Abnormal postexcitatory and interhemispheric motor cortex inhibition in writer's cramp

Area of Science:

• Neuroscience
• Motor Control
• Clinical Neurology

Background:

• Writer's cramp is a focal dystonia affecting fine motor skills.
• The underlying pathophysiology of writer's cramp, particularly motor cortex excitability, remains incompletely understood.

Purpose of the Study:

• To investigate abnormalities in motor cortex inhibition in patients with writer's cramp.
• To differentiate between excitatory and inhibitory corticospinal and interhemispheric effects using transcranial magnetic stimulation (TMS).

Main Methods:

• Focal TMS was applied to the motor cortex in 25 writer's cramp patients and 25 healthy controls.
• Motor evoked potentials (MEPs) were recorded from the first dorsal interosseus muscles.
• Excitatory and inhibitory effects, including post-excitatory inhibition and interhemispheric inhibition, were assessed.

Main Results:

• Motor thresholds, MEP amplitudes, and latencies were normal in writer's cramp patients.
• Post-excitatory inhibition duration was significantly shortened in patients compared to controls (168±55 ms vs. 198±39 ms).
• Interhemispheric inhibition duration was significantly prolonged in patients compared to controls (30.3±6.6 ms vs. 26±3.9 ms).

Conclusions:

• Writer's cramp is associated with decreased motor cortex inhibition, affecting both corticospinal and transcallosal outputs.
• These inhibitory deficits appear generalized, present in both hemispheres and during non-dystonic motor tasks.
• Findings suggest abnormal motor cortex inhibition is a key feature of writer's cramp pathophysiology.