Somatosensory evoked high-frequency oscillation in Parkinson's disease and myoclonus epilepsy

A high-frequency oscillation in the range of 600-900 Hz has been shown to be a component of the somatosensory evoked potential (SEP) in humans. In the present communication, we studied these oscillation potentials in two neurological disorders.

Subjects were 20 healthy volunteers, 17 patients with Parkinson's disease (PD) and 3 with myoclonus epilepsy (ME). Median nerve SEPs were recorded using filters set at 0.5 and 3000 Hz. Several peaks of oscillation were obtained by digitally filtering raw SEPs from 500 to 1000 Hz, and their amplitudes and onset latencies were measured.

In normal subjects, several oscillation potentials were observed at the latency of 0 to 8 ms after the onset of N20. In PD patients, the oscillation potentials at normal latencies were significantly larger than those of normal subjects. Moreover, in 7 of 17 PD patients, they were extremely enlarged (>mean +/- 3 SD of normal values). In contrast, in patients with ME, abnormally enlarged oscillation potentials were seen at longer latencies (7-14 ms) in spite of normal-sized early oscillation potentials. Magnetoencephalographic analyses showed that any oscillation potentials originated from the primary sensory cortex.

There are at least two mechanisms for producing the oscillation potentials of SEP. Those around N20 have some relation with the basal ganglia function and are enlarged in PD patients, the others around P25-N33 are enhanced in ME patients.