A Systematic Review of Devices and Techniques that Objectively Measure Patients' Pain

Assessment of pain is important in daily clinical practice and as an endpoint in clinical studies. Because pain perception is highly subjective, pain measurement is complex. Self-rating pain scales are currently of great importance but have limitations. They depend on many more factors than pain, which could lead to an incorrect assessment of therapies or clinical studies. Therefore, there is need for valid, reliable, safe, and low-cost methods to determine and quantify patients' pain more objectively.

To provide an overview of devices and techniques that can be used to administer a pain stimulus with similar intensity as the endogenous pain experienced by the patient, in order to quantify and subsequently follow patients' pain more objectively.

In this systematic review, articles from PubMed, EMBASE, Cochrane library and Scopus were reviewed for eligibility.

Studies that described a device or technique that could be used to induce a variable, controlled, and measurable pain stimulus were included. Studies that made correlations with established pain scales or those who compared outcomes in multiple tests were selected to assessvalidity and reliability.

A total of 1,308 manuscripts were initially retrieved. After independent screening by a team of 4 reviewers, 19 studies were eventually included describing 15 different devices or techniques. These devices could be divided into groups based on stimulus administration: electrical, external pressure (probe) and miscellaneous pain stimulators. Electrical stimulators were found to be tested extensively and proven to be both valid and reliable.

To correlate new techniques with older methods such as the Numeric Rating Scale (NRS) and Visual Analogue Scale (VAS) for which an improvement is desired, is debatable. To (partially) address this problem, the reliability is added as an additional primary outcome to assess which device works best. Further limitations include the heterogeneity of studies found in both the types of pain measured as in outcome measures presented. In addition, it is important to note that part of the devices described cannot directly be used for clinical practice due to products that have cease to exist or the description of solely techniques rather than testing ready-to-use devices.

Several devices and techniques compared pain intensity experienced by patients with an external pain stimulus that potentially could be used as a new objective pain measurement tool. Given the results of our review, electrical stimulators that have been tested extensively with high validity, reliability, and feasibility would be recommended for use for clinical and research purposes. Moreover, normalization of pain intensity scores for current perception is important. Pain intensity normalization leads to higher correlations with established pain scales and possibly to increased inter-patient reliability.Registration number: Registered in the PROSPERO database (PROSPERO 2016:CRD42016041974)KEY WORDS: Systematic review, objective pain measurement, pain scales; devices, techniques, validity, reliability, safety, feasibility.