Sensor-based analysis of eye drop instillation: effects of bottle stability on instillation success

Abstract

Proper self-administration of eye drops is essential for glaucoma management, yet many older adults struggle with technique, leading to missed doses, wasted medication, and bottle-tip contamination. We developed a bottle-mounted inertial sensing approach to quantify the motion of eye-drop instillation using three-dimensional stability metrics that extend beyond conventional planar measures. Eighty-eight healthy older adults (mean age 74.7±6.0 years) completed 18 instillation trials across standing, sitting, and supine postures, yielding 1,699 attempts. Motion data from a bottle-mounted sensor were used to define the instillation period and compute movement magnitude, directional, and trajectory-dispersion metrics, as well as bottle tilt and instillation duration. Directional metrics were calculated in four reference frames: bottle-fixed, world-vertical, principal component analysis-based, and velocity-defined. Associations of posture and reference frame with stability metrics were evaluated using generalized estimating equations, and posture-stratified logistic models tested associations between stability metrics and three instillation outcomes: drop reaching the eye, bottle-tip contamination, and single-versus multiple-drop dispensing. All reference-frame-dependent metrics differed across frames, indicating that axis definition materially affects directional and planar stability measures. Associations with instillation outcomes were posture-specific. Predictors of successful drop delivery were concentrated in the supine posture, where greater movement magnitude and dispersion were associated with lower odds of success. Multiple-drop dispensing was consistently associated with longer instillation duration across all postures and with greater total excursion in sitting and standing. In upright postures, greater bottle tilt was associated with a lower risk of bottle-tip contamination. These findings support the use of three-dimensional bottle-motion metrics for objective monitoring of eye-drop technique and highlight the importance of consistent reference-frame definition when interpreting directional stability features.