Within-day variability of pain in youth with juvenile idiopathic arthritis and non-arthritic pain conditions

Purpose: Describe and explain patterns of within-day variability of pain intensity in youth with JIA and non-arthritic pain conditions and within-day relationships between physical activity, mood and pain intensity. Methods: Two complementary studies were conducted. In Study 1 pain intensity data previously collected 3 times per day for 7 days from 112 youth age 8 to 18 years with juvenile idiopathic arthritis (JIA) were examined for within-day patterns using cosinor analysis and generalized estimating equations (GEE). In Study 2, an electronic diary application for the iPod Touch was developed to collect momentary pain and mood data 7 times per day for 4 days from 28 youth age 8 to 17 years with JIA (n=11) or non-arthritic pain conditions (n=17). Physical activity data were collected by accelerometry. GEE analysis was used to examine relationships between pain intensity, physical activity and mood. Results: A cosine pattern of systematic variability in pain intensity was identified in 22.4% of youth in Study 1 (n=85) and 25% in Study 2 (n=28). Age (Study 1: β=0.28, p=0.039), and diagnosis of systemic onset JIA (Study 1: β=2.46, p=0.022) were significant predictors of a cosine pattern of systematic variability. Within-day patterns of pain other than a cosine pattern are identifiable, as time of day (TOD) was a significant predictor of pain on GEE. The relationship between TOD and pain intensity differed by sex and disease subtype (Study 1). On average males had a higher probability of having moderate or severe pain in the morning compared to other times of day. On average females exhibited a U shaped within-day pain pattern with a higher probability of moderate to severe pain in the morning and evening and lower probability in the afternoon. Youth with enthesitis, psoriatic or undifferentiated subtypes of JIA had higher probability of moderate to severe pain in the evening; whereas for all other subtypes this probability was highest in the morning. Pain intensity was related to physical activity level; however, this relationship varied by time of day (Study 2 - combined JIA and non-JIA data). Youth had a higher probability of moderate to severe pain if they were sedentary in the morning, or more physically active in the evening. Higher pain intensity was significantly related to negative mood (β=1.16; p=0.004 [Study 2]) and higher numbers of body locations in pain (β=0.75, p