Somatosensory dysfunctin in fibromyaligia : Implications for pathophysiological mechanisms

Sammanfattning: SOMATOSENSORY DYSFUNCTION IN FIBROMYALGIA. IMPLICATIONS FOR PATHOPHYSIOLOGICAL MECHANISMS. Eva KosekDissertation from the Department of Rehabilitation Medicine, Karolinska Institure/Hospital, Stoclcholm, Sweden Fibromyalgia is a chronic pain syndrome characterized by generalized pain, tenderness,disturbed sleep and pronounced fatigue. The pathophysiology is unknown but peripheralmechanisms (i.e., muscle ischemia) as well as central mechanisms (dysfunction in endogenouspain modulation) have been implicated. The aim of this thesis was to increase theunderstanding of pathophysiological mechanisms in fibromyalgia by studying the location oftenderness, the perception of somatosensory stimuli, the relationship between ongoing pain andperception of somatosensory stimuli and, furthermore by studying the modulation ofsomatosensory perception during isometric muscular contraction, vibratory stimulation andheterotopic noxious conditioning stimulation.Methods: Thirty-one female fibromyalgia patients, all meeting the classification criteriaproposed by the American College of Rheumatology, and 26 healthy females participated inthe study. Von Frey filaments were used to test low-threshold mechanoreceptive function.Pressure pain sensitivity was assessed with a pressure algometer and thermal sensitivity with aThermotest. A computerized isokinetic system was used for determination of maximalisometric strength and force registration during contraction. Vibratory stimulation (lOOHz)was delivered by an electromechanical vibrator and a submaximal effort tourniquet test wasused for heterotopic noxious conditioning stimulation. Skin hypoesthesia was induced by alocal anesthetic cream (EMLA).Results: Fibromyalgia patients exhibited a generalized, non-modality specific increase in painsensitivity and a further increase in sensitivity to pressure pain and innocuous warmth in areaswith high intensity of spontaneous ongoing pain. The increased pressure pain sensitivity infibromyalgia patients was located deep to the skin but not restricted to muscle tissue. Anabnormal increase in pressure pain sensitivity over the contracting muscle during and followingisometric contraction was demonstrated in fibromyalgia patients, as opposed to a decrease insensitivity in healthy controls. No abnormalities were found in the endogenous modulation ofsomatosensory perception by activity in large myelinated fibers set up by vibratory stimulation.Decreased sensitivity to innocuous thermal stimulation was seen during and following thetourniquet test in both groups alike, but no modulation of pressure pain sensitivity was foundin fibromyalgia patients during the tourniquet test, as opposed to healthy controls, suggesting adysfunction of 'diffuse noxious inhibitory controls'.Conclusions: The lack of normal modulation of pressure pain sensitivity in fibromyalgiapatients during isometric contraction could be explained by peripheral as well as centralmechanisms. However, the findings that fibromyalgia patients had a generalized, non-modalityspecific increase in pain sensitivity, only partially related to the intensity of ongoing pain in theassessed area, and that the increased pressure pain sensitivity was not restricted to muscletissue support central mechanisms. The absence of normal inhibition of pressure pain sensitivityduring heterotopic noxious conditioning stimulation in fibromyalgia patients can only beexplained by a central nervous system dysfunction. Key words: allodynia, diffuse noxious inhibitory controls (DNIC), fibromyalgia, hyperesthesia, isometric contraction, pain modulation, pain inhibition, pressure algometry, quantitative sensory testing, sensitivity, sensitization, tourniquet test, vibration. ISBN 91-628-2081-8