What abnormalities in the central nervous system have been implicated in FM?
Most investigators believe that the pathophysiology of FM is due to central sensitization within the CNS, manifesting as amplified pain perception. Thus, physical stresses to the musculoskeletal system that in the normal individual are perceived as non-tender touch, position sense and non-tender temperature sensation are perceived as pain in the patient with FM. It appears that the pain threshold is lower in patients with FM. The underlying abnormalities within the CNS leading to amplified pain perception are not completely understood, but a number of specific abnormalities have been observed in the afferent pain processing areas of the CNS in some patients with FM:
- • Increased excitability of dorsal horn nuclei due to abnormal windup: increased levels of substance P, nerve growth factor, glutamate, and aspartate measured in cerebrospinal fluid (CSF) of patients with FM
- • Expanded receptive fields for central pain perception (central sensitization): fMRI shows expanded fields in the insula, anterior cingulate cortex, and somatosensory cortex.
- • Abnormalities within the descending analgesia system: decreased levels of pain inhibitory neurotransmitters (antinociceptive) including norepinephrine, serotonin, and dopamine in the CSF of patients with FM
- • Suppression of normal activity of dopamine-releasing neurons in limbic system.
Note: the levels of opioids are increased in the CSF of patients with FM; this may explain why narcotics have not proven to be effective in controlling the widespread pain typical of FM.