Background: Multiple chemical sensitivity (MCS) is a chronic multisystem condition characterized by low levels of multiple chemical susceptibility inducing a spectrum of central nervous system symptoms, including dizziness. Thus, considering (i) the overlapping psychogenic and organic burdens shared in MCS development and in vestibular disorders; (ii) the number of previous studies describing central processing impairment related to inner ear inflow in this syndrome; and (iii) the lack of literature with respect to clinical evidence of the presentation of MCS dizziness, the purpose of the present study was to highlight the possible hidden aspects of vestibular impairment by applying the recent contribution of implemented otoneurological testing, inferential statistic and principal component (PC) analysis in 18 MCS and 20 healthy subjects (HC); Methods: Both groups filled in a dizziness and environment exposure inventory and underwent the Rod and Disc and Rod and Frame Test, video Head Impulse Test (vHIT) and Static Posturography Test (SPT) with fast Fourier Transform (FFT). Between-group analysis of variance and PC analysis implemented on otoneurological variables were performed; Results: Defective vestibular processing was identified in 18 MCS patients (11 female and 7 male; mean age 49.5 ± 9.3 years) by finding a significant increase in SPT and FFT parameters and in Visual Dependency (VD) behaviour and a decrease in vHIT scores. Component correlation analysis in MCS showed a positive correlation of FFT parameters in PC1 and SPT parameters in PC2 with a negative correlation of vHIT and VD values in PC2. HC subjects demonstrated a positive correlation of VD and SPT parameters in PC1 and FFT parameters in PC2. Conclusion: Inferential and PC analysis provided the opportunity to disclose such possible hidden phenomena to (i) support that MCS physiopathological cascades could lead to a vestibular decay; and (ii) suggest rearrangement of the dimension of the variables as an aspect of near-optimal re-weighting, possibly underpinning the dizzy symptoms complained of by MCS patients.
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