Emergent and Re-emergent Tremor in Dystonia: A Case Series and Neurophysiological Characteristics

Re-emergent tremor (ReT), classically described in Parkinson's disease (PD) as the delayed reappearance of tremor after assuming a posture, may occur in dystonic tremor (DT: tremor in a dystonic body part). We identified four patients with dystonic upper limb tremor (DT), demonstrating visible delayed tremor emergence during rest-to-posture or posture-to-posture transitions. Tremor latency was brief (1-2 seconds), shorter than typically reported in PD-related ReT (9-10 seconds), and tremor amplitude increased during sustained specific postures, accompanied by dystonic posturing of the tremulous limb without parkinsonian signs. Neurophysiological studies revealed regular tremor bursts superimposed on dystonic co-contractions, with a mean peak frequency of postural tremor of 4.36± 0.56 Hz, along with persistent electromyographic co-contraction during transient pauses in tremor. This suggests that ReT can be observed in DT, but its characteristics are distinct from those in PD. We propose a pathomechanism where voluntary movements may transiently modulate dystonic co-contractions and suppress tremor, followed by reactivation of the cerebello-thalamo-cortical (CTC) pathway once the new posture is stabilized.