Pain Measurement in Adult Patients

Introduction

Measurement is central to accurate diagnosis and management. Measurement of pain is therefore essential to determine the intensity, perceptual qualities, and time course of the pain so that the differences among pain syndromes can be ascertained and investigated. Furthermore, measurement of these variables provides valuable clues that help in the differential diagnosis of the underlying causes of the pain. They also help determine the most effective treatment necessary to control the pain and are essential to evaluate the relative effectiveness of different therapies. Measurement of pain, then, is important to

• Determine pain intensity, quality, and duration

• Aid in diagnosis

• Help decide on the choice of therapy

• Evaluate the relative effectiveness of different therapies

• Study the mechanisms of pain and analgesia

Dimensions of the Pain Experience

Research on pain, since the beginning of the 20th century, has been dominated by the concept that pain is purely a sensory experience. Yet pain also has a distinctly unpleasant, affective quality. It becomes overwhelming, demands immediate attention, and disrupts ongoing behavior and thought. It motivates or drives the organism to activity aimed at stopping the pain as quickly as possible. To consider only the sensory features of pain and ignore its motivational–affective properties is to look at only part of the problem. Even the concept of pain as a perception—with full recognition of past experience, attention, and other cognitive influences—still neglects the crucial motivational dimension.

These considerations led to suggest that pain has three major psychological dimensions: sensory–discriminative, motivational–affective, and cognitive–evaluative. They proposed, moreover, that these dimensions of the pain experience are subserved by physiologically specialized systems in the brain.

• The sensory–discriminative dimension of pain is influenced primarily by the rapidly conducting spinal systems.

• The powerful motivational drive and unpleasant affect characteristic of pain are subserved by activities in the reticular and limbic structures that are influenced primarily by the slowly conducting spinal systems.

• Neocortical or higher central nervous system processes, such as evaluation of input in terms of past experience, exert control over activity in both the discriminative and motivational systems.

postulated that these three categories of activity interact with one another to provide perceptual information on the location, magnitude, and spatiotemporal properties of the noxious stimuli; motivational tendency toward escape or attack; and cognitive information based on past experience and probability of the outcome of different response strategies. All three forms of activity could then influence the motor mechanisms responsible for the complex pattern of overt responses that characterize pain.

The Language of Pain

Clinical investigators have long recognized the varieties of the pain experience. Descriptions of the burning qualities of pain after peripheral nerve injury or the stabbing, cramping qualities of visceral pain frequently provide the key to diagnosis and may even suggest the course of therapy. Despite the frequency of such descriptions and the seemingly high agreement that they are valid descriptive words, studies of their use and meaning are relatively recent.

Anyone who has suffered severe pain and tried to describe the experience to a friend or to the doctor often finds herself or himself at a loss for words. The reason for this difficulty in expressing the pain experience actually is not because the words do not exist. As we shall soon see, there are an abundance of descriptive words. Rather, the main reason is that fortunately, they are not words that we have occasion to use often. Another reason is that the words may seem absurd. We may use descriptors such as splitting, shooting, gnawing, wrenching, or stinging as useful metaphors, but there are no external objective references for these words in relation to pain. If we talk about a blue pen or a yellow pencil, we can point to an object and say “that is what I mean by yellow” or “the color of the pen is blue.” However, to what can we point to tell another person precisely what we mean by smarting, tingling, or rasping? A person who experiences terrible pain may say that the pain is burning and add that “it feels as if someone is shoving a red-hot poker through my toes and slowly twisting it around.” These “as if” statements are often essential to convey the qualities of the experience.

If the study of pain in people is to have a scientific foundation, it is essential to measure it. If we want to know how effective a new drug is, we need numbers to say that the pain decreased by some amount. Yet although overall intensity is important information, we also want to know whether the drug specifically decreased the burning quality of the pain or if the especially miserable, tight cramping feeling is gone.

Pain Rating Scales

Traditional methods of pain measurement treat pain as though it were a single unique quality that varies only in intensity ( ). These methods include the use of verbal rating scales (VRSs), numerical rating scales (NRSs), and visual analog scales (VASs). These simple methods have all been used effectively and have provided valuable information about pain and analgesia. VRSs, NRSs, and VASs provide simple, efficient, and minimally intrusive measures of pain intensity that have been used widely in clinical and research settings in which a quick index of pain intensity is required and to which a numerical value can be assigned.

VRSs typically consist of a series of verbal pain descriptors ordered from least to most intense (e.g., no pain, mild, moderate, and severe) ( ). Patients read or are read the list and choose the one word that best describes the intensity of their pain at the moment (or over some time interval such as a day or a week). A score of 0 is assigned to the descriptor with the lowest rank, a score of 1 is assigned to the descriptor with the next lowest rank, and so forth.

NRSs typically consist of a series of numbers ranging from 0–10 or 0–100, with end points intended to represent the extremes of the possible pain experience and labeled “no pain” and “worst possible pain,” respectively. Patients choose the number that best corresponds to the intensity of their pain. VRSs and NRSs are simple to administer and have demonstrated reliability and validity ( ).

The most common VAS consists of a 10-cm horizontal or vertical line with the two end points labeled “no pain” and “worst pain ever” (or similar verbal descriptors). Patients are required to place a mark on the 10-cm line at the point that corresponds to the level of pain intensity that they presently feel (or felt over the past day, week, etc.). The distance in centimeters from the low end of the VAS to the patient’s mark is used as a numerical index of the severity of the pain.

VASs for pain affect have been developed in an effort to include domains of measurable pain experience other than the sensory intensity dimension. The patient is asked to rate the unpleasantness of the pain experience (i.e., how disturbing it is). End points are labeled “not bad at all” and “the most unpleasant feeling imaginable” (Price et al 1987).

A major advantage of the VAS as a measure of sensory pain intensity over NRSs and VRSs is its ratio scale properties ( ). In contrast to many other pain measurement tools, equality of ratios is implied, which makes it appropriate to speak meaningfully about percent differences between VAS measurements obtained either at multiple points in time or from independent samples of individuals. Despite this advantage, there has been a recent trend away from use of the VAS in clinical and research settings, largely because of its requirement for additional material (e.g., paper and pencil, computer) and empirical data showing that VRSs and NRSs have sound psychometric properties ( ).

Standard VASs also have several limitations and disadvantages, including difficulty of administration to patients who have perceptual–motor problems; impractical scoring method in a clinical setting, where immediate measurement of the patient’s response may not be possible; the occasional patient who cannot comprehend the instructions; and problems with use in telephone surveys or with electronic devices that are not equipped with the scale.

Although VRSs, NRSs, and VASs have all been shown to have adequate or better than adequate psychometric properties (i.e., validity and reliability), comparisons of the three scales generally show that VRSs lack sensitivity to detect changes in pain intensity when compared with VASs or NRSs ( ). However, despite the advantages associated with unidimensional pain rating scales, they fail to capture the complexity of the pain ( ) and, whenever possible, should be co-administered with a multidimensional measure of pain.

The Mcgill Pain Questionnaire

Development and Description

developed procedures to specify the qualities of pain. In the first part of their study, physicians and other university graduates were asked to classify 102 words obtained from the clinical literature into small groups that describe distinctly different aspects of the experience of pain. On the basis of the data, the words were categorized into three major classes and 16 subclasses ( Fig. 21-1 ). These classes consist of

• Words that describe the sensory qualities of the experience in terms of temporal, spatial, pressure, thermal, and other properties

• Words that describe affective qualities in terms of the tension, fear, and autonomic properties that are part of the pain experience

• Evaluative words that describe the subjective overall intensity of the total pain experience

Each subclass, which was given a descriptive label, consists of a group of words that were considered by most subjects to be qualitatively similar. Some of these words are undoubtedly synonyms, others seem to be synonymous but vary in intensity, and many provide subtle differences or nuances (despite their similarities) that may be of importance to a patient who is trying desperately to communicate to a physician.

The second part of the study by was an attempt to determine the pain intensity implied by the words within each subclass. Groups of physicians, patients, and students were asked to assign an intensity value to each word by using a numerical scale ranging from least (or mild) pain to worst (or excruciating) pain. When this was done, it was apparent that several words within each subclass had the same relative intensity relationships in all three sets. For example, in the spatial subclass, shooting was found to represent more pain than flashing , which in turn implied more pain than jumping . Although the precise intensity scale values differed for the three groups, all three agreed on the positions of the words relative to each other. Figure 21-1 shows the scale values of the words for patients based on the precise numerical values listed by .

Because of the high degree of agreement on the intensity relationships among pain descriptors by subjects who have different cultural, socio-economic, and educational backgrounds, a pain questionnaire ( Fig. 21-2 ) was developed as an experimental tool to study the effects of various methods of pain management. In addition to the list of pain descriptors, the questionnaire contains line drawings outlining the body to show the spatial distribution of the pain, words that describe temporal properties of the pain, and descriptors of the overall present pain intensity. The present pain intensity is recorded as a number from 1–5, with each number being associated with the following words: 1, mild; 2, discomforting; 3, distressing; 4, horrible; and 5, excruciating. The mean scale values of these words, which were chosen from the evaluative category, are approximately equally far apart so that they represent equal scale intervals and thereby provide anchors for specification of the overall pain intensity ( ).

In a preliminary study, the pain questionnaire consisted of the 16 subclasses of descriptors shown in Figure 21-1 , as well as additional information deemed necessary for the evaluation of pain. It soon became clear, however, that many of the patients found certain relevant words to be absent. These words were then selected from the original word list used by , categorized appropriately, and ranked according to their mean scale values. A further set of words—cool, cold, and freezing—were used by patients on rare occasion but were indicated to be essential for an adequate description of some types of pain. Thus, four supplementary, or miscellaneous, subclasses were added to the word lists of the questionnaire ( Fig. 21-2 ). The final classification, then, appeared to represent the most parsimonious and meaningful set of subclasses without at the same time losing subclasses that represent important qualitative properties. The questionnaire, which is known as the McGill Pain Questionnaire (MPQ) ( ), has become the most widely used clinical and research tool for measuring pain.