Defining Mild, Moderate, and Severe Pain in Persons with Multiple Sclerosis

Chronic pain is a common problem in persons with multiple sclerosis (MS) (1, 2). Among persons with MS, pain is often experienced in multiple sites (2). In some locations pain is only experienced during a short period of time, while pain in other sites may be persistent pain of either neuropathic or musculoskeletal origins, or some combination of the two. In clinical research and in many clinical settings, pain intensity is commonly measured with numerical rating scales (NRS), where patients rate their pain intensity from 0 to 10 or 0 to 100 (3). However, treatment guidelines (e.g., cancer guidelines by the World Health Organization (4, 5) and Agency for Health Care Policy and Research (6)) are often developed based upon categorical ratings of pain (e.g., mild, moderate, severe). Despite the development of category-based treatment guidelines, there is no clearly defined mechanism for classifying pain intensity into these categories, which can make it difficult to determine when patients are experiencing the level of pain specified in treatment guidelines. This important gap has inspired a number of studies to gain an understanding of how NRS pain ratings relate to categorical pain ratings, and cutoffs for classifying pain intensity into discrete categories for a number of populations have been identified (7-16). However, the extent to which existing cutoffs are valid for classifying pain intensity in persons with MS is not known. In a seminal study, Serlin and colleagues (13) evaluated NRS pain intensity ratings with respect to the interference of pain with daily activities among persons with cancer. They found that classifying mild pain as 0-4, moderate pain as 5-6, and severe pain as 7-10 was most consistent with the pattern of nonlinear associations between (worst) pain intensity ratings and pain interference in their sample. This study has been subsequently replicated in samples of individuals with a variety of pain conditions and diagnoses. Patient groups that have yielded the same cutoffs as those found by Serlin and colleagues include those with post-operative pain (8) and low back pain (10). However, a number of studies have found variations in cutoffs, primarily surrounding whether 4 should be classified as mild or moderate and 7 should be classified as moderate or severe. For example, relative to the study by Serlin and colleagues, a study of patients with diabetic peripheral neuropathy found a lower cutoff between mild and moderate pain (i.e., 0-3, 4-6, 7-10, for mild, moderate and severe) (11), while two replications in cancer populations (9, 15) reported a higher cutoff between moderate and severe pain (0-4, 5-7, 8-10). Beyond these between-group differences, additional studies have shown differences based on the type of pain measured. Cutoffs have been found to vary between general (0-3, 4-6, 7-10), phantom limb (0-4, 5-7, 8-10), and back pain (0-4, 5-6, 7-10) (7). Thus, despite some consistencies across populations for some intensity ratings, research shows that the best classification scheme varies to some extent as a function of patient population and type of pain measured. It is unclear, however, which scheme is most useful for persons with MS and chronic pain. As research on pain cutoffs has evolved, Anderson (17) recommended that researchers consider cutoffs for both average pain and worst pain, given (1) the variability of pain for many patients and (2) that cutoffs may vary as a function of the pain intensity domain being considered. Using average pain is likely to adequately measure pain severity among persons who experience stable pain, but asking patients to rate worst pain in addition to average pain could provide additional critical information in individuals whose pain is highly variable. However, thus far, results are mixed: Cutoffs have not varied for average vs. worst pain for patients with cancer (9) or diabetic peripheral neuropathy (11), but the cutoff between moderate and severe pain has been found to be one point lower for “average” vs. “worst” pain in patients with primarily musculoskeletal pain (0-3, 4-5, 6-10 for average and 0-3, 4-6, 7-10 for worst pain in neck pain patients (14) and 0-3, 4-6, 7-10 for average and 0-3, 4-7, 8-10 for worst pain in veterans (16)). The principle aim of the study was to identify the optimal classification scheme for average and worst pain for individuals with MS, using the method described by Serlin and colleagues (13). Secondary aims included determining if the schemes would be the same for measures of average and worst pain intensity, and assessing how the optimal classification scheme for individuals with MS relates to those identified in other patient populations (7-16).