With the advent of electronic diary methods that capture the dynamic nature of symptom levels, it is becoming increasingly evident that a patient's “average” or “usual” levelof pain, interference, or affectrepresents only one dimension of persistent pain [41
]. In this report, we focus on individual differences in the magnitude of fluctuations in pain and other daily measures across multiple days in two studies. Several results are noteworthy. First, day-to-day fluctuationsare sizeable in patients with persistent pain. Second, the magnitude of variabilityis similar in both studies using different rheumatology diagnostic groups. In both samples, day-to-day fluctuations range on average from 13 to 17 points (100-point scale) across all daily measures. Indeed, variability in daily pain and other variables has been found across patients with rheumatoid arthritis[38
], chronic pain clinic patients[21
], and fibromyalgia [15
]. This documents the need to understandday-to-day symptom fluctuation in a variety of persistent pain conditions, including OA,which has been assumed to be a more stable disease [2
]. Taken together, these findings suggest that such fluctuations are an important feature of the pain experience and deserve more attention from pain clinicians and researchers.
Furthermore, substantial individual differences
in the magnitude of the variabilityare observed in both studies. Whereas some patients experience little fluctuation in the intensity of daily pain and other measures, other patients' reports show pronounced day-to-day shifts. Across our two studies, we were able to examine several constructs with the potential to explainthese individual differences in day-to-day variability: depression, anxiety, self-efficacy, and pain catastrophizing, though not all variables were available in both studies. Depression emerges as a significant predictor of greater variability in pain intensity in Study 1. Depression predicts more frequent”acute” day-to-day changes in pain, that is, daily shifts exceeding a threshold that has been considered clinically meaningful[13
]. Specifically, patients with low levels of depression experiencedtheseacute daily shifts in pain intensity on every fifth day, whereas patients with moderate or severe depression experienced them approximately every third day.
Depression also predicts daily variations in ratings of happiness and frustration in Study 1, consistent with depression being viewed as a mood regulation problem.The effect is stronger for daily shifts in frustration than for happiness, in line with cognitive theory suggesting that depression involves biased attention and processing particularly of negative emotional information.[4
]Previous studies have similarly documented greater affective instability in patients during depressive episodes [7
].In Study 2, depression is a significant predictor of daily variations in how much arthritis interfered with social relationships and how satisfied patients were with their daily accomplishments. However, depression does not predict pain variability in Study 2. This may be due to the fact that Study 2 patients were less depressed with only 11% in the moderate to severe range of depression on the BDI (compared to 22% in Study 1), thus attenuating a possible relationship. Anxiety does not predict variability in either study for any of the patient ratings. These results are consistent with prior research where depression is a more consistent predictor of adjustment to pain than anxiety [27
]. Considered overall, these findings suggest that patients who have persistent pain and who have higher levels of depression are more likely to have highly variable day-to-day experiences in pain and well being with more frequent highs and lows.
In Study 2, we examine two indices of pain coping (self efficacy and pain catastrophizing) that might explain variability. First, patients who are more confident about their ability to control pain (self-efficacy) display more stability in their satisfaction with daily accomplishments and quality of their day. Thus, self-efficacy may serve as a buffer that prevents frequent vacillation in patients' perceived quality of life and daily accomplishments. These results are consistent with findings that self-efficacy buffers the effects of stressful events on emotion and well-being[31
]. Second, pain catastrophizing predictsmore variability of interference with social relations. The dominant theory of pain catastrophizing is a communal model that maintains that catastrophizing initially develops because it helps patients to maintain proximity and support from others[42
]. However, the communal model and research studies further suggest that, when pain persists over time, catastrophizing has a detrimental impact on support and social relationships[9
]. Our finding that catastrophizing predicts more variability in interference with social relations, thus, fits both theory and prior research.
Our results have several clinical implications. First, they suggest that fluctuations in pain and other pain-related measures are a common experience forpatients, even those with “stable” disease.Thus, to best capture how patients are adjusting to pain, clinical assessments should go beyond asking patients to report their usual or average pain level and include assessment of how their pain varies from day to day and how this variability impacts them.Second,clinicians might consider targeting a reduction in variability as an important treatment target. For example, patients may find the pain experience to be more predictable and manageable if it varies less from day to day. Our results suggest that individual differences in variability are moderately stable over four weeks (Study 1). The 4-month stability estimate(Study 2) ismoderate to low;but comparison of these reliabilities must be interpreted cautiously, since fewer observations were available and a different methodology was used in Study 2. Third, clinicians need to recognize that patients with certain characteristics (e.g., depression) are more likely to experience daily fluctuations.Thus, a treatment focus on depressionmay also mitigate variability in pain and related variables.
There are several important future directions for research in this area. First, there is a need to explore additional factors that might explain daily variability. The current study focused on several psychological factors, but future studies should examine biological factors (e.g. genetic background, disease severity, medications) as well as social and environmental factors (social support, cultural background, activity level) that may be important. Second, future studies should examine the degree to which fluctuations in pain and related experiences are, in and of themselves, predictive of treatment outcome. Along these lines, there is evidence that people who show greater symptom fluctuations may be more likely to respond to drugs and placebos. For example, Harris[15
] reported that fibromyalgia patients with larger pain variability (defined by within-person SD) were more responsive to a placebo intervention than patients with smaller pain variability. Similarly, Scott-Lennox [35
]reported that patients with initially more intense pain “flares” were more responsive to placebo.
There are several strengths of these studies. First, the daily ratings of pain and related variables were collected electronically with time and date stamping to insure the internal validity of the measurements [40
]. Second, the hypotheses were examined in two somewhat different samples of rheumatology patients, increasing generalizability of the findings. Third, the samples were relatively large (N=106, N=194) increasing the reliability of the findings. Fourth, the analytic strategies usedin this report are especially suitedto understand individual differences in symptom variability. Importantly, the described multilevel models can readily be expanded to research questions involving longitudinal change in variability, for example, to analyze the extent to which patients exhibit increasing stability over time in response to medical or psychological treatment[16
There are also limitations of these studies. The samples are predominantly female, as would be expected in rheumatology samples, White, and well educated; thus, the results may not generalize to other demographic and diagnostic groups. Second, the analyses are based on end-of-day diaries. The results may not generalize to fluctuations that occur within days, such as moment-to-moment instability. Third, the relatively small number of observations per person (7 days) in Study 2 may have limited the statistical power to detect significant effects of person-level predictors in this study, and may also have attenuated the test-retest reliability estimates of individual differences in variability. Fourth, the analysesare cross-sectionaland cannot establish whether depression and coping appraisals are a cause or a consequence of fluctuations in pain and daily adjustment, or whether both have reciprocal effects on each other.
In conclusion, fluctuations in pain, interference in activities, and mood are observed to be a fundamental aspect of the daily experience of patients with persistent pain. However, little is still known about the clinical importance of these fluctuations as an individual-difference marker. Whereas this research was focused on associations of variability with psychological factors, future research will be necessary to examine more closely the clinical utility of measuring individual differences in variability. This requires more understanding, for example, about the role of symptom variability in the frequency of medication usage or healthcare utilization, and the patient's overall view of the manageability of the disease. It is conceivable that optimal treatment effects, as perceived by patients,may require both reductions in level and variability in pain.