Essential and aging-related tremor: Differences of central control

For essential tremor, the distribution of age of onset is bimodally distributed, with peaks in adolescence and another in late adulthood. The latter is here referred to as aging-related tremor, and it is considered to be associated with earlier aging and increased mortality. We hypothesize that different tremor networks detected by multichannel electroencephalography (EEG) underlie these two tremor groups.

We investigated 20 patients with essential tremor separated into two groups and 10 age- and sex-matched aging-related tremor patients using pooled coherence spectra of the maximally coherent EEG electrodes during a holding task, a pinch grip task, and whilst performing slow hand movements. Functional and effective connectivity at the coupling frequency was estimated.

The maximal coherence was significantly higher in early-onset essential tremor compared with aging-related tremor during the three tasks. Compared with the patients with aging-related tremor, 2 Hz to 40 Hz power, spectral peak frequency, and relative signal-to-noise ratio were not different, but the electromyographic amplitude was significantly greater in essential tremor. The source analysis revealed the well-known cortico-brainstem-cerebello-thalamo-cortical network for classical essential tremor, but patients with aging-related tremor showed a cortico-thalamic network only. The connections between the sources for both tremors were bidirectional. Only the cerebellum and the brainstem showed unidirectional connections in essential tremor and the thalamus in aging-related tremor patients. A second essential tremor group with similar electromyographic amplitudes confirmed the differences between both tremor types.

We show that the oscillating cerebral networks underlying classical essential tremor and aging-related tremor differ, suggesting a pathophysiological difference.