![]() ![]() For example beta-band (15–30 Hz) coherence is dependent on an intact corticospinal tract ( 15). Moreover, there is growing evidence that coherence at specific frequencies is mediated via specific pathways. ![]() Intermuscular (EMG-EMG) and corticomuscular (EEG-EMG) coherence (IMC, CMC) have both been used extensively to describe changes in brain connectivity in various movement disorders ( 14). Whilst PPI of the blink reflexes is reportedly abnormal in craniofacial dystonia ( 11), cervical dystonia and blepharospasm ( 12, 13), in a previous study of 3 WC patients PPI was normal ( 8). This is thought to be mediated via an inhibitory brainstem reticular pathway involving the pedunculo-pontine nucleus (PPN), and is typically diminished in disorders affecting the PPN or adjoining structures ( 9, 10). In healthy people, the blink reflex is normally inhibited if conditioned by a preceding peripheral nerve stimulus, a phenomenon referred to as PPI. One such technique is pre-pulse inhibition (PPI) of the blink reflex. This supposition is supported to some extent by the lack of strong evidence of brainstem involvement using techniques that are able to probe brainstem physiology non-invasively. In WC, given the importance of the corticospinal tract in the control of skilled finger and hand movements such as writing ( 7), loss of inhibition within sensorimotor cortex is thought to be the almost exclusive cause of the dystonia ( 6, 8). In some patients with WC, dystonic posturing is almost imperceptible and there is focal tremor more prominent whilst writing, similar to dystonic tremor ( 5).įocal dystonia is thought to emerge because of the loss of central inhibition at various anatomical levels including cerebral cortex, brainstem and spinal cord ( 6). WC is more formally classified as a task specific focal hand dystonia ( 2, 3), of which there are other examples reported in typists, musicians and golfers ( 4). These signs are associated with co-contraction of antagonistic muscles ( 1). These patients usually present with difficulty in writing, abnormal posturing, painful spasms and muscle cramps or tremor affecting the forearm or hand whilst writing but not during other activities involving the same muscle groups. In some, this fine balance of activity is lost, writing becomes increasingly difficult and a disorder popularly known as writer's cramp (WC) emerges. Writing is a highly developed motor task requiring fractionated contraction of hand and forearm muscles. IMC may merit further development as an electrodiagnostic test for focal dystonia. Moreover, the lack of PPI implicates abnormalities in brainstem inhibition in the emergence of WC. IMC at 3–7 Hz could reliably discriminate WC patients from healthy controls.Ĭonclusion: Cortical or sub-cortical circuits generating theta (3–7 Hz) oscillations might play an important role in the pathogenesis of WC. Compared to healthy controls, in WC patients the R2 component of the blink reflex showed significantly less PPI. Results: Significant IMC at 3–7 Hz was present in WC patients, but not in healthy controls. PPI involved conditioning this stimulation with a prior shock to the right median nerve (100 ms interval), and measuring the reduction in the R2 component of the blink reflex. Blink responses were evaluated in 9 healthy controls and 10 WC patients by stimulating the right supraorbital nerve and recording surface EMG from the orbicularis oculi muscles bilaterally. Methods: IMC was measured in 10 healthy controls and 20 WC patients (10 with associated tremor) while they performed a precision grip task at different force levels. We explored the involvement of cortical and brainstem circuits by measuring intermuscular coherence (IMC) and pre-pulse inhibition (PPI) of the blink reflex. ![]()
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