Internal tremor: characterization and quantification in a cohort of neurological patients

Abstract

Background: Internal tremor (IT) is an underexplored symptom. It may be present in up to 44% of patients with Parkinson's Disease (PD) and 55% of patients with essential tremor (ET). Some authors suggest a possible associated with anxiety. Objectives: We aimed to ascertain the presence of IT in a cohort of patients with PD and ET. Secondly, we intended to characterize its phenomenological features (specifically tremor frequency), to study its correlation with postural and resting tremors, and to analyze its association with anxiety. Methods: This observational cross-sectional study enrolled 50 patients diagnosed with PD or ET. Patients reporting IT completed a questionnaire regarding its characteristics, and their postural and resting tremors were quantified using a smartphone application. We further inquired whether they could mimic this sensation orally, and then recorded and analyzed their vocal imitations with acoustic analysis software. Internal tremor frequency was subsequentially calculated and compared with the patients' postural and resting tremors. Results: IT was reported in seven patients (14%). Five patients (71.4%) noted a correlation with anxiety. Five patients (71.4%) were able to characterize IT through sound, with frequencies generally falling within the range of 4-6 Hz. Conclusions: IT appears to be an infrequent phenomenon in patients with overt tremor syndromes. It could be associated with anxiety, and it can also reflect a subclinical tremor. Our study is compatible with the possibility of a central pattern generator driving the tremor observed (and felt) in patients with PD and ET. Larger studies are required to better understand IT.