Smartphone Apps Provide a Simple, Accurate Bedside Screening Tool for Orthostatic Tremor

Abstract

Background: Orthostatic Tremor (OT) is characterized by the presence of a sensation of instability while standing, associated with high frequency (13-18 Hz) lower extremity tremor. Diagnosis is confirmed with surface electromyography (EMG). An accurate screening tool that could be used in the routine clinical setting, without any specialized equipment, would be useful in earlier detection of OT and judicial use of additional testing.

Objective: The objective of this study was to evaluate OT diagnostic test characteristics at bedside using iPhone's built-in accelerometer and available applications for tremor recordings.

Methods: We obtained recordings using iPhones (Model 5, 5s, and 6) and free Applications ("LiftPulse" by LiftLabs [App1] and "iSeismometer" by ObjectGraph LLC [App2]) at default settings.

Results: 24 EMG-confirmed OT subjects (mostly females, 22/24) and 15 age-matched controls (mostly males, 11/15) were evaluated. App1 detected OT range tremor in 22/24 patients and none of the controls. (Sensitivity = 92%, Specificity = 100%, NPV = 88%). App2 detected OT range tremor in 21/24 patients and in 1/13 controls (Sensitivity = 88%, Specificity = 92%, NPV = 80%). When combined, 24/24 patients and 1/13 controls had OT range tremor (Sensitivity = 100%, Specificity = 92%, NPV = 100%).

Conclusions: Smartphone apps that use the built-in accelerometer provide a simple, accurate and inexpensive bedside screening diagnostic tool for patients with OT.

Keywords: accelerometer; application; orthostatic tremor; smartphone; tremor frequency.

Figure 1

Example of using the iPhone apps. Panel A shows the iPhone tied to the subjects leg and the app is open on the screen. Panel B shows the subjects standing and the app running.

Figure 2

Screen shots from App1 and App2. Panels A and B show screen shots of recordings from App1 with a single dominant frequency given. Panel C‐F show screen shots from App2. Panel C shows a recording from a subject with corresponding analysis giving three peak frequencies (1 for each direction of leg tremor X, Y, and Z) below in panel D. The subject recording in panel E corresponds to the analysis below in panel F.

Figure 3

Bland‐Altman plot showing average frequency between App1 result and EMG report on x‐axis verses the difference between the frequency on App1 and EMG report on y‐axis for each subject with all the available data.

Figure 4

Bland‐Altman plot showing average frequency between App2 and EMG report on x‐axis verses the difference between the frequency on App2 and EMG report on y‐axis for each subject with all the available information. We used the highest reported frequency on App2 of the three axes. The two outliers are both subjects with EMG results in the OT tremor frequency range but tremor frequencies lower than OT range in all axes using App2.