Exploring The Evidence For Using TENS Unit To Relieve Pain
As far back as 2500BC, stone carving depict a species of catfish with organs that produce electrical charge being used to treat pain. The physician to the Roman Emperor Claudius in AD46 was the first to document that standing on an electric fish could relieve some symptoms of pain.
Several studies have shown an improve- ment in the rate of wound healing when electrodes have been applied locally around sacral and leg ulcers). It has been hypothesised that this could be due to TENS Unit inducing vasodila- tion or a possible inhibition of sympa- thetic impulses by the release of brain endorphins.
A systematic review evaluating the effec- tiveness of TENS Unit in chronic pain found that in 13 of 22 inactive control (placebo) studies, there was a positive analgesic out- come in favour of active TENS device treatments. However, for multiple-dose treatment comparison studies, only eight out of 15 were in favour of active TENS treatments.
Lower back pain: the National Institute for Health and Clinical Excellence (2009) recommended that TENS Unit should not be offered for the early management of per- sistent non-specific low back pain that has lasted for less than a year. In addition, a systematic review concluded there was level-A evidence that TENS should not be recommended for chronic lower back pain, according to levels of evidence from the NHS R&D Centre for Evidence-Based Medicine.
A Cochrane review could identify only four RCTs that the authors considered methodologically suitable for review, and concluded that the evidence did not sup- port the use of TENS machine in the routine man- agement of chronic lower back pain.
Osteoarthritis: Rutjes et al reviewed 18 trials of TENS machine use for knee pain and concluded there was a lack of adequate evidence to support the use of any type of transcutaneous electrostimulation in patients with knee OA.
Neuropathic pain: the European Federation of Neurological Societies pub- lished guidelines following a review of the available literature by Cruccu et al , which concluded that “standard high- frequency TENS Unit is possibly better than placebo although prob- ably worse than acupuncture-like or any other kind of electrical stimulation ”.A systematic review by Dubinsky and Miyasaki concluded there was level-B evidence that TENS Unit should be con- sidered specifically to treat painful dia- betic neuropathy.
Quality of evidence
The majority of authors reviewing TENS Unit trials, for all types of pain, concluded there were great limitations to their review find- ings in terms of methodological issues.
Bennett et al looked at the meth- odological quality of RCTs using TENS Unit and argued that several aspects of study design throughout the literature could lead to bias towards a result of “no effect” including:
-Small sample size – often fewer than 50 patients;
-Lack of blinding of patients, therapists and outcome assessors;
-Inadequate TENS intervention
-Incorrect electrode placement, and inadequate intensity and treatment duration;
-Inadequate assessment of compliance with treatment in home trials;
-Poorly defined outcome measures – the effect of TENS Unit was often not measured while it was in use;
-Inconsistency around permissible concurrent analgesics;
-A variety of disease states and condi- tions are often looked at within a category, such as neuropathic or low back pain, which are difficult to compare;
-Lack of instruction to patients about self-administration and expectation of sensation – active TENS Unit produces electrical paraesthesia whereas placebo TENS does not and it is obviously difficult to blind study participants to this. To overcome this, Bennett et al suggested informing patients that some TENS machines do not produce sensation during stimulation
The literature available on the use of TENS Unit for pain relief often provides conflicting evidence, perhaps due in part to the inherent problems with trial design.
This gives rise to a dilemma: in a climate where evidence-based medicine pre- vails, should the decision to use TENS Unit be based solely on research findings or, instead, should clinical experience, exper- tise and anecdotal patient report be trusted? As Binder and Baron argued, an “absence of evidence is not [always] evidence of absence” and went on to say that “there seems to be considerable empirical evidence that, at least in some patients, TENS Unit is useful”. Box 1 gives details on contraindications and cautions.
Despite the lack of robust evidence from RCTs on the efficacy of TENS Unit, it is still useful to consider trying it as an additional method of pain management, as it has no side-effects and therefore a “favourable benefit-risk ratio”.