THE PATHOPHYSIOLOGY OF PRIMARY DYSTONIA

Go-contraction and overflow of EMG activity of inappropriate muscles a re typical features of all dystonic movements whether voluntary or inv oluntary. Voluntary movements are slow and more variable than normal, and there is particular difficultly switching between component moveme nts of a complex task, Reduced spinal cord and brainstem inhibition is common to many reflex studies (long-latency reflexes, cranial reflexe s and reciprocal inhibition), These reflex abnormalities may contribut e to the difficulties in voluntary movements but cannot be causal as t hey can occur outside the clinically involved territory, Clinical and neurophysiological studies have emphasized the possible role of sensor y feedback in the generation of dystonic movements, Abnormalities of c ortical and basal ganglia function have been described in functional i maging 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 volu ntary movements, whilst magnetic brain stimulation has revealed change s in motor cortical excitability, Functional imaging of the brain in p rimary dystonia has suggested reduced pallidal inhibition of the thala mus with consequent overactivity of medial and prefrontal cortical are as and underactivity of the primary motor cortex during movements. The se 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 dysto nia results from a functional disturbance of the basal ganglia, partic ularly in the striatal control of the globus pallidus (and substantia nigra pars reticulata), This causes altered thalamic control of cortic al motor planning and executive areas, and abnormal regulation of brai nstem and spinal cord inhibitory interneuronal mechanisms.