Pain phenomena and possible mechanisms in patients with painful polyneuropathy

Painful polyneuropathy is a common neuropathic pain condition characterized by different typical pain phenomena and symptoms. The present study determined the frequency of pain phenomena and signs in painful polyneuropathy, and compared the symptomatology in patients with signs of increased small fiber response with that in patients with signs of deafferentation. Eighty-one consecutive patients with painful polyneuropathy were studied. The most common pain phenomena were deep aching pain (88%) and pain on pressure (69%), followed by pain paroxysms (59%) and less frequently pain on light touch (31%). Patients with increased cold and heat detection thresholds were more likely to have pain paroxysms (odds ratio (OR) 2.5 and 5.2) and patients with pain summation on repetitive mechanical stimulation more often had touch-evoked pain (OR=4.0) than patients without these phenomena. Findings compatible with increased small fiber response were found in six patients (7.4%), in 41 (50.6%) unequivocal signs of deafferentation were found, and 34 patients (42%) could not be classified. There was no significant difference in presenting symptoms between these groups. In conclusion, in painful polyneuropathy, (1). deep aching pain is the most frequently reported pain symptom; (2). the association between pain paroxysms with decreased small fiber function and touch-evoked pain with abnormal pain summation on mechanical stimulation indicate that central nervous system mechanisms are responsible for these symptoms; (3). sensitized small fibers as the single mechanism of pain is rare; and (4). pain symptomatology cannot predict pain mechanisms as being mainly deafferentation or sensitized small fibers.