Abnormal cortical motor excitability in dystonia

Hemispheric differences in motor cortex excitability during a simple index finger abduction task in humans

Transcranial magnetic (TMS) and electrical (TES) stimulation was used to assess the contribution of the corticospinal pathway to activation of the first dorsal interosseous muscle (FDI) in each hand of 16 right-handed subjects. TMS was applied at relaxed threshold intensity while the subject performed isometric index finger abduction at seven force levels [0.5 N to 50% maximal voluntary contraction (MVC)]. In a separate session, TES of equivalent intensity was applied to each hemisphere in 5 of these subjects while they performed the same force-matching protocol. In the resting state, mean threshold intensity for a muscle-evoked potential (MEP) in FDI using TMS was similar for the hemispheres controlling the dominant and nondominant hands. The size of the threshold MEPs in resting FDI after TMS and TES were also similar in each hand. With TMS, contraction-induced facilitation of the MEP in FDI was significantly larger when the nondominant hand was used for index finger abduction. In the pooled data, the nondominant/dominant ratio of MEP areas (normalized to the maximum M wave) ranged from 1. 7 in the weakest contraction (0.5 N) to 1.1 in the strongest (50% MVC). Eight subjects had significant differences between hands in favour of the nondominant hand, whereas in two subjects contraction-induced facilitation of MEPs was larger in the dominant hand. In five subjects for whom detailed motor unit data were available from a previous study, lateral differences in MEP facilitation were positively correlated with differences in FDI motor unit synchronization between hands. With TES, contraction-induced facilitation of the MEP was similar in each hand, suggesting that spinal excitability was equivalent on both sides. For the group of five subjects tested with both stimulation techniques, contraction-induced facilitation of the MEP was significantly larger after TMS than that obtained with TES when the contraction was performed with the nondominant hand, but not when the dominant hand was used to perform the task. We conclude that the extent of corticospinal neuron involvement in the command for simple index finger abduction in right-handed subjects is generally greater when the nondominant hand is used, compared with the same task performed with the dominant hand.

Change in lateralization of the P22/N30 cortical component of median nerve somatosensory evoked potentials in patients with cervical dystonia after successful treatment with botulinum toxin A

The precentral P22/N30 cortical component of the median nerve somatosensory evoked potentials (SEPs) was recorded in 16 patients (11 women and five men) suffering from cervical dystonia before and after botulinum toxin therapy. Cervical dystonia was diagnosed as idiopathic in all patients: 13 patients suffered from right-sided torticollis, and three suffered from left-sided torticollis. The amplitude of the P22/N30 component and the side-to-side ratio of amplitude values were measured. Normal values were obtained by acquiring measurements in two groups of healthy volunteers (n1 = 20 and n2 = 20). The recordings in the first control group were done with the patient's head in a normal position, whereas, in the second control group, the patient kept the head intentionally rotated 60 degrees to the right. Patients were treated with local injections of botulinum toxin A (BTX-A). The mean duration of treatment was 8.3 months, and the mean total amount of BTX injected was 295 U. The P22/N30 precentral component was repeatedly recorded in patients after head posture had been corrected to the normal plane by BTX-A treatment. The recordings showed that the amplitude of the P22/N30 precentral component recorded contralaterally to the direction of head deviation was significantly higher in patients before treatment than after treatment. Contralateral pretreatment amplitudes were also significantly higher (p < 0.01 and p < 0.05, respectively) than amplitudes in both groups of healthy volunteers. The mean side-to-side ratio of precentral P22/N30 component amplitudes was significantly higher in patients before treatment compared with after treatment and also compared with both control groups. These changes in dystonic patients probably reflect the direction of head rotation, the muscle pattern of torticollis, and the change in force of dystonic contraction after the treatment. The changes presumably could be the result of higher excitability of the precentral cortex contralateral to head rotation in patients with cervical dystonia and its change after successful BTX-A treatment.

Alterations of motor cortical inhibition in patients with dystonia

Cortical inhibitory mechanisms were investigated with the technique of paired transcranial magnetic stimulation in 10 patients with dystonia of the right arm: six patients had focal, task-specific dystonia (writer's cramp) and three had segmental and one had generalized dystonia. Paired stimuli were delivered in a conditioning-test design during slight voluntary activation of the target muscle, with subthreshold conditioning stimuli at short intervals (3-20 ms) and suprathreshold conditioning stimuli at long intervals (100-250 ms). The amount of inhibition at short interstimulus intervals did not differ significantly between patients and normal subjects. With long interstimulus intervals, patients showed more inhibition of the test response, which was significant at the 150-ms interval. The cortical silent period following a single suprathreshold magnetic stimulus was slightly shorter in patients. No significant difference was detected between the affected side and the unaffected side in patients with unilateral task-specific dystonia, neither in the duration of the silent period nor in the response to paired magnetic stimuli. These results indicate that the different types of motor cortical inhibition are produced by different inhibitory circuits. We propose that the alterations observed in patients with dystonia are the result of impaired feedback from the basal ganglia to motor cortical areas, with the ultimate effect of a flattening of the excitability curve of the cortical motoneuron pool during voluntary muscle activation.

Excitability of the central masticatory pathways in patients with painful temporomandibular disorders

Much is unclear about the pathophysiological mechanisms underlying painful temporomandibular disorders. In addition to various other theories, masticatory muscle dysfunction and pain have also been attributed to primary central nervous system hyperactivity. We assessed this possibility in a study using recent neurophysiological techniques. From among outpatients whose diagnosis of temporomandibular disorders had been obtained in stomatognathic facilities, we studied 10 patients with bilateral pain and 15 patients with unilateral pain, in whom electromyographic examination of the trigeminal reflexes disclosed normal findings except for absence or amplitude asymmetry of the jaw jerk. Transcranial magnetic stimulation yielded masseter motor evoked potentials of normal latency and amplitude, but five patients had to exert a near-maximum contraction to obtain their responses. The masseter silent periods elicited by the double-shock technique recovered normally. Because these tests measure the excitability of the masticatory system (including motor cortex, corticobulbar and corticoreticular connections, reticular interneurones and lower motoneurones), the lack of facilitation in these patients' responses excluded central hyperactivity as the primary cause of their masticatory dysfunction and pain.

Stimulus/response curves as a method of measuring motor cortical excitability in man

We investigated whether input/output curves of human motor cortex could provide similar information to cortical mapping under two conditions where the motor maps are known to change dramatically: ischaemic anaesthesia and amputation. Stimulus/response curves were constructed by recording the size of EMG responses evoked in arm muscles with transcranial magnetic stimulation at a single site using a range of intensities. Changes in the slope of this relationship during ischaemic anaesthesia (6 normal subjects) or amputation (two patients) were compared to changes in cortical motor maps produced by stimulating different sites at the same intensity. At rest both interventions increased map areas, as well as the slope of the stimulus/response curves. During voluntary activity they had no effect. We conclude that stimulus/response curves can detect changes in cortical motor maps, and discuss potential mechanisms for this effect.

Post-exercise depression of motor evoked potentials as a function of exercise duration

Post-exercise facilitation and post-exercise depression are phenomena described in motor evoked potentials (MEPs) elicited to transcranial magnetic stimulation. Brief, non-fatiguing muscle activation produces post-exercise facilitation, and prolonged fatiguing muscle activation produces post-exercise depression. We studied 12 normal subjects to determine whether post-exercise depression occurs before fatigue is reached. We recorded MEPs from the resting extensor carpi radialis muscle after increasing the duration of isometric wrist extension, at 50% of maximum voluntary contraction, until the muscle fatigued. Fatigue was defined as the inability to maintain that force. The mean exercise duration before the muscle fatigued was 130 s, and post-exercise depression occurred only beyond 90 s of exercise. We conclude that post-exercise depression is detectable only after prolonged muscle activation.

Low striatal D2 receptor binding as assessed by [123I]IBZM SPECT in patients with writer's cramp

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