Myofascial pain and fibromyalgia: Spatial summation of heat pain is not abnormal in patients with fibromyalgia syndrome

Abstract

Spatial summation of heat sensations (SPAS) is positively correlated with the stimulated surface area and this effect has been clearly demonstrated for non-painful and painful stimuli in normal controls subjects (NC). In animal and human experiments the application of increasingly painful stimuli has been shown to recruit not only strong nociceptive but also anti-nociceptive mechanisms including diffuse noxious inhibitory controls (DNIC). In addition, pain coding by central nociceptive neurons is not only dependent on stimulus intensity but also on stimulated surface area. Abnormal pain modulatory and spatial summation mechanisms have been implicated in patients with fibromyalgia syndrome (FMS). We therefore applied thermal stimuli to the upper extremities (UE) to explore the hypothesis that FMS patients have abnormal SPAS mechanisms. 37 FMS patients and 22 NC immersed pre-determined areas of the UE in hot water (46C) for 5s and 20s in counterbalanced order. A mechanical visual analogue scale (VAS) was used for pain ratings. The area of thermal stimulation was varied across UE areas to test the contribution of dermatomes to SPAS. The results of pain ratings during 5s and 20s immersions were similar for each diagnostic group (p > .05). All experimental pain ratings of FMS patients were higher than NC (p < .05). Immersion of each individual finger (2-5) resulted in similar VAS ratings of pain (p > .5) within each group (baseline). Pain ratings, however, increased in NC and FMS subjects when the immersed areas were expanded within or across dermatomes (p < .04). The increase in pain ratings was greater within than across dermatomes. In addition, the increase in pain ratings during SPAS was comparable for FMS and NC subjects (p > .05) for all test conditions. Spatial summation of heat pain is not different between FMS patients and NC. A lack of difference between pain ratings after hot water immersions suggests effective anti-nociceptive mechanisms in both groups. These findings indicate that the clinical pain of FMS patients may not be related to abnormal spatial coding of peripheral nociceptive input.