Hypomimia which refers to a reduced degree in facial expressiveness is a common sign in Parkinson's disease (PD). The objective of our study was to investigate how hypomimia affects PD patients' facial expression of pain. The facial expressions of 23 idiopathic PD patients in the Off-phase (without dopaminergic medication) and On-phase (after dopaminergic medication intake) and 23 matched controls in response to phasic heat-pain and a temporal summation procedure were recorded and analyzed for overall and specific alterations using the Facial Action Coding System (FACS). We found reduced overall facial activity in response to pain in PD patients in the Off which was less pronounced in the On. Especially the highly pain-relevant eye-narrowing occurred less frequently in PD patients than in controls in both phases while frequencies of other pain-relevant movements, like upper lip raise (in the On) and contraction of the eyebrows (in both phases), did not differ between groups. Moreover, opening of the mouth (which is often not considered as pain-relevant) was the most frequently displayed movement in PD patients, whereas eye-narrowing was the most frequent movement in controls. Not only overall quantitative changes in the degree of facial pain expressiveness occurred in PD patients but also qualitative changes were found. The latter refer to a strongly affected encoding of the sensory dimension of pain (eye-narrowing) while the encoding of the affective dimension of pain (contradiction of the eyebrows) was preserved. This imbalanced pain signal might affect pain communication and pain assessment.
Keywords: Caudate nucleus; Facial expression of pain; Hypomimia; Nigrostriatal dopamine system; Non-verbal pain communication; Pain; Parkinson's disease.
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