FMRI shows deficient somatosensory activation in orofacial dystonia

Background: Previous fMRI studies indicated cortical dysfunction not only in motor but also in somatosensory areas of patients with orofacial or other focal dystonias during motor tasks. In order to find out whether sensory dysfunction is related to motor execution or an inherent abnormality in orofacial dystonia, we investigated the central processing of passive somatosensory stimuli.

Methods: Standardized tacile stimuli were applied to the face (forehead and upper lip) and hand in 16 patients with orofacial dystonia and 15 healthy controls by a new MR-compatible stimulation device in an event-related fashion. Patients were scanned twice, i.e. before and after treatment with botulinumtoxin (BTX). The functional data were acquired on a 1.5 T-Siemens Symphony MR scanner and analysed using SPM2.

Results: A within-group analysis showed that patients as well as controls activated a distributed somatosensory network including the contralateral primary and bilateral secondary somatosensory cortex as well as the insula, thalamus and inferior parietal cortex. The between-group comparison of patients and controls revealed a reduced activation of the primary somatosensory cortex in patients with orofacial dystonia. This deficient somatosensory activation was not reversed after BTX treatment.

Conclusion: This study found deficient somatosensory activation in patients with orofacial dystonia compared to healthy controls. Since a passive non-motor task was used and the impaired somatosensory activation was not reversed by BTX treatment, we conclude that sensory dysfunction is a primary abnormality in orofacial dystonia. Therefore, orofacial dystonia is more than a disorder of the motor system as it also shows alterations within the sensory system.