The patchy tremor landscape: recent advances in pathophysiology

PURPOSE OF REVIEW: We focus on new insights in the pathophysiology of Parkinson's disease tremor, essential tremor, tremor in dystonia, and orthostatic tremor. RECENT FINDINGS: Neuroimaging findings suggest that Parkinson's disease resting tremor is associated with dopaminergic dysfunction, serotonergic dysfunction, or both. Not all tremors in Parkinson's disease have the same pathophysiology: postural tremor in Parkinson's disease can be subdivided into pure postural tremor, which involves nondopaminergic mechanisms, and re-emergent tremor, which has a dopaminergic basis. Unlike Parkinson's disease tremor, essential tremor has an electrophysiological signature suggestive of a single (or several tightly coupled) oscillators. Visual feedback increases essential tremor and enhances cerebral activity in the cerebello-thalamo-cortical circuit, supplementary motor area, and parietal cortex. Little is known about dystonic tremor but the available evidence suggests that both the basal ganglia and the cerebellum play a role. Finally, recent work in orthostatic tremor points towards the role of the pontine tegmentum and dysfunctional cerebellar-SMA circuitry. SUMMARY: Many pathological tremors involve the cerebello-thalamo-cortical circuitry, and the clinical and pathophysiological boundaries between tremor disorders are not always clear. Differences between tremor disorders - or even individual patients - may be explained by the specific balance of neurotransmitter degeneration, by distinct circuit dynamics, or by the role of regions interconnected to the cerebello-thalamo-cortical circuit.