The mutual relationship between movement and pain: a study by functional near-infrared spectroscopy and laser evoked potentials in normal subjects and fibromyalgia patients

Recent studies suggested that the activation of motor cortex can decrease subjective pain and nociceptive induced brain responses in patients with chronic pain. Physical exercise would seem to improve quality of life and fitness of patients with Fibromyalgia (FM), an invalidating disorder characterized by chronic diffuse muscle-skeletal pain. This study aimed to examine the interaction between the activity of motor and somatosensory cortex during concurrent painful stimulation and motor task in FM patients and control subjects. The simultaneous recording by functional Near Infrared-Spectroscopy (fNIRS) and Laser Evoked Potentials (LEPs) was used to evaluate the effects of motor cortex activity on the subjective pain and LEPs. The fNIRS is a portable optical method to detect cerebral hemodynamic changes in real time. We used 20 fNIRS channels positioned on the primary motor cortex according to Predefined Montage Tab by NIRStar 14.2. and 63 scalp electrodes for LEPs recording. The experimental protocol consisted of a basal session of 30 laser stimuli delivered over the right hand back, followed by analogue stimulation modality during a concurrent finger tapping task. The participants were 23 patients with FM (4 males and 19 females, Mage =43,17 SD=10,56 , age range from 23 to 60) and 13 healthy subjects (5 males and 8 females, Mage=37,38 SD=15,50, age range from 19 to 60 ). The analysis of fNIRS data confirmed the activation of primary motor cortex both in patients and controls during finger tapping task. A significant reduction of oxyhemoglobin levels corresponding to the contralateral motor cortex was present in FM patients during fast movement with a trend toward reduced changes during concurrent laser stimulation. The finger tapping speed was reduced in patients and independent from concurrent laser stimulation. The N1 amplitude was reduced during movement in patients and controls, with a similar trend for the N2P2 complex. There was a trend toward reduced changes in patients. In FM patients the general level of motor activation was reduced, as for effect of a chronic inhibition. A trend toward a reduced modulation of both N1 and N2P2 components emerged during fast movement, thus preliminary confirming the self sustained circuit of pain persistence and motor inhibition. The study confirmed the mutual relationship between motor and nociceptive cortex, as physical activity may be considered a beneficial support in pain control.