Stability of The Head in Response to Sudden Tilts in Normal and Labyrinthine Defective Patients

Abstract

This study investigated the mechanisms that help to stabilize the head during head tilt in normal subjects and in patients with absent labyrinthine function. Seven normal subjects and four patients were examined. They were seated in a gimbal with limbs and torso restrained and head free to move. The gimbal could be tilted from upright to pitch or roll about earth horizontal axes. Tilt stimuli were delivered under two conditions: i) while performing mental arithmetic with eyes closed. ii) while reading fixed N six test type. During tilt, position and acceleration of the trunk and head and neck EMGs were recorded simultaneously. Initial responses was similar for all subjects; following gimbal tilt, the had tended to lag behind the trunk because of intertial lag. After that, movement of the head on the trunk varied widely with three broad types. i) Head held relatively fixed to trunk. This tactic was deployed by normal subjects and one patient. ii) Unstable head on trunk. This response was typical of patients with labyrinthine defect. iii) Effective compensatory head movement. Only normal subjects had effective compensatory movement. Regarding head stability in space, all subjects had superior stability during reading than mental arithmetic. All patients had significantly poorer stability than normal subjects during pitch tilt backwards. The stability of the two patients tested in roll tilt was at the upper limit of the normal range. The latency of the initial large burst of EMG varied depended on the mean rate of change in head acceleration (jolt). The pattern of the latency in proportion to jolt did not show a clear difference between normal subjects and patients. These results suggest that the short bursting of the neck muscles during sudden tilts is non-stretch and non-vestibular in origin and other candidates for the origin could exist. © 1995, Japan Society for Equilibrium Research. All rights reserved.