Multiple oscillators are causing parkinsonian and essential tremor

The tremors of Parkinson's disease (PD) and essential tremor (ET) are tradi tionally considered to depend on a central oscillator producing rhythmic ac tivation of the motoneurones of all extremities. To test this hypothesis, w e have compared electromyographic tremor activity in different muscles of t he affected limbs using cross spectral analysis, including coherence and ph ase. Surface electromyographic recordings from both arms, legs, and the nec k were analyzed in 22 patients with PD and 28 patients with ET. Volume cond uction between neighboring muscles producing artificial "coherence" has bee n found to be an important methodologic problem. We have developed a mathem atical test to exclude data that could distort the results. According to th is test, 10% or 25% of muscle combinations from the same limb had to be exc luded from further analysis in PD or ET, respectively. In both, patients wi th PD and ET, we found a considerable number of muscle combinations oscilla ting at virtually the same frequency (Delta frequency <0.4 Hz) without show ing a significant coherence. Thus, the frequency difference between differe nt muscles is not sufficient to measure the correlation between two muscles . Significant coherencies between muscles within the same arm or lee were f ound in 70% or 90% of patients with PD or ET, respectively, whereas only on e patient with PD and not a single patient with ET showed significant coher encies between muscles from different limbs. The phase between coherent mus cles of the same arm of patients with PD showed a preference of either a re ciprocal alternating pattern for antagonistic muscles in forearm flexor and upper arm extensor as opposed to a co-contraction pattern between the hand flexors and the triceps brachii, In patients with ET such clear difference s were lacking. We conclude that multiple oscillators are responsible for t he tremor in different extremities of patients with PD and ET. Differences between PD and ET concerning the phase relation within the arm may either b e related to the topography within the basal ganglia or to differently invo lved-spinal pathways.