The pathophysiology of primary dystonia

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.