Hyperalgesia and laser evoked potentials alterations in hemiparkinson: Evidence for an abnormal nociceptive processing

A number of patients with Parkinson's Disease (PD) complain of painful sensations that might be related not only to peripheral factors (muscle spasms, postural abnormalities) but also to an abnormal processing of nociceptive inputs in the Central Nervous System (CNS). To test this hypothesis, we recorded scalp CO₂ laser evoked potentials (LEPs) to foot skin stimulation in 11 pain-free treated PD patients affected by hemiparkinson (during the off state), in 6 pain-free drug-naïve hemiparkinsonian patients and in 11 healthy subjects. After each LEP recording, both patients and controls were asked to rate pain due to laser stimuli. In all subjects, CO₂ laser stimulation gave rise to a main negative N2 potential followed by a positive P2 response at vertex peaking at a latency of about 250 and 350 ms respectively which are thought to originate from several brain structures devoted to nociceptive input processing, including the cingulate gyrus and insula. ANOVA showed that the N2/P2 amplitude was significantly lower and pain rating significantly increased in treated PD patients than in controls in both the affected and unaffected sides, while in drug-naïve PD patients the reduction of the N2/P2 amplitude and the increase in pain rating were observed only in the affected side. These results suggest that in pain-free PD patients there is an abnormal nociceptive input processing that may be independent of the clinical expression of parkinsonian motor signs.