Mechanisms of VOR Suppression in Brainstem Pathology: Insights from the Absence of Anti-Compensatory Saccades Despite Normal VOR Gain

<b>Background/Objective</b>: The Suppression Head Impulse Paradigm (SHIMP) is a specialized variant of the Head Impulse Test (HIT), designed to evaluate the suppression of the angular Vestibulo-Ocular Reflex (aVOR) by central mechanisms. These mechanisms are primarily mediated by brainstem structures, including the vestibular nuclei, their projections to ocular motor nuclei, and modulatory inputs from the cerebellum. Damage to these areas can impair the generation of anti-compensatory saccades (ACs), even when the peripheral vestibular apparatus remains intact. The present study explores this phenomenon in a cohort of patients with neurological disorders known to potentially involve the brainstem, including multiple sclerosis, severe traumatic brain injury, stroke, and Parkinson's disease. <b>Methods</b>: This cross-sectional study included 119 patients with multiple sclerosis (PwMS), severe traumatic brain injury (PwTBI), stroke (PwS), and Parkinson's disease (PwPD). The video Head Impulse Test was performed to assess the aVOR gain across all semicircular canals using both the HIMP and SHIMP. The presence, absence, or delay of ACs was systematically recorded. <b>Results</b>: Among the 119 patients evaluated (238 semicircular canals), 24 (20%) demonstrated normal aVOR gain but failed to generate ACs during SHIMP. The absence of ACs was observed in seven PwMS, five with PwTBI, six with PwS, and six with PwPD. <b>Conclusions</b>: The absence of ACs despite normal aVOR gain suggests a potential impairment in the central pathways controlling saccadic responses, independently of peripheral vestibular function. These findings underscore the clinical relevance of integrating the SHIMP into vestibular assessments to improve the identification of central vestibular dysfunction in neurological disorders.