Experimental pain sensitivity in multiple system atrophy and Parkinson's disease at an early stage

Background: Chronic spontaneous pain is a clinically relevant non-motor symptom in multiple system atrophy (MSA) and Parkinson's disease (PD). Experimental pain sensitivity, reflecting the mechanisms of nociception and pain perception leading to clinical pain, is known to be enhanced in both diseases at advanced stages. Also, this study aimed at investigating experimental pain sensitivity already at an early stage (i.e. symptom duration <= 5 years). Methods: Experimental pain sensitivity was assessed by investigating the nociceptive flexion reflex (NFR, reflecting spinal nociception) and heat and electrical pain thresholds. 'Off-drug' MSA (n = 11) and PD (n = 14) patients selected at an early stage of the disease were compared to healthy controls (HC, n = 27). MSA patients had either parkinsonian (MSA-P, n = 5) or cerebellar (MSA-C, n = 6) subtypes. Results: Compared to HC, MSA patients had lower heat pain sensitivity, whereas PD patients had reduced NFR threshold. MSA and PD patients did not differ from HC regarding other variables. MSA-P and MSA-C patients did not differ, either. Conclusions: Impaired sensory discrimination and attention deficits could contribute to the reduced perception of heat pain in MSA, whereas in PD, local changes in spinal excitability or a diminished dopaminergic descending inhibition might impact on the motor efference of the NFR to reduce its threshold to nociceptive afferent information. What does this study add?: This study investigated experimental pain sensitivity at an early stage in MSA and PD.