Direct and Indirect Benefits Reported by Users of Transcutaneous Electrical Nerve Stimulation for Chronic Musculoskeletal Pain: Qualitative Exploration Using Patient Interviews

Direct benefit subtheme 1: pain relief.

Eight of the 9 participants reported pain relief as a result of TENS use, as shown in Table 3. The following data extract illustrates the difference between pain relief as a direct benefit and the indirect benefits: It doesn't take the pain away, total. What it does, it makes it manageable…there's a big difference between managed pain and chronic pain. Chronic pain just totally immobilizes you, whereas managed pain gives you some normality. You are able to continue with your life, and I think…from a pain perspective, as I have already said, it empowers me, the TENS enables me, to have that control over my disability. (Fran)

Managed pain is contrasted by Fran with “chronic pain,” used here as a lay term to mean severe pain. The “managed pain” is equivalent in meaning to pain relief as a direct benefit of TENS, whereas the benefits of mobility, continuing with life, normality, and control are reported as consequences of the managed pain and so were developed into themes as indirect benefits.

There was evidence from the case-level analysis that TENS helped with some types of pain quality more than others. In particular, pain of a sharp and shooting nature associated with faster or larger movements was reported as being helped less by TENS than constant pain, limiting the indirect benefits of TENS for some activities: I think it would be too much an activity…when you're doing a sport…because I play a lot of (a ball sport), so it would be when you are batting or throwing, those jerky movements, I don't think it would really work. (Irene)

The case-level analysis indicated that experiences of the participants of TENS use during a pain flare-up differed to some extent. Two participants (Oliver and Claire) reported that TENS was of lesser value as the pain escalated. However, they would still persist with its use, in addition to other pain relief methods, to gain any possible benefit, however slight. Other participants (Fran, Jack, Naomi, and Sally) routinely used TENS to help during flare-ups of pain, with more benefit. For example, Jack reported having used TENS for flare-ups: If…I am doubled up, if I can't get out of the chair, nothing cures it, nothing. But this thing I can aim at the dead spot, and it will move it, and make things a lot easier for me.

Direct benefit subtheme 2: distraction from pain.

Several participants suggested that the TENS sensation provided a helpful distraction from pain. During the thematic analysis, it was important to decide whether this distraction should be classified as a direct benefit of TENS use or to consider it only as a hypothetical mechanism of TENS action leading to the direct benefit of pain relief. This issue was explored by examining the actual words used by participants. For example, Claire described both distraction and pain relief operating separately: It's quite good for the distraction thing apart from anything else, just sort of having the impulses sort of takes your mind off the pain. But I think it does help with the pain as well.

Naomi explained her experience, saying that she would: …turn it up to kind of cover, so it kind of covers up the pain.

This “covering up” of the pain did not depend on her being consciously aware of a sensation at all times. This finding suggests that distraction at a conscious level is not the only mechanism of operation. Naomi explained the process this way: Yeah,…it's almost that it numbs that area so that you don't notice it particularly, and if you change it, then you notice it. So…if you notice that…if you find that actually you're starting to feel the pain, then perhaps turn it up, and then you would notice it again, but again it would fade as well, then it would just fade again.

Oliver and Brian both suggested that distraction from the pain was a benefit of TENS use, but both also reported a reduction in pain. In contrast, Moira perceived the benefit of TENS only as a distraction from the pain, rather than pain relief as such: It distracts, disguises…changes the pain for a while. You get some time off from the pain if you get it right.What the TENS machine does is change my ability to cope with it. Or my ability to…put it further to the back of my mind so that I can then go about my business.

Of note, Moira reported that the distraction (or disguising) benefit of TENS relieved her continuous pain more than other types of pain such as shooting pain, which could be more severe at times. The case-level analysis, therefore, informed the thematic analysis and confirmed that distraction should be considered as a direct benefit of TENS use.

Direct benefit subtheme 3: reduced sensation of muscle tension and spasm.

A reduction in the sensation of muscle tension and spasm was reported by several participants. This benefit may represent a physiological change in the behavior of the muscles or an altered perception of the muscles, or both. From these interview data alone, we can conclude only that TENS influenced the perception of muscle tension and spasm, which is concordant with the World Health Organization International Classification of Functioning, Disability and Health³⁰ code b780, described as “sensations of muscle stiffness and tightness of muscles, muscle spasm or constriction, and heaviness of muscles.” The following data extract provides a clear indication of the link between this direct benefit and the indirect benefit of increased function: Because I feel they were all knotted…they were all knotted up, so with this machine, I feel that it's sort of made them a bit more open and relaxed to allow me to move a bit more freely. (Irene)

In summary, the case-level and thematic analysis of the data indicated that the influence of TENS on symptoms experienced by patients with chronic musculoskeletal pain is unlikely to be captured by the use of a unidimensional pain scale, and a more complex assessment is likely to be required.

Indirect benefit subtheme 1: reduction in medication.

The case-level analysis indicated that the 3 participants who reported a reduction in medication as an indirect benefit of TENS all experienced difficulties with medication, which increased the importance of this perceived benefit. In contrast, the other 6 participants used TENS as an adjunct to pain medication: they chose to use TENS when medication was insufficiently helpful. For Fran, pain medication caused significant sedation, which meant that she had been unable to drive and unable to work before first using TENS, as she explained: If I wasn't able to use the TENS in between, I would have to use those other 2 tablets…. I wouldn't be able to drive, I'd be dangerous, because it's very much like being outside your body…it affects my speech, it affects my vision…my coordination, my balance, everything. It's tremendous.

The use of TENS offered Fran significant pain relief, which she indicated on a 100-mm visual analog scale (VAS) as a reduction from 80 mm to 50 mm. This pain reduction allowed her to reduce her medication dose to a level where she could drive, function at work, and socialize. These indirect benefits are shown in Figure 2 as deriving from the direct benefits of TENS use within a context of poorly controlled pain and medication associated with side effects. The contexts of further indirect benefits reported by Fran are shown in Figure 2 and are described below.

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Figure 2.

Representation of Fran's perceived benefits of transcutaneous electrical nerve stimulation (TENS), illustrating the reduction in medication as a stepwise, contextualized process linking direct and indirect benefits of TENS use across several domains.

Indirect benefit subtheme 2: enhanced rest.

This subtheme is developed from 5 accounts of TENS being used to enhance the benefits of a planned rest period. This use is associated with a common pain management strategy known as “pacing” or “activity management,” which involves a planned alternation of physical and sedentary activities, sometimes linked with a change of posture (eg, lying down).^31,32 These periods of planned rest typically form part of a daily routine. Pre-emptive rest may be used in this way to facilitate activity that follows the rest period, as referred to in the extracts below. These periods of planned rest are different from those times when rest is used because pain and associated symptoms have escalated to a level where an individual feels unable to remain active, commonly known as a “flare-up” of pain. Irene mentioned using TENS while sitting to relax her back muscles so that she could carry on with activity later in the day: No, I wouldn't say it helped me sit, because I would sit down to use it, to make it, to make me, to make…I don't know what it does, but to sort of relax the muscles, to enable me to then carry on with the rest of the day.

The case-level analysis indicates that this use of TENS to enhance recuperative rest can be conceptualized as a mechanism operating within a specific context, as detailed in Figure 3.

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Figure 3.

Processes involved in the use of transcutaneous electrical nerve stimulation (TENS) to enhance planned rest, leading to potential benefits after the rest period.

Indirect benefit subtheme 3: psychological benefits.

Although this subtheme is constructed from data relating to the perceived benefits of TENS use that fall within the psychological domain, it should be noted that the psychological domain is diverse, and the perceived benefits reported relate only to a subset of psychological functions. One of the clearest examples of a psychological benefit from TENS use was the improvement in concentration reported by Naomi: Yeah, you're not in so much pain, so you can then concentrate on something else, you can actually, kind of, you can read a book, which takes your mind off the fact that you're in pain…you can go and do something else, it's the bit that you're not conscious of it all the time, of how much pain you're in, so yeah…it does, it works on a lot of different levels.

There is case-level evidence here of a process involving different stages, which are represented in the eFigure. The direct benefit (pain relief) facilitates an indirect benefit (improved concentration), which facilitates Naomi's ability to read, leading to further pain relief via a distraction mechanism.

Sally made reference to the psychological benefits of TENS use indirectly by describing the emotional suffering that she experienced during a pain flare-up: Yeah, they're awful. Those days I've just, I have said I would rather not be there on my bad days 'cause they are just awful. They are horrible.

The modest pain relief that she reported helped to ameliorate this suffering. Sally completed the pain VAS, indicating pain severity of 100 mm during a flare-up (the anchor described as “pain as bad as possible”); she indicated a reduction in pain severity to 84 mm as a result of TENS use. This 16% pain reduction falls into the 10% to 20% pain reduction indicated by IMMPACT³³ as a minimal clinically important difference: for Sally, it is clearly enough of a difference for the use of TENS to be worthwhile.

Fran described the psychological challenges in the early days of her pain and the importance of TENS in helping her to make a transition toward control and acceptance. This transition included physical and psychological elements: Psychologically, it's really hard when you're first diagnosed with something…or even prior to being diagnosed…you're suffering, really really bad pain…. I've found that when that's happened to me, the adjustment with coming to terms with the pain, chronic pain may never ever be cured, all right…the most that you could hope for is for it to be managed, or maintained…. That transition is very very tough to take, and I…was pumped with loads and loads of drugs, to begin with…and my quality of life was zero, until I started experimenting with the TENS, and it's through that, that I got that control, and I was able to do that transition psychologically. So it's not only the physical [effects], it's the psychological effects it has on you as well, but I think you've got to understand what the TENS does: it's not going to cure your pain, it enables it to be managed. (Fran)

In summary, this subtheme brings together case-level data related to improved concentration, reduced suffering, acceptance, control, and empowerment. This is an important but heterogeneous group of perceived benefits.

Indirect benefit subtheme 4: function.

The case-level analysis of the influence of TENS use on daily function indicated complexity. Some participants indicated an increase in function as a result of TENS use, whereas others used TENS to help them to sustain their usual level of function when it might be threatened by an increase in pain. An additional complication to the analysis of the functional benefits was the extent to which specific activities were helped or whether the benefits were generalized to most day-to-day activities. A clear example of the latter case was provided by Oliver, who reported help with general function when his pain was worse. He tended not to use TENS when his pain was less intrusive because of the problems associated with use. When asked if there were any activities that TENS does not help with, Oliver explained: I don't see, because the whole thing is, I keep saying, about day-to-day function, you know, function, just existing day-to-day, working and so on when it flares up, so the answer to that is probably “no,” because it contributes towards that. There's nothing else that I can think of, well, sticking the TENS on doesn't help with me boiling an egg…well, it probably does, because I've got to stand up (laughs), so it's all to do with the overall picture, the day-to-day, just doing your day-to-day stuff.

In contrast, some users reported that TENS would be used to facilitate increased levels of specific activities, such as sitting, standing, and walking. Increases in these specific activities led to improved involvement with tasks such as shopping and housework and participation in work and social activities. A case-level example of this indirect benefit was provided in a data extract from Fran, represented in Figure 2, which indicated that TENS could increase sitting tolerance, especially in uncomfortable chairs, which facilitated attendance at social events, leading to a reduction in social isolation: If you've actually got chronic pain in your back,…just sitting or climbing the stairs or just standing even for short periods of time can be excruciating. Now, if you've actually got your TENS on, and you can fiddle around with it to get the pulses very inconspicuously, it enables you to do things that able-bodied people are able to do with the least discomfort…because not all cinemas, all seating areas, are specifically made for you, whereas in my home and even in my work environment, I've got special chairs. I can't afford that luxury when I go out socializing, so I've got a choice really: I either use my TENS…so I can be as able as everyone else, or I don't use my TENS, and I become isolated, so it's given me more freedom.

Sleep was reported as an indirect benefit of TENS. Using TENS before falling asleep can be thought of as a special case of using TENS to enhance rest, with the aim of reducing sleep latency, but several users also reported that they would sleep with the TENS machine switched on, as Brian explained: Yeah, sometimes it will be on solid for 2 days. I know because I've got to try and go to sleep with it on as well, so if I can't go to sleep, I will put the TENS on and go to sleep.

Case-level analysis indicated that this was not a universal benefit and may be negated by the user's tendency to move around at night and any consequent difficulty in keeping the pads and leads in place.

Improved ability to read also was reported as an indirect functional benefit of TENS, as shown in the eFigure. It is possible that the reduced interference of pain with cognitive capacity as a result of TENS use might lead to improvements in other cognitive activities, but such improvements were not spontaneously reported by participants.

In summary, TENS use could be directed toward improvements in function or maintaining function. Function involving dynamic movements such as sports, reaching, and heavy lifting was notable by their absence.