SUMMARY1. In ten normal volunteers, a transcranial magnetic or electric stimulus that was subthreshold for evoking an EMG response in relaxed muscles was used to condition responses evoked by a later, suprathreshold magnetic or electric test shock. In most experiments the test stimulus was given to the lateral part of the motor strip in order to evoke EMG responses in the first dorsal interosseous muscle (FDI).2. A magnetic conditioning stimulus over the hand area of cortex could suppress responses produced in the relaxed FDI by a suprathreshold magnetic test stimulus at interstimulus intervals of 1-6 ms. At interstimulus intervals of 10 and 15 ms, the test response was facilitated.3. Using a focal magnetic stimulus we explored the effects of moving the conditioning stimulus to different scalp locations while maintaining the magnetic test coil at one site. If the conditioning coil was moved anterior or posterior to the motor strip there was less suppression of test responses in the FDI. In contrast, stimulation at the vertex could suppress FDI responses by an amount comparable to that seen with stimulation over the hand area. With the positions of the two coils reversed, conditioning stimuli over the hand area suppressed responses evoked in leg muscles by vertex test shocks.4. The intensity of both conditioning and test shocks influenced the amount of suppression. Small test responses were more readily suppressed than large responses. The best suppression was seen with small conditioning stimuli (0 7-0 9 times motor threshold in relaxed muscle); increasing the intensity to motor threshold or above resulted in less suppression or even facilitation.5. Two experiments suggested that the suppression was produced by an action * Present address: Third Department of Internal Medicine, Division of Neurology, Yamagata University, School of Medicine, 2-2-2 Iida-Nishi, Yamagata City 990-23, Yamagata, Japan.

Bradykinesia means slowness of movement and is one of the cardinal manifestations of Parkinson's disease. Weakness, tremor and rigidity may contribute to but do not fully explain bradykinesia. We argue that bradykinesia results from a failure of basal ganglia output to reinforce the cortical mechanisms that prepare and execute the commands to move. The cortical deficit is most apparent in midline motor areas. This leads to particular difficulty with self-paced movements, prolonged reaction times and abnormal pre-movement EEG activity. Movements are often performed with normally timed EMG bursts but the amount of EMG activity is underscaled relative to the desired movement parameters. There are also abnormalities in sensory scaling and sensorimotor integration. The brain appears to be able to compensate to some degree for the basal ganglia deficit. There is overactivity in the lateral premotor areas during task performance and movements can be speeded by giving sensory cues. Attention to movement is also beneficial. However, we propose that the engagement of compensatory processes may also lead to reduced performance in other tasks. For example, patients' problems in performing more than one task at the same time could result from lack of sufficient resources both to compensate for their basal ganglia deficit and to run two tasks simultaneously. Surgical therapies are unlikely to work solely by normalizing basal ganglia output to that seen in healthy individuals. It seems more plausible that surgery removes an interfering signal that allows more efficient compensation by other structures.

Co-contraction and overflow of EMG activity of inappropriate muscles are typical features of all dystonic movements whether voluntary or involuntary. Voluntary movements are slow and more variable than normal, and there is particular difficulty switching between component movements of a complex task. Reduced spinal cord and brainstem inhibition is common to many reflex studies (long-latency reflexes, cranial reflexes and reciprocal inhibition). These reflex abnormalities may contribute to the difficulties in voluntary movements but cannot be causal as they can occur outside the clinically involved territory. Clinical and neurophysiological studies have emphasized the possible role of sensory feedback in the generation of dystonic movements. Abnormalities of cortical and basal ganglia function have been described in functional imaging and neurophysiological studies of patients with dystonia and in animal models of primary dystonia. Studies of cortical function have shown reduced preparatory activity in the EEG before the onset of voluntary movements, whilst magnetic brain stimulation has revealed changes in motor cortical excitability. Functional imaging of the brain in primary dystonia has suggested reduced pallidal inhibition of the thalamus with consequent overactivity of medial and prefrontal cortical areas and underactivity of the primary motor cortex during movements. These findings are supported by preliminary neuronal recordings from the globus pallidus and the thalamus at the time of stereotaxic surgery in patients with dystonia. All this evidence suggests that primary dystonia results from a functional disturbance of the basal ganglia, particularly in the striatal control of the globus pallidus (and substantia nigra pars reticulata). This causes altered thalamic control of cortical motor planning and executive areas, and abnormal regulation of brainstem and spinal cord inhibitory interneuronal mechanisms.

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Twenty index patients with hereditary essential tremor and their kindreds were studied to define the phenotype of this condition. Ninety-three first degree and 38 more distant relatives were examined; 53 definite and 18 possible secondary cases were identified. The age of tremor onset was bimodally distributed with a median at approximately 15 years. Segregation analysis indicated autosomal dominant inheritance and penetrance was virtually complete by the age of 65 years. There were no examples of the disease skipping a generation. Men and women were affected in equal proportions. About 50% of cases were alcohol responsive. In the majority of families alcohol responsiveness was either consistently present or did not occur, but in 20% of kindreds definite heterogeneity of responsiveness was encountered within each family. The typical phenotype was a mild symmetrical postural tremor of the upper limbs. Tremor of the legs, head, facial muscles, voice, jaw and tongue occurred but never in isolation and rest, task specific (e.g. primary writing tremor) and primary orthostatic tremors were not found. Head tremor was invariably mild and 75% was of a 'no-no' type. Dystonia (e.g. torticollis and writer's cramp) were not encountered, a finding which strongly suggests that many previous studies of 'essential tremor' were contaminated by cases of idiopathic or hereditary torsion dystonia. No association with Parkinson's disease was found but classical migraine occurred in approximately 26% of cases and co-segregated with tremor. The severity of arm tremor (assessed using a clinical rating scale and by scoring tremor in Archimedes spirals) and disability increased with advancing age and increasing tremor duration, but there was no correlation between age at tremor onset and either tremor severity or disability. Men and women were affected with equal severity. The sex of the affected parent had no influence on the severity of tremor or the degree of disability experienced by an affected child. Disability commenced in the second decade and progressively increased. All the index patients and 59% of the definite secondary cases had tremor induced disabilities. Eighty-five percent of index patients and 38% of secondary cases also reported some degree of social handicap. Twenty-five percent of index patients and 12% of secondary cases had been compelled to change jobs or retire. Biological fitness was normal.

Rhabdomyolysis, a syndrome of skeletal muscle breakdown with leakage of muscle contents, is frequently accompanied by myoglobinuria, and if sufficiently severe, acute renal failure with potentially life-threatening metabolic derangements may ensue. A diverse spectrum of inherited and acquired disorders affecting muscle membranes, membrane ion channels, and muscle energy supply causes rhabdomyolysis. Common final pathophysiological mechanisms among these causes of rhabdomyolysis include an uncontrolled rise in free intracellular calcium and activation of calcium-dependent proteases, which lead to destruction of myofibrils and lysosomal digestion of muscle fiber contents. Recent advances in molecular genetics and muscle enzyme histochemistry may enable a specific metabolic diagnosis in many patients with idiopathic recurrent rhabdomyolysis.

The presenting features and their subsequent evolution in 36 patients with pathologically proven or clinically probable corticobasal degeneration are described. The most common initial complaint was of a unilateral 'clumsy, stiff or jerky arm' (n = 20). Typically the arm was akinetic, rigid and apraxic. In about a third of these the arm was held in a striking and characteristic fixed dystonic posture. Jerking of the arm, due to action and stimulus-sensitive myoclonus accompanied these symptoms in about a third of the cases. The next most common presentation (n = 10) was difficulty walking due to clumsiness and loss of fine motor control of one leg due to apraxia or dysequilibrium, or a combination of both. Sensory symptoms in the affected arm heralded the onset of illness in three and accompanied a motor disturbance in two cases. Presentation with dysarthria or a behavioural syndrome were rare. The symptoms progressed slowly, usually involving first the ipsilateral arm and leg, but gradually spread to affect all four limbs. After a mean follow-up of 5.2 years (range 2-8 years) gait difficulties and a supranuclear ophthalmoplegia had emerged in most patients and dysarthria and pyramidal signs were common. Higher mental function was relatively preserved in most and a cortical sensory loss was evident in a quarter of cases.

The stiff man syndrome (SMS) and its variants, focal SMS, stiff limb (or leg) syndrome (SLS), jerking SMS, and progressive encephalomyelitis with rigidity and myoclonus (PERM), appear to occur more frequently than hitherto thought. A characteristic ensemble of symptoms and signs allows a tentative clinical diagnosis. Supportive ancillary findings include (1) the demonstration of continuous muscle activity in trunk and proximal limb muscles despite attempted relaxation, (2) enhanced exteroceptive reflexes, and (3) antibodies to glutamic acid decarboxylase (GAD) in both serum and spinal fluid. Antibodies to GAD are not diagnostic or specific for SMS and the role of these autoantibodies in the pathogenesis of SMS/SLS/PERM is the subject of debate and difficult to reconcile on the basis of our present knowledge. Nevertheless, evidence is emerging to suggest that SMS/SLS/PERM are manifestations of an immune-mediated chronic encephalomyelitis and immunomodulation is an effective therapeutic approach.