Objective: To test if (1) abnormal right dorsal premotor cortex (dPMC) to left primary motor cortex (M1) interaction during writing with right hand contributes to abnormal action selection and  abnormal movements in right-handed patients with writer’s cramp (WC), and (2) loss of right M1 to left M1 interhemispheric inhibition (IHI) contributes to mirror dystonia in right hand during writing with left hand. 

Background: Dominant M1 is under facilitatory and inhibitory influences from contralateral motor areas for selecting wanted from unwanted motor commands.

Methods: 37 right-handed WC patients with (n=21) and without (n=15) mirror dystonia, and 14 healthy controls (n=14) were tested using dual site TMS during a cued writing task with one hand first and then with the other. Left M1 excitability was tested during preparation for writing, 75 and 100 ms after a visual cue  indicating hand to be used. Motor evoked potentials were recorded from  right first dorsal interosseous (FDI), abductor digiti minimi, and extensor carpi radialis (ECR). Right dPMC-left M1 interactions at inter-stimulus intervals of 8ms and 10ms, and IHI from right M1 to left M1 at inter-stimulus intervals of 10ms (SIHI) and 40ms (LIHI), were tested

Results: Without conditioning, patients showed increased excitability in left M1 during preparation to write with either hand, independent of presence of mirror dystonia. The level of left M1 excitability correlated with severity of WC. With conditioning, the right dPMC inhibited the left M1 at rest equally in controls and WC patients. During preparation to write with the left hand, the dPMC-M1 interaction was similar in both controls and WC patients. However, during preparation to write with the right hand, the right dPMC-left M1 inhibition was replaced by facilitation in FDI in controls while this did not occur in WC patients. Unlike the intrinsic hand muscle, dPMC-M1 inhibition in ECR was replaced by facilitation during preparation to write with right hand only in patients. Patients without mirror dystonia differed from those with mirror dystonia in only one aspect:  LIHI from right to  left M1 for ECR was exaggerated in patients without mirror dystonia, probably as a compensatory mechanism. 

Conclusions: WC might be the combined effect of abnormal excitability in left M1, deficient left dPMC-M1 interaction to facilitate intrinsic hand muscles, and excessive interhemispheric facilitation to forearm extensors

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MDS Abstracts - https://www.mdsabstracts.org/abstract/abnormal-patterns-of-action-selection-in-the-dominant-motor-cortex-in-writers-cramp/