Voluntary control of facial musculature in Parkinson's disease

Aside from being measured in the context of producing facial expressions of emotion, the ability to voluntarily control a range of facial muscles in Parkinson's disease (PD) has not been systematically measured. We used in three enrollment phases an adaptation of the Upper and Lower Face Apraxia test, a measure of the ability to make voluntary movements of the upper and lower face in PD patients and healthy controls. Errors were scored due to (1) pauses prior to movement initiation, (2) loss of individuation, (3) impoverished movement, (4) no movement at all, or (5) content errors (likened to ideational apraxia errors). The results show impaired voluntary control of facial musculature in most but not all with PD (with large effect sizes) which correlated positively and highly with disease severity. Errors by PD patients were predominantly due to impoverished movement and individuation loss whereas those made by controls were predominantly due to individuation loss. Patients committed more errors than controls due to impoverishment and no movement, with negligible differences between groups in other errors. In summary, similarly to spontaneous and voluntary emotional expressions, voluntary non-emotional facial movements are impoverished in PD; impoverishment of all movement types will likely contribute to the mask-like facial appearance that is seen with disease progression. These findings also illustrate the utility of an adapted Face Apraxia test as a practical and sensitive measure of voluntary facial musculature control in PD. The test can be used to supplement clinical observations and as a research tool.