Functional MRI reveals enhanced somatosensory and impaired motor activation in orofacial dystonia

Background: Etiology and pathophysiology of the focal dystonia blepharospasm which causes increased blinking frequency and involuntary eyelid spasms are unknown. In Meige syndrome, blepharospasm is associated with oromandibular dystonia. Previous studies have suggested deficient sensorimotor integration and cortical dysfunction in dystonia. We used fMRI to investigate cortical functional changes in these patients during skilled orofacial actions.

Methods: We studied 13 patients with blepharospasm and 13 patients with Meige syndrome before and after botulinumtoxin (BTX) therapy compared to 13 matched controls. Whistling served as a model for a skilled orofacial movement with a high demand on sensorimotor integration. The task occurred during the silent interval of a silent event-related fMRI paradigm to reduce task-related imaging artefacts. Performance was controlled by microphone recordings during functional imaging.

Results: (a) Subject performance: Acoustic analysis of the whistle sounds approved equivalent task performance between the groups. (b) Within-group comparisons: whistling activated the sensorimotor and ventral premotor cortex, the basal ganglia and the rostral paravermal cerebellum bilaterally in all groups. (c) Between-group comparisons: Both forms of orofacial dystonia had overactivity of the postcentral gyrus and caudal SMA compared to controls in common. Meige patients additionally showed deficient activation of the primary motor (M1) and ventral premotor cortex (PMv) bilaterally when compared to controls and blepharospasm patients. In Meige syndrome, postcentral overactivity was reversed by BTX treatment while motor dysfunction was not affected despite clinical improval.

Conclusion: Overactive somatosensory areas during a motor task are particularly noteworthy in blepharospasm where whistling tested a clinically asymptomatic motor system. This finding corroborates the hypothesis that the somatosensory system is involved in the pathophysiology of dystonia. Deficient M1 and PMv activation may be due to cortical reorganization or a primary deficit in orofacial dystonia. They could reflect reduced activation of inhibitory neurons during motor execution in this disorder.