The relation between upper limbs action tremor asymmetry, midline tremor and gait disorders in essential tremor: an exploratory study

Upper limbs action tremor represents the clinical hallmark of Essential Tremor (ET), which showed varying degrees of asymmetry. However, the possible role of the upper limbs action tremor asymmetry in the context of the broader ET motor phenotypical spectrum has never been addressed to date. The aim of the present study was to assess the possible relation between upper limbs action tremor asymmetry and other motor aspects which may characterize the ET syndrome. Clinical tremor scores and instrumental kinematic gait parameters were assessed. An asymmetry index (AI) was computed based on clinical severity of each upper limbs action tremor component. Symmetric (S-ET, AI = 0) and asymmetric (A-ET, AI ≠ 0) patients were defined and compared based on the most asymmetric action tremor component. Thirty-seven tremor patients [26 men (70.3%); mean age ± SD: 71.4 ± 6.5] were enrolled. Forward outstretched (FO) postural tremor represented the action tremor subtype showing the greater clinical asymmetry across the whole tremor population. Based on FO tremor AI, two patients’ subgroups were defined: A-ET (N = 21, 56.8%) and S-ET (N = 16, 43.2%), the latter showing higher midline tremor severity (i.e. head and voice) and worse instrumental gait parameters. The study highlights the possible role of upper limbs action tremor asymmetry as an adjunctive feature to be considered in ET clinical phenotyping.