Arm-trunk coordination as a measure of vestibulospinal efficiency

When arm and trunk segments are involved in reaching for objects within arm's reach, vestibulospinal pathways compensate for trunk motion influence on arm movement. This compensatory arm-trunk synergy is characterised by a gain coefficient of 0 to 1. Vestibular patients have less efficient arm-trunk synergies and lower gains. To assess the clinical usefulness of the gain measure, we used a portable ultrasound-based device to characterize arm-trunk coordination deficits in vestibular patients. Arm-trunk coordination without vision was measured in a Stationary Hand Task where hand position was maintained during trunk movement, and a Reaching Task with and without trunk motion. Twenty unilateral vestibular patients and 16 controls participated. For the Stationary Hand task, patient gains ranged from g=0.94 (good compensation) to 0.31 (poor compensation) and, on average, were lower than in controls (patients: 0.67 ± 0.19; controls: 0.85 ± 0.07; p< 0.01). Gains were significantly correlated with clinical tests (Sensory Organization; r=0.62, p< 0.01, Foam Romberg Eyes Closed; r=0.65, p< 0.01). For the Reaching Task, blocking trunk movement during reaching modified hand position significantly more in patients (8.2 ± 4.3 cm) compared to controls (4.5 ± 1.7 cm, p< 0.02) and the amount of hand position deviation was correlated with the degree of vestibular loss in a sub-group (n=14) of patients. Measurement of the Stationary Task arm-trunk gain and hand deviations during the Reaching Task can help characterize sensorimotor problems in vestibular-deficient patients and track recovery following therapeutic interventions. The ultrasound-based portable device is suitable for measuring vestibulospinal deficits in arm-trunk coordination in a clinical setting.