Physical Activity and Pain Modulation in Gulf War Veterans with Chronic Unexplained Muscle Pain

Abstract

Approximately 30% of US Veterans of the First Gulf War are affected by debilitating multisymptom illnesses of which chronic unexplained muscle pain (CMP) is a primary complaint. Large numbers of Veterans of the more recent Iraq War are also dealing with CMP. Previously our lab demonstrated a positive relationship between physical activity (PA) and pain modulatory responses in civilian patients suffering from CMP. To our knowledge, similar relationships have not been explored in Gulf Veterans (GVs) with CMP. PURPOSE: To examine the association between pain modulatory responses and self-reported and objectively-measured PA in GVs with CMP. METHODS: Thirty-three GVs (17 CMP; 16 healthy) underwent thermal heat pain testing with and without distraction. Self-reported PA was assessed via questionnaire; objectively quantified PA with hip-mounted accelerometers. During testing, participants were exposed to 9 heat stimuli (20-s duration) relativized to their perception of ‘slightly intense’ pain. Heat stimuli were administered alone or with distraction in the form of the congruent (CS) and incongruent versions (IS) of the Stroop Task. Pain intensity and unpleasantness ratings were collected following each stimulus. Pain modulation was operationalized as difference scores of ratings for pain stimuli presented alone and with distraction. Correlation coefficients (Pearson r; α = 0.05) for PA measures and pain modulatory responses were calculated separately for each group. RESULTS: Self-reported PA, using either total or intensity measures, was not significantly related to pain modulatory responses (p > 0.05) for either group. Accelerometer measures of time spent in PA at recommended intensity (moderate or greater) and duration (>10 min) were significantly (p > 0.05) and positively related to pain modulatory responses during CS (r > 0.56) and IS (r = 0.60) in healthy GVs only. PA, regardless of how it was measured or quantified, was not related to pain modulation in GVs with CMP. CONCLUSIONS: Unlike healthy GVs, PA levels in GVs with CMP do not appear to be related to effective pain modulation to experimental pain stimuli. Although GVs with CMP bear some symptomatic similarity to civilian CMP patients, a growing body of evidence suggests their condition is unique. Supported by Dept. of Veterans Affairs grant: 561-00436