transcutaneous electrical nerve stimulation ( tens ) is widely used in western and developed countries to relieve a wide range of painful conditions , including non - malignant acute and chronic pain and pain resulting from cancer and its treatment [ 13 ] .
tens can be self administered by patients following simple training and because there is no potential for toxicity , patients can titrate the dosage on an as - needed basis . during tens
pulsed electrical currents are generated by a small battery operated tens device that can be kept in the pocket or attached to the user 's belt .
currents from the tens device are delivered through the skin by two self - adhering electrode pads ( figure 1 ) . a standard tens device .
maximal pain relief is achieved when tens generates a strong non - painful electrical sensation beneath the electrodes .
pain relief is usually rapid in onset and stops shortly after tens is turned off .
for this reason patients are encouraged to deliver tens for as long as needed , which may be for hours at a time and throughout the day .
, tens devices can be purchased without prescription , although this is not the case in some european countries .
tens devices , including electrode leads , pads and battery , retail for approximately  30gbp although bulk buying can markedly reduce cost .
interestingly , tens does not appear to be widely available for patient use in developing countries . in this review
the basic science behind tens will be discussed , the evidence of its effectiveness in specific clinical conditions will be provided and a case for its use in pain management in developing countries will be made .
the ancient egyptians are usually acknowledged as the first people who used electrogenic fish to apply electricity for pain relief . yet , the first documented use of this kind of pain relief is of a roman physician in 46 ad . in 1786 , luigi galvani , an italian doctor , demonstrated that the leg of a frog contained electricity . this observation and other advancements in generating electricity lead to a resurgence in the use of electricity to treat different illnesses and relieve pain . however , increased use of pharmacological agents to manage pain resulted in the decline of the electrotherapy at the end of the 19th century . in 1965 , ronald melzack from mcgill university in montreal canada and patrick wall from university college london uk , published their seminal paper which proposed a gating mechanism in the central nervous system to regulate the flow of nerve signals from peripheral nerves en - route to the brain . according to this gate - control theory of pain , activity in large diameter low threshold mechanoreceptive ( touch - related )
nerve fibers could inhibit the transmission of action potentials from small diameter higher threshold nociceptive ( pain - related ) fibers through pre and post synaptic inhibition in the dorsal horn of spinal cord .
because nociceptive fibers ( a - delta and c - fibers ) have a higher threshold of activation than mechanoreceptive fibers ( a - beta fibers ) melzack and wall proposed that it would be possible to selectively stimulate mechanoreceptive fibers by titrating the amplitude of electrical currents delivered across the surface of the skin ( ie tens ) .
this would prevent signals from nociceptive fibers from reaching higher centres of the brain , thus reducing pain ( figure 2 ) .
diagrammatic representation of the principle of conventional tens . by selectively activating a - beta fibers ,
tens shuts the pain gate on a - delta and c fibers preventing pain - related signals reaching the brain .
in addition to interrupting nociceptive signals , at spinal cord dorsal horn , we now know that tens analgesia also involves a descending inhibitory mechanism that could be partially prevented by spinalization .
activity in nerve fibers descending from the brain can also block onward transmission of peripheral nerve signals within the spinal cord .
humans utilise this mechanism whenever they mentally distract themselves to not feel pain despite the presence of tissue damage ( figure 2 ) evidence gathered from animal studies suggested that low frequency tens effects may be due to release of endogenous opioids .
this explains why analgesia may persist for hours after electrical stimulation has stopped because endorphins have long lasting effects in the central nervous system .
the released opioids may generate their analgesic action at peripheral , spinal and supraspinal sites [ 7 , 8 ] .
however , other neurochemicals have been implicated in tens analgesia including gaba , acetylcholine , 5-ht , noradrenaline   and adenosine .
in health care the term tens refers to the delivery of currents using a  standard tens device ( table 1 ) . however , there are a variety of devices that deliver electrical currents through the skin but have technical output characteristics that differ from a standard tens device .
we have previously described these as  tens - like devices  and include trancutaneous spinal electroanalgesia , interferential therapy , microcurrent stimulation and pain gone pens ( see [ 3 , 14 , 15 ] for a review of these devices ) .
low - intensity pulsed currents are administered at high - frequencies ( between 10200 pulses per second , pps ) at the site of pain . the user experiences a  strong , non - painful tens sensation often described as  tingling or pleasant  electrical paraesthesiae. physiologically , conventional tens activates large diameter non - noxious afferents which has been shown to close the pain gate at spinal segments related to the pain .
another technique , which is used less often is acupuncture - like tens ( al - tens ) using high - intensity and low - frequency ( less than 10pps , usually 2pps ) administered over muscles , acupuncture and trigger points .
the purpose of al - tens is to activate small diameter afferents which has been shown to close the pain gate using extra - segmental mechanisms .
tens can also be used as a counter - irritant , termed intense tens , using high - intensity and high - frequency currents ( table 2 , figure 3 ) .
the user can control the amplitude ( intensity ) , duration ( width ) , frequency ( rate ) and pattern ( mode ) of the pulsed electrical currents .
in western clinical practice tens has been shown to have a role in pain management .
there are many systematic reviews on tens although evidence is often inconclusive because of shortcomings in rct methodology .
early systematic reviews suggested that tens was of limited benefit as a stand alone pain therapy for acute pain .
judged there to be no benefit of tens for postoperative pain because 15 of 17 rcts found no differences in pain relief between active and placebo tens .
re - assessed the evidence and concluded that tens reduced post - operative analgesic consumption if tens was applied using adequate tens technique .
systematic reviews have also concluded that there was no evidence for tens producing beneficial analgesic effects during childbirth [ 19 , 20 ] and insufficient evidence to determine the effectiveness of tens in reducing pain associated with primary dysmenorrhoea .
rcts suggest that tens is effective for acute orofacial pain , painful dental procedures , fractured ribs and acute lower back pain ( for review see .
previously , systematic reviews suggested that tens may be of benefit for chronic pain but definitive conclusions were hindered by shortcomings in rct methodology [ 23 , 24 ] .
reviews on rheumatoid arthritis of the hand , whiplash and mechanical neck disorders , chronic low back pain , poststroke shoulder pain   and chronic recurrent headache   were inconclusive .
in contrast , systematic reviews have demonstrated tens efficacy for knee osteoarthritis   and chronic musculoskeletal pain .
rcts have also demonstrated effects for a range of other chronic pain conditions including localized muscle pain , post - herpetic neuralgia , trigeminal neuralgia , phantom limb and stump pain and diabetic neuropathies ( for review see . a recent cochrane review by robb et al .
concluded that there is insufficient available evidence to determine the effectiveness of tens in treating cancer - related pain [ 32 , 33 ] .
the international association for the study of pain ( iasp ) speculate that  the prevalence of most types of pain may be much higher in developing countries than in developed countries  , although epidemiological evidence is lacking .
it is known that there is a higher incidence of pain conditions associated with epidemics such as hiv / aids in the developing world .
an iasp developing countries task force , which included africa and the middle east reported that pain management in the general population was inadequate , although there was considerable variations between regions .
limited resources , ignorance by health care professionals and a lack of pain specialists were contributing factors .
this has impacted significantly on pharmacological therapy with many drugs commonly used in the developed world being unavailable to the general public because of the weak economy and limited purchasing power of citizens .
in addition , drugs even when available may be fake , adulterated , passed their expiry date or perished due to inadequate storage .
the cost of a tens device is  30gbp , although devices are available for less than  15gbp if bought in bulk .
once purchased a tens device will not perish or deteriorate and devices in the developed world are used for many decades without the need for further servicing or repair .
often clinics purchase tens devices in bulk and loan them to patients for short and long term use , on the proviso that the patient returns the device to the clinic when it is no longer needed .
manufacturers recommend that individual pads have longevity of one month on daily use , although patients often use them for far greater lengths of time , especially if they take care to store them carefully .
electrode costs can be reduced by using carbon rubber electrodes which are smeared with electrode gel and attached to the skin with micropore tape , rather than using self adhering electrodes . in general ,
tens has no known drug interactions and so can be used in combination with pharmacotherapy to reduce medication , medication - related side effects and medication costs .
tens has very few side effects with no incidents of serious or adverse events reported in the literature .
tens has a rapid onset of action , unlike medication , and there is no potential for toxicity or overdose .
clearly , there is a case to use tens for pain management in the developing world .
however , research is needed to determine the feasibility of tens use in developing countries .
perhaps health promotion authorities should alert the public and heath care practitioners to the role of tens as an important aid in the fight against pain .
 transcutaneous electrical nerve stimulation ( tens ) refers to the delivery of electrical currents through the skin to activate peripheral nerves . 
 the technique is widely used in developed countries to relieve a wide range of acute and chronic pain conditions , including pain resulting from cancer and its treatment . 
 there are many systematic reviews on tens although evidence is often inconclusive because of shortcomings in randomised control trials methodology . in this overview the basic science behind tens will be discussed , the evidence of its effectiveness in specific clinical conditions analysed and a case for its use in pain management in developing countries will be made . 
Introduction
Physiological principle of TENS induced pain relief
TENS and TENS-like devices
TENS Techniques
Clinical effectiveness of TENS
Pain Management in developing countries: Could TENS help?