Review

Correspondence *Spine Center, Schulthess Klinik, Lengghalde 2, 8008 Zürich, Switzerland

Email anne.mannion@kws.ch

Introduction

Pain is a major cause of morbidity, with the low back being one of the most common locations of symptoms. Low back pain (LBP) has a considerable impact on both the individual sufferer and society at large. The accurate assessment of pain is a prerequisite for its effective management,¹ yet the systematic quantification of this common symptom is rare in clinical practice.² It is difficult for the busy clinician to keep abreast of the continually emerging scientific literature on pain assessment; the intention of this article is to provide a clear and practical overview of the current information. This Review is not exhaustive, but it is based on an extensive search of the literature and the knowledge and experience of a multidisciplinary group of authors. We aim to provide recommendations for daily clinical practice. The main focus of this Review is LBP, but many of the principles discussed and tools recommended are applicable to other musculoskeletal conditions.

Why measure pain?

In some specialties, such as cardiology or oncology, primary outcomes might be mortality or survival. Such end points are not applicable in LBP. The importance of pain was highlighted in the 1990s when the American Pain Society declared it to be the fifth vital sign of medical examination.³ In LBP, pain has been described as one of the cardinal domains to be assessed along with back-specific function, generic health status, work disability, and patient satisfaction.^{4, 5, 6} Pain is one of the best determinants of disability due to LBP^{7, 8} and is predictive of work resumption within the year following related short-term absence.⁹

Traditionally, within the biomedical model, pain has been regarded as a subjective measure, while physical examination, laboratory tests, and imaging studies are considered more objective measures. Correlations between self-reported pain and both self-reported disability and objective measures of function (e.g. range of motion, muscle performance tests) are sometimes significant but are invariably weak,^{6, 7, 10} highlighting the necessity of measuring pain itself. In patients with chronic LBP, the use of pain medication such as opiates is not always associated with pain severity,¹¹ indicating that drug intake might not be appropriate as a surrogate measure of pain.

Measurement of pain often yields much greater treatment effect sizes, or responsiveness, than physical variables^{7, 12} or condition-specific instruments¹³ (i.e. it is the more 'sensitive' measure for evaluating the effects of treatment). Finally, and of importance, the patient typically seeks care due to pain, and hence this is the variable that needs to be carefully assessed both at baseline and in response to treatment. In other words, pain itself is the most relevant variable in the syndrome of LBP.

Can pain be measured?

Pain has been defined as "an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage."¹⁴ Pain is a highly personal experience and the patient is thus the best informant;^{2, 15, 16} it is a multidimensional phenomenon that includes physiologic, sensory, affective, cognitive, behavioral, and sociocultural aspects. The understanding and interpretation of symptoms (beliefs and cognitions) is well known to modulate the pain experience. This experience might be associated with emotional reactions, such as anxiety, distress, or depression, which could in turn give rise to illness behaviors that then influence the individual's private, social, and professional activities. The social environment might or might not allow or reinforce the expression of symptoms and their consequences. Consideration of these various dimensions acknowledges the multifaceted nature of pain and confirms that the global evaluation of pain is not a straightforward matter.

The influence of various factors that modify patients' pain perception and reporting—such as age, sex, race, income, type of health insurance, comorbidity, education, work satisfaction, anxiety, and depression—appears to be variable.^{16, 17, 18, 19, 20, 21} The identification of confounders in the measurement of pain is, however, difficult because of the lack of any external reference or gold standard. Typically, predictors are sought that (statistically) account for the differences in pain levels of patients with a given clinical condition or in response to experimental pain. Whether this model is valid (i.e. whether the 'exposure' or pain stimulus can otherwise be considered the same in all individuals) is, however, not certain.

A review of pain assessment in clinical and health services research highlighted the pros and cons of the most frequently used tools.²² The recall of key parameters of chronic recurrent pain (average intensity, interference with activities, activity-limitation days, and days with pain) had acceptable validity for at least a 3-month recall period.²² Hence, despite its intrinsic complexity, it is still possible for clinicians and researchers to gather useful information on pain.

The available tools

An exhaustive description of all the available pain measurement tools is beyond the scope of this article. Interested readers are referred to a review on the topic published elsewhere.²³ In many patients with back disorders, back pain and leg pain coexist with different intensities and respond differently to treatment; as such, the severity of each should always be measured separately.⁶

Pain intensity and pain affect

Various instruments have been developed to evaluate the two key dimensions of the pain experience—pain intensity (how much a person hurts) and pain affect (how much a person suffers).^{22, 24} These dimensions are conceptually and statistically different but are not wholly independent.²² Three methods have traditionally been used to measure pain intensity: visual analogue scales (VASs; Box 1), verbal rating scales (VRSs; Box 2), and numerical rating scales (NRSs; Box 3). VASs and VRSs are also commonly used to assess pain affect.^{22, 24, 25}

Box 1 The visual analogue scale and graphic rating scale.

 

The visual analogue scale (VAS) consists of a line, usually 100 mm long, whose ends are labeled as the extremes ('no pain' and 'pain as bad as it could be'); the rest of the line is blank.⁶² The patient is asked to put a mark on the line indicating their pain intensity (at the present time, over the past week, or over the past 2 weeks, etc.). The distance between that mark and the origin is measured to obtain the patient's score.

Sometimes descriptive terms, such as 'mild', 'moderate' and 'severe', or numbers are provided along the scale for guidance, as shown below, and the scale is then referred to as a graphic rating scale.^{24, 62}

Box 2 The verbal rating scale.

 

Verbal rating scales (VRSs) consist of a list of adjectives that describe different levels of pain intensity.⁶² A VRS for pain includes adjectives that reflect the extremes (e.g. 'no pain' to 'pain as bad as it could be'), and sufficient adjectives to capture the gradations in between. VRSs are most frequently five-point or six-point scales. The patient is asked to select in a questionnaire or state verbally the adjective that best describes his or her level of pain intensity. In behavioral rating scales, different pain levels are described by sentences including behavioral parameters.

 

SF-36 Bodily Pain subscale¹²

How much physical pain have you had during the past 4 weeks? (Please tick one box.)

• None
• Very mild
• Mild
• Moderate
• Severe
• Very severe

McGill Pain Questionnaire¹²

How strong is your pain? People agree that the following five words represent pain of increasing intensity. They are:

• Mild
• Discomforting
• Distressing
• Horrible
• Excruciating

Which word describes your pain right now? (At its worst? When it is least?)

 

Behavioral rating scale^{24, 28}

Please select the statement that best describes the level of your pain (today, over the last week, over the past 2 weeks, etc.):

• No pain
• Pain present, but can easily be ignored
• Pain present, cannot be ignored, but does not interfere with everyday activities
• Pain present, cannot be ignored, interferes with concentration
• Pain present, cannot be ignored, interferes with all tasks except taking care of basic needs such as going to the toilet and eating
• Pain present, cannot be ignored, rest or bed rest required

Box 3 The numeric rating scale.

 

The numeric rating scale (NRS) involves asking patients to rate their pain intensity by selecting a number on a scale from 0–10 (11-point scale), 0–20 (21-point scale), or 0–100 (101-point scale) by filling in a questionnaire or stating verbally a numerical level.⁶² For example²⁸:

"Please indicate on the line below the number between 0 and 100 that best describes your pain. A zero (0) would mean 'no pain' and a one hundred (100) would mean 'pain as bad as it could be'. Please write only one number." An empty box or line is provided for the corresponding number to be entered. A slight variation of the NRS²⁴ is the box scale, where each number (e.g. 0–10) is written in a box and patients are asked:

"If a zero (0) means 'no pain' and a ten (10) means 'pain as bad as it could be', on this scale of 0–10, what is your level of pain? Put an "X" through that number."²⁸

For each type of scale, several versions are available, with different numbers of response levels (grades) for VRSs and NRSs and different space orientations and anchor words for VASs.²² Sometimes the traditional VAS is supplemented with descriptive terms or numbers lined up along the scale for anchorage, producing what is then known as a graphic rating scale (Box 1).²⁴ Some still refer to the graphic scale as a VAS, which can cause confusion when discussing the merits of different scales. Both horizontal and vertical versions of the VAS exist, although some advise against the use of vertical scales in persons with chronic LBP.²⁶ Careful instructions regarding how to rate pain intensity on a VAS are imperative for its proper completion.^{20, 25} A short written introduction to the scale is, however, usually sufficient and further oral explanations do not seem to be necessary.²⁶ For all scales, the instructions should also be formulated in a manner that clearly states whether the question refers to current pain, usual pain, worst pain, average pain over a given time period, and so on.²⁶ The most appropriate time frame for measurement varies depending on the circumstances: for the assessment of acute pain or postoperative pain on the ward, current pain is most appropriate; for chronic pain, with its day-to-day fluctuations, an average rating over the preceding weeks (between 1 and 4) is recommended, with the precise period being influenced, in part, by the duration of any planned intervention and timing of the next follow-up. In chronic LBP it is not uncommon to inquire about current pain and worst and least pain in preceding weeks and then to take an average of these values as the representative value.²⁷

In one study on patients with chronic pain, six different pain scales (traditional VAS, 101-point NRS, 11-point box scale, 6-point behavioral rating scale, 4-point VRS, and 5-point VRS) were compared in relation to their ease of administration of scoring, rates of correct responding, sensitivity (as defined by the number of available response categories), and responsiveness to change, as well as in terms of the predictive relationship between each scale and a linear combination of pain intensity indices.²⁸ The scales yielded similar results in terms of their predictive validity and the proportion of patients not responding as instructed (e.g. leaving response blank, marking between two categories, marking two answers, etc.). When considering the remaining criteria (responsiveness to change, ease of administration, sensitivity) the 101-point NRS proved to be the most practical index. In patients with osteoarthritis, VAS and VRS responses were shown to be highly correlated (r 0.7–0.8) and the scales yielded similar effect sizes after treatment; however, the VRS was easier to administer and interpret.²⁹ Herr et al.²⁰ used an experimental pain model to compare five common pain scales. Although all scales were psychometrically sound, the VRS emerged as the overall scale of choice in both younger and older cohorts.

Proponents of VASs and 101-point NRS highlight the purportedly larger response ranges as a key advantage of these tools,³⁰ though whether such a high resolution really delivers more accurate results is uncertain.²⁸ In practice, many patients treat the 101-point NRS as a 21-point or an 11-point scale, consistently providing their responses in multiples of 5 or 10.³¹ Furthermore, the error of measurement (Box 4) imposes a restriction on the minimal score difference that can be reliably discriminated using the scale. Hence, 11-point and 21-point scales appear to provide sufficient levels of discrimination.³¹

Box 4 Key psychometric properties of outcome instruments: the basics.

 

The quality of any questionnaire or instrument used to assess patients' perceptions is determined by its psychometric properties: score distribution, reliability, validity, and responsiveness.

 

Score distribution

A good score distribution with low (<15%)⁶³ floor and ceiling effects (i.e. percentage of people with the lowest and highest scores, respectively) shows the instrument's ability to assess the full range of severity.

 

Reliability

Two indices of reliability are of interest, and their coefficients should be >0.7 for the reliable interpretation of group change and >0.90 for the reliable interpretation of individual change:⁶⁴

• Internal consistency (given by Cronbach's ): the degree of homogeneity of individual items in an instrument/scale (i.e. how well the items correlate with each other and the whole scale)
• Reproducibility or test–retest reliability: the extent to which consistent scores are recorded on successive occasions, when no relevant change has occurred inbetween.⁶² The intraclass correlation coefficient (ICC) is one measure of reproducibility, but its value is sensitive to the heterogeneity of the sample examined.⁶⁵ The standard ('typical') error of measurement (SEM) provides a more practical indicator and allows calculation of the instrument's 'minimal detectable change' (MDC) (i.e. degree of change denoting 'real' change, beyond measurement error (MDC_95% being 2.77 SEM).⁶⁵

Validity

Validity is the capacity of the instrument to measure what it purports to measure and comes in many guises (content, construct, criterion),⁶² a thorough discussion of which is beyond the scope of this review. It is typically evaluated by examining the correlation between the instrument's scores and those of conceptually related (or clearly unrelated) variables or outcome events.

 

Responsiveness

Responsiveness (or "sensitivity to change") refers to the accurate detection of clinically relevant change in disease status when it has occurred⁶²; it is typically evaluated by means of effect sizes (Cohen's d) after intervention (where d 0.8 is large, 0.5–0.8 is moderate, 0.2–0.5 is small, and <0.2 is trivial).

VAS methods are sometimes criticized on the grounds of their being difficult to understand, with 7–16% higher failure rates being reported for VASs than for the VRSs and NRSs.^{28, 32, 33} The problem is exacerbated in individuals with physical or cognitive impairment²⁰ and in the elderly.³³ Evidence shows that the visuospatial abilities required for the use of VASs are more affected by age than are the lexical abilities required for use of the NRSs or VRSs.³³ The VAS is also less reliable in illiterate patients.³⁴ The addition of markers to the traditional pain VASs (to form a graphic rating scale; Box 1) might render the scale more understandable; studies in other fields of medicine indicate that such anchors improve the reliability and sensitivity of the scale³⁵ and do not necessarily result in notable marker bias³⁶ (i.e. the tendency to be 'drawn' towards the markers when completing the scale).

The Faces Pain Scale, which was originally developed for use in children, incorporates schematic faces depicting increasing severity of expressed pain, each of which is associated with a number from 0 to 6;³⁷ this scale also appears to be a valid and reliable instrument for use in the elderly,¹⁵ although it is not necessarily preferable to VRSs or NRSs.²⁰ The visual numeric scale was developed with the aim of reducing the amount of missing data in pain assessment.³⁸ This scale includes a horizontal line with anchors at both ends ('no pain' and 'severe pain'). The line is graded from 0 to 10, and vertical bars of increasing height and shading-depth are associated with each increasing number. The visual numeric scale was shown to be easier to administer and code than VASs and was sensitive to changes in pain after treatment.³⁸

When and how often to measure?

Due to its episodic nature, chronic LBP cannot be treated as a static phenomenon; both the intensity of pain and its episodic nature influence an individual's ability to function in work and social life.⁸ Acknowledging that the episodic expression of back pain is common and complex, von Korff⁵² reflected on what might be the most valid and useful summary measures to encapsulate its clinical course: percentage or number of days with LBP, with severe LBP, with LBP exceeding a definite severity level, or with LBP preventing the performance of major life activities; or average level of activity limitation or pain intensity when experiencing LBP. Unfortunately, there is still no consensus as to how such a summary measure is best formed, although new ideas are continuously emerging, based on cluster analysis of longitudinal pain patterns in large groups of individuals (i.e. the sorting of cases into homogeneous subgroups with 'typical characteristics').⁵³ It is hoped that such studies will ultimately lessen the confusion surrounding the definition of chronic LBP⁵² and provide a better basis of classification for assessment and intervention.⁵³

A longitudinal study of spinal fusion patients included both a prospective and a retrospective evaluation of the preoperative pain intensity. The authors showed that at long-term follow-up, the retrospective ratings of baseline pain were significantly higher than the actual preoperative estimations by the same patients.⁵⁴ This finding is relevant to the evaluation of effect sizes after treatment: a cross-sectional postoperative study in this group would have led to an overestimation of the effectiveness of surgery.⁵⁴ The retrospective overestimation of pretreatment pain is not uncommon.^{32, 55} Experimental pain studies⁵⁶ suggest that pain recall might be more influenced by recollection of one's emotional state at the time of the initial experience than by factual information associated with it. LBP should hence be assessed prospectively, especially when evaluating treatment effectiveness.

Longitudinal studies on patients with acute LBP or sciatica⁵⁷ or undergoing spinal surgery^{58, 59} have shown significant reductions in pain during the first few months, with a leveling off thereafter. The pain levels recorded in the early postoperative phase are reliable indicators of the longer-term outcome.^{58, 59}

These examples indicate that it is important to carefully select the timing and frequency of the evaluation and the time period over which the patient is asked to reflect; these will depend on the characteristics of the patient and their pain, as well as the type of intervention under evaluation.⁶⁰

Conclusions

Pain is a prominent feature of many musculoskeletal disorders, and its accurate and reliable measurement is a prerequisite for its effective management. For the assessment of pain intensity—one of the most important pain variables—VRSs and NRSs appear to be preferable overall to traditional VASs. No single tool is best, however, and valid instruments are available for patients of all ages and mental abilities.^{15, 20, 61} The clinician should be aware of the properties of the tool or tools that he or she intends to use, including its level of imprecision and its MCID (Box 4). Brief measures for the other core outcome domains are available^{5, 6, 40} (Box 5) and should complement the pain assessment.

The course of LBP, assessed in long-term longitudinal studies, is currently the subject of much investigation. An improved understanding of issues such as the natural history of LBP, factors that predispose to chronic pain, and the dependence of clinically relevant change on initial severity, duration, and type of intervention should assist with clinical decisions regarding the necessity for and timeliness of intervention.

Acknowledgments

A Mannion gratefully acknowledges funding support from the Swiss National Science Foundation National Research Programme 53 'Musculoskeletal Health – Chronic Pain' and the Schulthess Klinik Research Funds. Désirée Lie, University of California, Irvine, CA, is the author of and is solely responsible for the content of the learning objectives, questions and answers of the Medscape-accredited continuing medical education activity associated with this article.

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