Matching the perceived benefits of Transcutaneous Electrical Nerve Stimulation (TENS) for chronic musculoskeletal pain against Patient Reported Outcome Measures using the International Classification of Functioning, Disability and Health (ICF)

Abstract

Background

There is no consensus regarding the effectiveness of Transcutaneous Electrical Nerve Stimulation (TENS) for chronic musculoskeletal or low back pain. A review of previous trial methodology identified problems with treatment fidelity. Qualitative research with experienced TENS users identified specific contexts for TENS use, leading to individualised outcomes. There is little information available to guide the selection of Patient Reported Outcome Measures (PROMs) appropriate for TENS evaluation.

Objective

To determine the capability of previously used PROMs to capture the perceived benefits of TENS reported by secondary care Pain Clinic patients who successfully used TENS to manage chronic musculoskeletal pain.

Design

The World Health Organisation International Classification of Functioning, Disability and Health (ICF) was used to match the perceived benefits of TENS against previously used PROMS.

Methods

Semi-structured interviews conducted with nine patients (6 women) as well as three other qualitative datasets (88 patients in total) generated patient-reported benefits which were matched against previously used PROMs using the ICF.

Findings

There were 18 items in the final list of benefits, and none of the four functional outcome measures used in previous RCTs captured more than 8 of these 18 items. The data analysis complemented the inductive thematic analysis but could not replace it, indicating the value of both forms of analysis.

Conclusions

This study highlights a low level of match between outcome measures used in previous TENS studies, and the benefits perceived by experienced TENS users. This suggests that further work is required if the patient-reported benefits of TENS are to be evaluated.

Introduction

Transcutaneous electrical nerve stimulation (TENS) is a portable, inexpensive and low risk form of electrostimulation which has the potential to improve the quality of life of people who live with chronic musculoskeletal pain [1], [2]. Whilst there is evidence to support the hypoalgesic effect of TENS from laboratory studies on healthy humans using experimental pain models [3], [4], [5], [6], [7], there is no consensus regarding its effectiveness for chronic musculoskeletal pain [8] or low back pain [9], [10]. A review of the methodologies of randomised controlled trials (RCTs) for acute, chronic and cancer pain [11] identified significant problems with elements of implementation fidelity such as limited duration of TENS application, insufficient intensity and limited instruction in TENS use that could explain the lack of evidence of effectiveness. In addition to these methodological issues, Bennett et al. [11] identified outcome assessment as a key issue which should be addressed to improve the quality of future research.

The Initiative on Methods, Measurement, and Pain Assessment in Clinical Trials (IMMPACT) [12] recommended core Patient Reported Outcome Measures (PROMs) for chronic pain clinical trials, including the Brief Pain Inventory [13], the Multidimensional Pain Inventory [14] and the Short Form 36 [15]. The Roland and Morris Disability Questionnaire [16] was recommended as an additional disease-specific PROM for low back pain. PROMs can be judged against eight criteria [17] including validity, reliability, precision, interpretability, feasibility, acceptability, and responsiveness. The eighth criterion of ‘appropriateness’ describes the “match” of a measure to the “purpose and questions of a trial” [17]. One risk of a poorly matched PROM was highlighted by a clinical audit of long-term users of TENS [18] which indicated that improved sitting tolerance was one important reported benefit of TENS. None of these four IMMPACT-recommended measures include items related to sitting tolerance, so a possible benefit of TENS could be overlooked by these PROMs.

A study using semi-structured interviews explored the benefits reported by nine experienced TENS users with chronic musculoskeletal pain [19], [20]. A thematic analysis [21] of these data indicated that pain relief, distraction from pain, and a reduction in the sensations associated with muscle tension/spasm were separate direct benefits. These direct benefits led to a range of indirect benefits that were dependent upon the way that individuals chose to use their device, including medication reduction, enhanced function, psychological benefits and an enhanced ability to rest. This complex pattern of TENS use, including the number and variability of outcomes, number of behaviours required, and the degree of tailoring of the intervention suggested that it should be considered as a complex intervention [22]. The study also generated a wealth of data regarding the ways in which patients learned to use TENS in their day to day lives, including the problems associated with TENS and how users managed these problems [23]. These data can inform a process evaluation [24] for pragmatic TENS use, informing both clinical practice [25] as well as the design of studies focussed on determining TENS effectiveness.

The present article reports on a secondary analysis of these qualitative data and three other qualitative datasets: an audit [18], a focus group and a service evaluation (88 participants in total) [20] which were linked with the International Classification of Functioning, Disability and Health (ICF) [26]. The outcome of this matching exercise was then compared with PROMs used in previous TENS studies identified by Cochrane reviews [8], [9]. The ICF is a taxonomy providing a standardised description of health and health-related states, the primary aim being to provide a standardised language and coding system to facilitate communication across disciplines and around the world. The ICF structure consists of two parts, each part consisting of two components. Part 1 provides two descriptive lists, body structures and body functions, grouped as one component, “Body Functions and Structures”. The second component of Part 1 is a list of “Activities and Participation”. Part 2 consists of two components: “Environmental Factors” influencing function and disability, and “Personal Factors”, which are internal influences on function: the taxonomy for this component has not yet been developed. The ICF uses an alphanumeric coding system. Each component is coded with a different letter: ‘b’ represents body functions, ‘s’ represents body structures, ‘d’ represents activities and participation and ‘e’ represents environmental factors. Following this initial letter, a numerical code begins with a chapter number (one digit), with the option of adding three further digits to indicate more detailed levels of coding.

The ICF was used as a reference framework for this secondary analysis, primarily to facilitate the crossmatching of the data against PROMs [27] to assess the extent of the match with the perceived benefits reported. The structure of the ICF also facilitated decisions about the amount of detail to preserve in the representation of the data. Using the ICF as a pre-existing code acted as a theoretical framework for deductive thematic analysis [28], [29] of the interview data: a form of methodological triangulation [30]. A number of overlapping concepts are used within outcome research, including health status, functional status, well-being, quality of life and health-related quality of life, which are often used synonymously: in contrast to this, the ICF provides a sharper focus on function [31], supporting the conceptual clarity of the data analysis.

The aim of this study was to determine the capability of commonly used outcome measures to capture the perceived benefits of TENS reported by secondary care Pain Clinic patients who successfully used TENS devices to help them to manage chronic musculoskeletal pain.