Accurate and objective determination of myalgic encephalomyelitis/chronic fatigue syndrome disease severity with a wearable sensor

Abstract

Background: Approximately 2.5 million people in the U.S. suffer from myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS). This disease negatively impacts patients' ability to function, often resulting in difficulty maintaining employment, sustaining financial independence, engaging socially with others, and in particularly severe cases, consistently and adequately performing activities of daily living. The focus of this research was to develop a sensor-based method to measure upright activity defined as time with feet on the floor and referred to as UpTime, as an indicator of ME/CFS disease severity.

Methods: A commercially available inertial measurement unit (IMU), the Shimmer, was selected for this research. A Kalman filter was used to convert IMU data collected by the Shimmer to angle estimates. Angle estimate accuracy was confirmed by comparison to a motion capture system. Leg angle estimates were then converted to personalized daily UpTime scores using a critical angle of 39º from vertical to differentiate between upright (feet on the floor) and not upright. A 6-day, case-control study with 15 subjects (five healthy controls, five moderate-level ME/CFS, and five severe-level ME/CFS) was conducted to determine the utility of UpTime for assessing disease severity.

Results: UpTime was found to be a significant measure of ME/CFS disease severity. Severely ill ME/CFS patients spend less than 20% of each day with feet on the floor. Moderately ill ME/CFS patients spend between 20-30% of each day with feet on the floor. Healthy controls have greater than 30% UpTime. IMU-measured UpTime was more precise than self-reported hours of upright activity which were over-estimated by patients.

Conclusions: UpTime is an accurate and objective measure of upright activity, a measure that can be used to assess disease severity in ME/CFS patients. Due to its ability to accurately monitor upright activity, UpTime can also be used as a reliable endpoint for evaluating ME/CFS treatment efficacy. Future studies with larger samples and extended data collection periods are required to fully confirm the use of UpTime as a measure of disease severity in ME/CFS. With the added perspective of large-scale studies, this sensor-based platform could provide a recovery path for individuals struggling with ME/CFS.

Keywords: Inertial measurement unit (IMU); Limb orientation; Myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS); Posture; UpTime; Upright activity.

Fig. 1

The angle of each lower leg is compared to the critical angle $\left({\mathrm{\theta }}_{\text{c}}\right)$ to determine uprightness. Accelerometer measurements (a_x, a_y, and a_z) and gyroscope (i.e. angular rate) measurements (p, q, and r) are in the local coordinate frame—x, y, and z. Roll $\left(\varphi \right)$ and pitch $\left(\mathrm{\theta }\right)$ are measured using the fixed global coordinate frame—X, Y, and Z

Fig. 2

Comparison of angle data from VICON and IMU for one subject

Fig. 3

a Mean plot of UpTime separated by disease level. b Mean plot of HUA separated by disease level. Error bars are 95% confidence intervals

Fig. 4

Group mean plots for UpTime. Error bars are 95% confidence intervals

Fig. 5

Correlation plots between UpTime and HUA, separated by disease level. Linear regressions were calculated using least squares by R Studio’s ggscatter function. Blue, grey, and red colored regions indicate the 95% confidence interval for each regression line. The R² values were: (Control) R² = 0.23, (Moderate) R² = 0.004, and (Severe) R² = 0.036

Fig. 6

Subject weekly average UpTime scores (left) with group UpTime averages in bold. Corresponding scale of expected UpTime scores for each disease group (right)