Chronic musculoskeletal pain : mechanisms and pain reports

Abstract

Background: The knowledge of underlying mechanisms for the maintenance and spread of musculoskeletal pain is limited. Pain is a complex subjective experience influenced by a variety of factors. The focus of the present thesis is on possible mechanisms associated with chronic generalized musculoskeletal pain, and factors of importance for the variation in reports of pain intensity and sensory symptoms in subjects with localized and generalized musculoskeletal pain. Aims: The specific aims were to investigate: 1) whether sympathoadrenal and cortisol responses were attenuated and associated with pain intensity and muscle fatigue during exercise in subjects with fibromyalgia (FM) compared with healthy controls; 2) the reliability and validity of two different pain assessment strategies of recalled pain intensity in subjects with localized (LP) and generalized (GP) musculoskeletal pain; 3) whether pain intensity, number of painful body areas and emotional distress were associated with neuropathic symptoms in subjects with musculoskeletal pain Materials and methods: Sympathoadrenal and cortisol responses, pain intensity and muscular responses were compared between subjects with fibromyalgia and their matched healthy controls during dynamic bicycling and static repetitive contractions (papers I and II). Pain intensity, neuropathic symptoms (LANSS), and number of painful body areas were reported over the first week in four subsequent months and compared in subjects with LP and GP (papers III and IV). Pain intensity was assessed as recalls of pain intensity during the last 24 hours (daily recalls) (papers I-III) and the last seven days (weekly recalls) (papers I-IV). Real-time pain intensity was assessed moment by moment during exercise (papers I and II) and during one week in every day life (paper III). Results: Compared with the healthy controls the FM patients exhibited lower peak oxygen uptake and lower MVC (papers I and II), similar physiological responses during dynamic exercise (paper I), but lower plasma adrenaline responses and higher relative EMG during static repetitive exercise (paper II). The catecholamine responses were not associated with real-time pain intensity and muscle fatigue during exercise (papers I and II). Real-time pain intensity increased during exercise, but no increase was reported in recalled pain comparing the week before and after exercise. Across four months the average of daily ratings of recalled pain intensity conducted over a week were lower and corresponded better with the average of multiple real-time ratings than single ratings of weekly recalls. The GP group obtained lower reliability of pain intensity than the LP group and overestimated weekly recalled pain compared to real-time pain. The overestimation increased with increasing pain intensity (paper III). The LANSS scores were stable over time and positively associated with number of painful body areas, pain intensity, and emotional distress. In multiple regression analysis emotional distress and the diagnosis of fibromyalgia remained the final predictors of neuropathic symptoms (paper IV). Conclusion: This study showed attenuated adrenaline responses in FM during static repetitive exercise, but no clear relationship between altered physiological responses and exercise related pain. Pain intensity varied considerably according to context and the assessment method applied. Generalized pain and emotional distress were the main factors influencing the reports of pain and sensory symptoms. In future studies, the causal relationship between emotional distress and development of generalized pain and associated symptoms need further exploration.