Clinical and Electrophysiological Heterogeneity of Orthostatic Tremor: An Individual Patient Data Analysis.

Background: Orthostatic tremor (OT) is traditionally classified using tremor frequency and the presence of associated neurological signs. However, increasing reports of overlapping frequencies, fluctuating electrophysiological profiles, and diverse clinical associations indicate that current classification systems may not fully capture the clinical heterogeneity of OT. Methods: A systematic review of PubMed/MEDLINE from inception to February 2026 identified case reports, case series, and original studies reporting electromyography-confirmed OT with individual patient data. Demographic, clinical, electrophysiological, and data on associated conditions were extracted. Cases were classified according to the 2018 Movement Disorder Society criteria and separately grouped as isolated or non-isolated OT. Associations between tremor frequency and clinical phenotype were assessed using receiver operating characteristic analysis and multivariable logistic regression. Results: One hundred studies comprising 244 patients were included. Mean tremor frequency was 13.32 ± 4.15 Hz (range 3-20 Hz). Seventeen patients (6.97%) could not be classified using current criteria because of frequencies exceeding 18 Hz or fluctuations across the 13-Hz threshold. Non-isolated OT had significantly lower tremor frequencies than isolated OT (11.03 Hz vs 14.80 Hz, p < 0.001). Tremor frequency independently predicted non-isolated phenotype (OR 0.78) (AUC 0.72). Parkinsonian disorders were the most common associated condition, but substantial frequency overlap was observed across neurological and systemic disorders associated with OT. Discussion: Most cases were classifiable using the current OT criteria. However, a small subset demonstrated boundary-spanning or out-of-range frequencies. These findings suggest that tremor frequency remains an important electrophysiological feature of OT but should be interpreted alongside the clinical context. Highlights: Electromyography-confirmed orthostatic tremor exhibited marked clinical and electrophysiological heterogeneity across neurological and non-neurological conditions. Most cases were classifiable using the current OT criteria. However, a small subset demonstrated boundary-spanning or out-of-range frequencies. These findings suggest that tremor frequency remains an important electrophysiological feature of OT but should be interpreted alongside the clinical context.