10.4103/0973-1075.150203
Source of Support: None, Conflict of Interest: None
Trancutaneous electrical nerve stimulation (TENS) and interferential therapy (IFT) have been a regular line of treatment for various types of acute and chronic pain. This review aims to compile the latest literature in pain management using these modalities which use low-frequency and medium-frequency currents. The Cochrane Library, Scopus, PubMed, MEDLINE, and CINAHL were searched and studies were examined from their inception till October 2013. After title and abstract screening the relevant studies were included for this review. We found through this review that even though TENS and IFT are used in management of pain, there is limited amount of high quality research available in this area. Most of the studies lack methodological quality and have a low sample size.
Keywords: Acute pain, Chronic pain, Electro physical modalities and electrotherapy, Interferential Therapy, Low-frequency currents, Medium frequency currents, Pain, Transcutaneous electrical nerve stimulation
Evidence-based practice is essential in clinical practice to hasten the recovery of a patient. In electrotherapy the applied energy is the trigger that stimulates or activates physiological events, which achieve therapeutic benefits that bring about pain relief.  In this review we would mainly focus on Transcutaneous Electrical Nerve Stimulation (TENS) and interferential therapy (IFT) which use low- and medium-frequency currents, respectively for pain relief.
The following databases were searched by the reviewers from their inception till October 2013, The Cochrane Library, Scopus, PubMed, MEDLINE, and CINAHL. After title and abstract screening randomized controlled trials and systematic reviews that compared active TENS/IFT in acute/chronic pain relief were included in this narrative review.
Transcutaneous electrical nerve stimulation
TENS relieves pain by inhibiting pain-related potentials on the spinal and supraspinal level, known as "gate control." It is alternating current (AC) or modulated DC, comprising rectangular impulses. The analgesic effects of TENS is seen in both the ipsilateral and contralateral spinal segmental regions. ,
Interferential therapy involves the use of "medium frequency" current to bring about the effect of a low-frequency (LF) current in the tissues. This is achieved by applying two "medium frequency" currents to the tissues, to generate LF interference current. Thus, the benefits of LF stimulation are achieved without the associated unpleasant side effects like pain, discomfort, skin irritation, etc. ,
The history of using electric currents to treat pain goes back to 2500 BC where some stone carvings depict a species of catfish with organs that produce an electrical charge used to treat pain. The physician to the Roman Emperor Claudius in AD46 claimed that standing on an electric fish could relieve symptoms of pain. 
0The gate-control theory
Melzack and Wall in 1965 published the gate-control theory with which increased the use of electroanalgesia.  This theory hypothesized that activity in small diameter nerve fibers causes pain and that, by stimulating the larger-diameter sensory nerve fibers, the perception of pain is reduced. They proposed that a physiological gating mechanism exists in the dorsal horn of the spinal cord. This "gate" can be opened or closed to allow or inhibit the transmission of painful stimuli through it, and up to the brain where it is processed. By selectively exciting A-beta nerve fibers in the skin with TENS, the amount of painful stimulation being transmitted by smaller diameter nerve fibers can be reduced, through segmental inhibition. ,
High frequency or conventional TENS (90-130Hz)
High frequency (HF) or conventional TENS (90-130Hz), causes the pain gate to close by stimulating the small A-beta sensory nerve fibers. Conventional TENS also acts by reducing the release of excitatory neurotransmitters such as aspartate and glutamate, increasing the release of inhibitory neurotransmitters such as gamma-aminobutyric acid (GABA) and serotonin. 
"Low frequency" or "acupuncture" TENS (2-5Hz)
LF TENS is also known as acupuncture TENS has a low pulse rate. It acts by stimulating the A-delta nerve fibers to produce endorphins which in turn relieve pain. 
Burst TENS stimulates both the A-beta and A-delta nerve types at the same time. In "burst" mode, conventional (HF) TENS is regularly interrupted by 2-3 "bursts" of lower frequency TENS. Different programs can be used interchangeably according to the preference of the patient. 
Low frequency TENS vs. high frequency TENS
Kocygit et al. in 2012 published a randomized controlled trial which compared 20 patients with subacromial impingement with patients being randomized into low TENS and Sham groups. Both the groups were given painful stimuli before and after TENS treatment. They found on functional magnetic resonance imaging (MRI) that in the LF TENS group, there was a statistically significant decrease in the perceived pain intensity and pain-specific activation of the contralateral primary sensory cortex, bilateral caudal anterior cingulate cortex, and of the ipsilateral supplementary motor area. They also reported in their results a statistically significant correlation between the change of Visual Analog Scale (VAS) value and the change of activity in the contralateral thalamus, prefrontal cortex, and the ipsilateral posterior parietal cortex. It was reported that the sham TENS group had no significant change in VAS value and activity of regions of interest. 
The results of this study support the efficacy of LF TENS in acute pain management. Although the sample size is low, MRI is a reliable tool in measuring the pain perceived by the individual.  Santos et al. in 2013 published a study done on rat paws wherein hyperalgesia and edema was induced by administering serotonin (5-HT). They applied LF and HF TENS on the right paw for 20 mins followed by serotonin induction. They used the Hargreaves method to measure nociception while the hydroplethysmometer was used to measure edema. Hargreaves method measures cutaneous hyperalgesia to thermal stimulation in animals. This study reported that neither HF nor LF TENS inhibited 5-HT-induced edema. However, LF TENS, but not HF TENS, completely reduced 5-HT-induced hyperalgesia. Pre-treatment of the paw with naltrexone, prior to application of TENS, showed a complete blockade of the analgesic effect induced by LF TENS.
This study supports the participation of peripheral endogenous opioid receptors in LF TENS analgesia in addition to its central action. 
Length of pain relief
LF TENS takes a longer time to achieve analgesia. Since the analgesia produced by the application of LF TENS is due to the release of endogenous opioids it lasts for a longer time. HF TENS or conventional TENS has a quick onset of analgesia but loses its effect quite rapidly when turning off the stimulation. The post-treatment analgesic effects of TENS can thus last anywhere between 5 minutes to 18 hours. It has been reported that in some patients' pain levels do not return to pre-stimulation levels even after 24 hours.
Post-stimulation analgesia has been widely attributed to the accumulation or depletion of endogenous opioids. There is a wide variation in post treatment pain relief experienced by patients and no reason for this has been documented yet. Cheing et al. reported a cumulative effect in pain reduction after repeated applications of TENS and suggested that the mechanisms underlying this may be related to changes in the neuronal pathway. Breaks between sessions and/or changing electrode positions is essential as with long-term use, the nervous system becomes habituated to TENS, causing poorer pain control. 
We have given a brief description of important studies that used TENS for pain relief in [Table 1], while in [Table 2] we have highlighted studies that focused on Low Frequency TENS.
Very few systematic reviews have examined the use of TENS for postoperative pain management. Out of 17 randomized controlled trials analyzed by Carroll et al. in a systematic review which included studies with pain outcomes, 15 concluded that TENS had no analgesic benefit in the acute postoperative period.  A systematic review conducted by Reeve et, included 20 studies of postoperative pain, and concluded that 12 of these had positive TENS outcomes. 
A Cochrane review published in 2009 excluded studies that allowed additional analgesics. The authors could extract data from only six of the 12 Randomized Controlled Trials RCT s that met their inclusion criteria. This review reported that only one out of five studies comparing TENS with placebo showed a statistically significant superior effect of active TENS. Owing to insufficient data this review could not come to a definitive conclusion about the effectiveness of TENS as a sole treatment for acute pain. 
Nnoaham and Kumbang in a cochrane systematic review evaluated the effectiveness of TENS interventions in chronic pain. Out of the 22 inactive control (placebo) studies they reviewed, 13 had a positive analgesic outcome and favored active TENS treatments. Whereas out of the 15 multiple-dose treatment comparison studies reviewed by them, only eight favored active TENS treatments. 
Cruccu et al. in a systematic review recommended the use of standard high-frequency TENS when compared with placebo treatment for neuropathic pain. This review also suggested that TENS should be considered specifically to treat painful diabetic neuropathy. This review was endorsed by the European Federation of Neurological Societies guidelines for the treatment of neuropathic pain. 
Carbonario et al. in 2013 in a clinical trial on 28 patients concluded that TENS as an adjuvant therapy is effective in relieving pain in fibromyalgia. 
IFT-Mechanism of action
It is suggested that by adjusting the frequency produced in the interference zone, it is possible to influence a range of different nerves. By changing the type of nerve which is primarily stimulated, the physiological outcome of the stimulation is modified, and hence, so is the therapeutic outcome. Frequencies can be utilized which primarily activate motor nerves, resulting in a muscle stimulation ranging from LF twitching (<15 Hz) to a tetanic, sustained contraction (>40 Hz) each of which have their therapeutic uses. 
There is at present, no evidence to suggest that muscle stimulation with electrical stimulation is anymore (or less) effective than by active exercise, but it can be utilized as a means of ensuring the muscle activity level is raised. This in turn will influence the local blood flow as a normal physiological response to an adjusted metabolic rate. Frequency ranges from 1-150 Hz or more can be employed in this respect, though it is suggested that clinically, the most appropriate ranges are between 10 and 20 or 25 Hz. At the lower end of this scale, a rapid muscular twitching will be produced, whilst at the upper end, a partial tetany will result. Using appropriate frequencies, sensory nerve stimulation can be achieved, thereby producing a mechanism to activate the pain gate (e.g. between 80-130 Hz) and opioid (<10 Hz) mechanisms which are associated with physiological pain relief. 
Fuentes et al. in 2010 in a systematic review and metanalysis reported that IFT was found to be effective in treating various musculoskeletal conditions. 
Ward et al. in 2009 in a single-blinded, within-group crossover study reported that medium frequency current is as effective as TENS in decreasing pain threshold. 
In [Table 3] we have highlighted some important studies that used IFT(Medium Frequency Current) for pain relief.
The published literature in this area is lacking in quality and there is a need for studies with good methodological quality in this area. In our clinical practise we have seen the benefits of TENS and IFT as an adjunct in pain relief in different conditions and in some cases like musculoskeletal pain a primary modality in pain relief. But this clinical experience should be supplemented by high quality research in this area.
Implacations in palliative care
Since the palliative care primarily revolves around giving pain relief, TENS and IFT could be of great benefit. They could be used as adjuncts or alternatives to pharmacological pain management. There is a need for research to complement the use and efficacy of these modalities in palliative care.
There is a plethora of evidence available to support the use of TENS and IFT of various frequencies in pain relief. Further research with more randomized controlled trials and studies with better methodological quality are warranted.
[Table 1], [Table 2], [Table 3]