Abstract
The cause of Adolescent Idiopathic Scoliosis (AIS) remains unclear, but one proposed cause of AIS is asymmetric vestibular function and the related descending drive to the spine musculature. The objective of this study was to determine if asymmetric vestibular function is present in individuals with AIS. Ten individuals with AIS (8F, 2M) and 10 healthy age- and sex-matched controls were exposed to 10s-long virtual rotations induced by monaural or binaural electrical vestibular stimulation (EVS), and 10s-long real rotations delivered by a rotating chair. Using a forced-choice paradigm, participants indicated their perceived rotation direction (right or left) to stimuli of varying intensity. A Bayesian adaptive algorithm adjusted the stimulus intensity and direction to identify a stimulus level, which we called the direction recognition threshold, at which participants correctly identified the rotation direction 69% of the time. For unilateral vestibular stimuli (monaural EVS), the direction recognition thresholds were more asymmetric in all participants with AIS compared to control participants [(0.22–1.00 mA) vs. (0.01–0.21 mA); p < 0.001]. For bilateral vestibular stimuli, however, the direction recognition thresholds did not differ between groups for either the real or virtual rotations (multiple p > 0.05). Previous reports of semicircular canal orientation asymmetry in individuals with AIS could not explain the magnitude of the vestibular function asymmetry we observed, suggesting a functional cause to the observed vestibular asymmetry. Thus, the present results suggest that a unilateral vestibular dysfunction is linked to AIS, potentially revealing a new path for the screening and monitoring of scoliosis in adolescents.
Highlights
Adolescent idiopathic scoliosis (AIS) is the most prevalent subtype of idiopathic scoliosis, with the onset of scoliotic curvature occurring between 10 and 16 years old [1]
There was no overlap in the absolute difference in direction recognition threshold for the monaural configuration between participants with AIS (0.22–1.00 mA) and controls (0.01– 0.21 mA) (U = 100.00, p < 0.001, r = 1.00; Figure 3A)
The direction recognition thresholds for virtual rotations ranged from 0.59 to 1.82 mA for participants with AIS and from 0.18 to 1.15 mA for control participants (t18 = 1.54, p = 0.14, d = 0.69) (Figure 4B)
Summary
Adolescent idiopathic scoliosis (AIS) is the most prevalent subtype of idiopathic scoliosis, with the onset of scoliotic curvature occurring between 10 and 16 years old [1]. When EVS is applied bilaterally over the mastoid processes (i.e., in a binaural bipolar EVS configuration), the net virtual rotation it induces is about a vector pointing posteriorly and ∼17–19◦ above Reid’s plane with negligible net linear acceleration from the otolith signals [17, 18, 26] (Figure 1A) This virtual motion is similar to head roll in headcentered coordinates. When participants maintain a neutral head posture (i.e., head upright), EVS generates the illusion of head roll without the corresponding change in the gravitational signal from the otoliths This mismatch is foreign to the brain, and Participants with AIS mean (SD)
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