Does abnormal interhemispheric inhibition play a role in mirror dystonia?

Abstract

The presence of mirror dystonia (dystonic movement induced by a specific task performed by the unaffected hand) in the dominant hand of writer's cramp patients when the nondominant hand is moved suggests an abnormal interaction between the 2 hemispheres. In this study we compare the level of interhemispheric inhibition (IHI) in 2 groups of patients with writer's cramp, one with the presence of a mirror dystonia and the other without as well as a control group. The level of bidirectional IHI was measured in wrist muscles with dual-site transcranial magnetic stimulation with a 10-millisecond (short IHI) and a 40-millisecond (long IHI) interstimulus interval during rest and while holding a pen in 9 patients with mirror dystonia 7 without mirror dystonia, and 13 controls. The group of patients without mirror dystonia did not differ from the controls in their IHI level. In contrast, IHI was significantly decreased in the group of patients with mirror dystonia in comparison with the group without mirror dystonia and the controls in both wrist muscles of both the dystonic and unaffected hand whatever the resting or active condition (P = 0.001). The decrease of IHI level in the group of patients with mirror dystonia was negatively correlated with the severity and the duration of the disease: the weaker the level of IHI, the more severe was the disease and the longer its duration. Interhemispheric inhibition disturbances are most likely involved in the occurrence of mirror dystonia. This bilateral deficient inhibition further suggests the involvement of the unaffected hemisphere in the pathophysiology of unilateral dystonia.

Cortical activation and inter-hemispheric sensorimotor coherence in individuals with arm dystonia due to childhood stroke

“…More recently, there has been a study of adult patients with secondary (due to focal lesions of caudate and putamen) hemidystonia (Trompetto et al, 2012) similarly indicating increased cortical excitability, and decreased inhibition compared to controls. Further, in patients with mirror dystonia (movement of the unaffected hand triggers dystonia in the affected hand), double-pulse TMS studies have shown reduced inhibition of the IL hemisphere from the CL hemisphere (Beck et al, 2009; Sattler et al, 2014). However, whether a reduction in inter-hemispheric connectivity causes dystonia, is a result of dystonia, or is related to an injury unassociated with dystonia is not clear from this study and is an important topic for future investigations.…”

Focal dystonia and the Sensory-Motor Integrative Loop for Enacting (SMILE)

“…This discovery suggests that interhemispheric inhibition is not deeply involved in the basic pathophysiology of dystonia, but only in its mirror aspect. In order to investigate the task-specificity of inter-hemispheric inhibition in mirror dystonia, Sattler et al (2013) extended the previous study with a rest versus pen-holding task. At rest, the inter-hemispheric inhibition levels of all three groups (healthy subject, mirror and non-mirror FHD) were similar, but mirror patients displayed a large bilateral decrease in inter-hemispheric inhibition in the pen-holding condition, inversely related to the severity and duration of symptoms.…”

Long-latency modulation of motor cortex excitability by ipsilateral posterior inferior frontal gyrus and pre-supplementary motor area

“…The dsTMS paradigm has been extensively used to investigate interhemispheric connections between homologous M1 sites2223252627. More recently, interactions between non-primary motor areas and M1 have started to be investigated20282930.…”

“…However, neural interactions within the motor system likely occur on different time-scales. Indeed, longer-latency interactions with ISIs up to 150 ms have been documented between M1 and contralateral motor-related areas3637 and studies have shown altered long-latency M1-M1 interhemispheric interactions (at an ISI of 40 ms) in neurological conditions affecting motor control2526. Thus, motor network functioning may be based on optimal tuning between short-latency, as well as long-latency, interactions.…”