Re-Emergent Tremor: What do we know so Far?

Abstract

Re-emergent Tremor (RET) is defined as a tremor of the upper limbs that appears after a varying latency during a posture. Although RET was first described almost 20 years ago [1], few studies have investigated the clinical features of RET.In 1999, Jankovic et al., first described RET in a small sample of patients affected by Parkinson's Disease (PD). The authors observed that PD patients with rest tremor may show a postural tremor starting after a variable delay while maintaining the upper limbs outstretched [1]. Given that RET and resting tremor showed similar clinical features,Jankovic et al., hypothesized that RET depends on the activity of the central oscillators responsible for resting tremor that is reset with some latency after the assumption of a new posture. In the same study, Jankovic et al., also observed that patients with essential tremor never showed RET, thus suggesting that RET was specific to PD. This hypothesis was supported by Schwingenschuh et al., [2] who compared upper limb tremor in patients with PD and patients with "Scans Without Evidence of Dopaminergic Deficit" (SWEDD) and reported that only patients with PD had RET. No studies have detected RET in patients with atypical parkinsonisms or dystonic tremor. By contrast, De Laat et al., [3] described a SWEDD patient who had RET.The presence of RET, therefore, seems to be strongly suggestive of PD but RET should be always assessed in patients with tremor syndromes.Only two studies have investigated the occurrence of RET in PD. The first, in which the aim was to evaluate occurrence and the clinical features of action tremor [4], showed that RET was present in 32% (63/197) of patients with PD. A second study [5] specifically designed to investigate RET in PD reported that 20% (42/210) of patients have RET. In the first study, the authors [4] did not, however, specify whether PD patients were evaluated while they were off or on pharmacological treatment. In the second study [5] instead, only PD patients who were on their usual therapeutic regimen were studied, which means any effect of pharmacological treatment on RET was not evaluated. In this regard, unpublished investigations from our group on the presence of RET in PD patients who were evaluated both off and on treatment showed that RET was more frequent when patients were evaluated off treatment.In PD, RET usually involves the upper limbs [1,[4][5][6] and is bilateral in 50% of cases [5]. Recent clinical reports have described patients with PD with RET even in the jaw [7] and tongue [8,9]. No studies have yet reported RET in the lower limbs, though this may be due to the fact that the investigation of postural tremor in the lower limbs is not a common clinical procedure.RET is considered as a clinical variant of resting tremor. The clinical features of patients with RET and patients with isolated resting tremor have been compared in two different studies [5,10]. Both studies showed that the two groups of patients, who were on an equivalent mean levodopa dose, did not differ significantly ...