Neurophysiological techniques are an objective way of investigating movement disorders and can support the clinical diagnosis as well as monitoring severity and the effects of treatment. Over the past 30 years there have been enormous advances in our understanding of the pathophysiology of movement disorders with a broad range of investigative techniques, which are still very useful for research purpose but not many of them are unfortunately applicable for diagnostic use.
The movement disorder investigation is needed to support the clinical diagnosis or to reveal conditions which cannot be detected with the clinical examination alone.
The most common movement disorders we investigate are tremor, jerks and spasms. In the case of tremor, the neurophysiological findings cannot be evaluated in isolation as a proper diagnosis often requires a close correlation with the clinical features (conditions favouring the tremor and the presence of associated neurological signs). This is particularly important in tremors with overlapping frequencies 4-6 Hz. However the neurophysiology has higher value in the diagnosis of high frequency tremor such as cortical tremor or orthostatic tremor where the tremor is not clinically visible. Investigation of tremor requires multichannel EMG recording + accelerometer and devices for entrainment and distractions.
The study of jerks needs the simultaneous recording of EMG and EEG. This allows analysis of the relationship between myoclonic and cortical events. In cortical myoclonus the EEG often shows multifocal or generalised spike and wave or multiple spike and wave discharges. When the EEG does not show clear abnormalities the EEG –EMG back-averaging can help in finding a cortical correlation to the jerks. The multichannel EMG recording is helpful in detecting the cranio-caudal progression of the short duration muscle bursts (<70 msec) or brief lapses of muscle activity (negative myoclonus). Diagnosis of cortical myoclonus is supported by Giant evoked potential and C reflex. Spinal myoclonus requires neurophysiological investigation to assess the pattern of muscles activation and the search for a Bereitschaftspotential in case the suspicion of non-organic origin is raised. Stimulus induced jerks as in startle require the recording of the pattern of muscle activation and measurement of the latency of the muscle jerks following sudden sound. Stiff-person syndrome together with the continuous muscle activity can be confirmed by the presence of the exteroceptive reflex. The diagnosis of psychogenic tremor can be supported by distraction manoeuvre and entrainment procedures.