A task force of the International Parkinson and Movement Disorder Society has defined tremors as an involuntary, rhythmic, oscillatory movement of a body part.1 The task force also proposed a definition for essential tremor (ET) as a tremor syndrome of at least 3 years duration, excluding isolated head or isolated voice tremors, in the absence of inherited dystonia, ataxia, or parkinsonism.1 Based on a 2014 epidemiological study, the estimated number of ET cases in the United States was 7 million (6.38 to 7.63 million), corresponding to approximately 2.2% of the population, making it one of the most common movement disorders.2 The prevalence of ET increases with age, with reported rates of 4% or higher in people over the age of 60 years.3 An exponential increase is seen with advancing age, with rates reaching in excess of 20% in those 95 years and older.3 The aim of this summary of evidence was to determine if external upper limb tremor stimulator therapy, also known as transcutaneous afferent patterned stimulation (TAPS) therapy, reduces ET symptoms immediately and 60-minutes after a therapy session, and if tremor stimulation therapy reduces ET symptoms in the longer term (≥ 3 months).
Food and Drug Administration (FDA) Approval
Cala Trio (External upper limb tremor stimulator):
A randomized, double-blinded, sham-controlled pilot trial, by Lin, et al.,4 compared the efficacy of either median and radial nerve stimulation with a non-invasive, wrist-worn, peripheral nerve stimulation device (n = 10), or a sham device (n = 13) for the symptomatic relief of hand tremors in participants with ET. Efficacy was measured using a change in the Tremor Research Group’s Essential Tremor Rating Assessment Scale (TETRAS) Archimedes spiral drawing task post-stimulation compared with pre-stimulation, after a single treatment session. In the treatment group, blinded rater scores significantly improved post-stimulation (1.77 + 0.21) compared to pre-stimulation (2.77 + 0.22; p = 0.01). In the sham group, scores did not change significantly following stimulation (2.37 + 0.22) compared to pre-stimulation (2.62 + 0.14; p = 0.37). The response to treatment corresponded to an estimated hand tremor amplitude reduction of 60% + 8.4% and was significantly greater in the treatment than in the sham group (p = 0.02). Limitations of the study included the small cohort, short duration, limited details on the study methodology, and possible confounding of results due to pharmacologic treatments in use.
A multi-center double-blinded, randomized, sham-controlled trial, by Pahwa, et al.,5 evaluated the safety and effectiveness of a wrist-worn peripheral nerve stimulation device in 77 patients with ET (treatment n = 40, sham n = 37). The primary outcome of the study was improvement in tremor severity in the hand as measured by the TETRAS task 6 Archimedes spiral score following stimulation compared to sham stimulation based on a single in office session. The study failed to reach its primary endpoint; there was not a significantly larger improvement in the Archimedes spiral rating with stimulation treatment compared to sham (p = 0.26). There were statistically significant improvements in some secondary endpoints compared to sham including the individual forward postural hold upper limb tremor task (TETRAS task 4) sub-score (p = 0.004), the combined upper limb tremor task (TETRAS task 4) score (p = 0.017), and the total TETRAS performance score (p = 0.015). Subject-rated improvements in activities of daily living (ADL) were significantly greater in 4 out of 7 ADL tasks in the treatment group compared with sham. A greater percentage of ET patients (88%) reported improvement in the stimulation group as compared to the sham group (62%) (p = 0.019) based on the clinical global impression-improvement (CGI-I) ratings. The study limitations included the small sample size, single treatment session, and sham device effects, which were not subtracted from the active scores.
The objective of a single-session, open-label study, by Yu, et al.,6 was to determine the time profile of therapeutic benefit from TAPS therapy for up to 60 minutes following a stimulation session in 15 patients (mean age 72.2 ± 8.6) with ET. Four hand tremor-specific tasks (postural hold, spiral drawing, finger-to-nose reach, and pouring) from the Fahn-Tolosa-Marin Clinical Rating Scale (FTM-CRS) were performed and assessed prior to, during, and 0-, 30- and 60-minutes following TAPS therapy. Mean FTM-CRS rating improved with TAPS therapy, with the peak tremor reduction occurring 30 minutes following the end of stimulation. Twelve of 15 patients had improvement in mean clinical rating at least 60 minutes following the end of stimulation (p = 4.6e-9). Accelerometer-measured tremor power improved with TAPS therapy across all tasks; the postural hold task had the greatest reduction in tremor power, with a median peak 5.9-fold improvement (83% reduction) in tremor power occurring 30 minutes following the end of stimulation. Median peak improvements for the other tasks were 2.4-fold (57% reduction) for spiral drawing, 1.6-fold (38% reduction) for finger-to-nose reach, and 2.5-fold (59% reduction) for pouring. Eleven (11) of 15 patients had at least 60 minutes of benefit for postural hold, spiral drawing, and finger-to-nose reach tasks (p = 9.8e–8), while 10 of 14 patients had at least 60 minutes of benefit for the pouring task (p = 5.8e–7). Limitations of this study include the small sample size, the open-label design, the short duration of follow-up (single-session), heterogenity in baseline ET severity, and 5 of 15 participants remained on their standard-of-care ET medication during the study, which may have confounded the results.
A prospective, multi-center, single-arm, open-label clinical trial, by Isaacson, et al.,7 evaluated the safety and efficacy of TAPS therapy in 205 patients over a 3-month period using a wrist-worn neuromodulation device. The study included 3 in-clinic visits consisting of a screening and enrollment visit, and a 1- and 3-month follow-up visit. Between visits, patients were instructed to use the device 2x/day for 40 minutes. The clinician rated TETRAS dominant hand score and patient-rated Bain & Findley Activities of Daily Living (BF-ADL) were the co-primary efficacy endpoints, while improvement in tremor power between the pre- and post-stimulation postural holds (measured by accelerometer) was the secondary efficacy endpoint. On average, patients completed at least one stimulation session per day for 78% of the days they were enrolled in the study and completed 68% of their total instructed stimulation sessions. The TETRAS dominant hand scores improved from baseline to the third study visit from a mean (standard deviation (SD)) of 12.6 (2.7) to 9.8 (3.5) (mean difference -2.8 (2.8), p < 0.0001). There was also a statistically significant improvement in the combined BF-ADL dominant hand score from a baseline mean (SD) of 18.4 (3.8) to 13.4 (4.4) (mean difference -5.0 (4.3), p < 0.0001). Accelerometer recordings before and after 21,806 treatment sessions showed that 92% of patients experienced improvement in tremor power, with 54% of patients experiencing ≥50% improvement; mean tremor power decreased from 1.1 ± 0.3 (m/s2)2 pre-stimulation to 0.3 ± 0.1 (m/s2)2 post-stimulation (p < 0.0001). No device-related serious adverse events (AEs) were reported, and 18% of patients experienced non-serious device-related AEs. Limitations of the study included its open-label, single-arm design, high drop-out rate, lack of sham control, and unblinded clinical raters. Although the data did produce statistically significant results, the authors noted a large variability in the magnitude of effect on different tasks.
A retrospective, post-market surveillance study, by Brillman, et al.,8 aimed to evaluate real-world usage and effectiveness of TAPS delivered by a non-invasive, wrist-worn, neuromodulation device in 321 patients (mean age 71 years) for the management of hand tremors associated with ET. Effectiveness was assessed by measurement of tremor power (i.e., frequency and amplitude of tremor motion to characterize tremor severity), via device-prompted postural hold tremor accelerometry measurements, immediately before and after a session of stimulation. Habituation of therapy was assessed in patients using the device for at least 1 year by evaluating effectiveness of therapy in their first 90 days of use compared to effectiveness after 90 days of use. Usage was defined as the average number of sessions, of at least 20 minutes duration, per week, and was gathered from data collected from integrated device logs. A total of 216 participants provided data that met effectiveness analysis criteria; immediately after TAPS therapy, 93% of patients had some reduction in tremor severity, with 59% of patients experiencing at least 50% reduction. Patients with the most severe pre-therapy tremors experienced the greatest reduction of tremor severity after stimulation (89% reduction). For patients who used TAPS therapy for greater than 1 year (n = 89), there was no significant habituation to therapy noted. Usage analysis identified total usage periods ranging from 90 to 633 days, with patients using the device 5.4 times per week, on average. Of the 69 participants (representing 21.5% of total patients) who returned a voluntary survey, 84% reported improvement in at least one key activity (i.e., eating, drinking, or writing), 65% reported improvement in overall quality of life, and 65% indicated a preference for TAPS therapy over both medication and surgical intervention for tremor management. The investigators recommended additional studies to better define TAPS dosing (i.e., number of sessions per week) and delivery. The study was limited by the retrospective design, lack of control for confounding, and the amount of missing or excluded data. Additionally, because postural hold data was only obtained immediately after a stimulation session, the duration of treatment effect is not known.
Evidence Based Guidelines
The American Academy of Neurology (AAN) first developed an evidenced based guideline for the treatment of Essential Tremor in 2005, which was updated in 2011 and reaffirmed in 2022. These guidelines do not offer a recommendation for or against the use of the Cala Trio.9
Professional Society Recommendations
No relevant Professional Society Recommendations were identified
International Essential Tremor Foundation (IETF) – Essential Tremor in Adult Patients10
Summary of Consensus Guide (in relevant part):
Figure 1. Overview of Management of Essential Tremor
Step 1 - Examination to establish presence of tremor
Step 2 – Rule out other causes of tremor, such as medications, Parkinson’s disease, dystonia, etc.
Step 3 - First-line pharmacological approaches: propranolol, primidone, or a combination. Optional add-ons: Cala Trio and/or other non-invasive devices
Step 4 – Cala Trio and/or other non-invasive devices (if not added during Step 3). Second-line and third-line pharmacological approaches: topiramate, gabapentin, other beta blockers, or benzodiazepines
Step 5 – DBS [Deep Brain Stimulation], FUS [Focused Ultrasound], or other surgical approaches
At any time during steps 3-5:
- Assistive devices can be used
- Discussions with patients regarding social stigmas, and for severe cases, social security/disability support