Inter-hemispheric inhibition is impaired in mirror dystonia

Abstract

Surround inhibition, a neural mechanism relevant for skilled motor behavior, has been shown to be deficient in the affected primary motor cortex (M1) in patients with focal hand dystonia (FHD). Even in unilateral FHD, however, electrophysiological and neuroimaging studies have provided evidence for bilateral M1 abnormalities. Clinically, the presence of mirror dystonia, dystonic posturing when the opposite hand is moved, also suggests abnormal interhemispheric interaction. To assess whether a loss of inter-hemispheric inhibition (IHI) may contribute to the reduced surround inhibition, IHI towards the affected or dominant M1 was examined in 13 patients with FHD (seven patients with and six patients without mirror dystonia, all affected on the right hand) and 12 right-handed, age-matched healthy controls (CON group). IHI was tested at rest and during three different phases of a right index finger movement in a synergistic, as well as in a neighboring, relaxed muscle. There was a trend for a selective loss of IHI between the homologous surrounding muscles in the phase 50 ms before electromyogram onset in patients with FHD. Post hoc analysis revealed that this effect was due to a loss of IHI in the patients with FHD with mirror dystonia, while patients without mirror dystonia did not show any difference in IHI modulation compared with healthy controls. We conclude that mirror dystonia may be due to impaired IHI towards neighboring muscles before movement onset. However, IHI does not seem to play a major role in the general pathophysiology of FHD.

Figure 1. Paired Pulse TMS APB

Mean and standard errors for the conditioned MEP in APB for all three groups (CON, FHD-D, and FHD-ND). While there was no difference among groups at rest, the FHD-D group shows loss of inhibition in the premotor phase, while the other two groups maintained some inhibition in this phase.

Figure 2. Paired Pulse TMS FDI

Mean and standard errors for the conditioned MEP in FDI for all three groups (CON, FHD-D, and FHD-ND). There was no difference in IHI in FDI among groups or phases.

Figure 3. IHI in APB during the premotor phase

The individual values for all participating subjects divided into the three groups (▲CON, ● FHD-D and ♦ FHD-ND) show that six of seven FHD-MM patients had an inter-hemispheric facilitation instead of an inhibition in the premotor phase. None of the subjects in the control group or the FHD-NM patients showed facilitation.