Functional and gain-based vestibulo-ocular measures across serum 25(OH)D categories: a dissociation between fHIT performance and vHIT gain

Background Conventional vestibular tests quantify canal and otolith function, whereas the functional head impulse test (fHIT) measures how effectively the vestibulo-ocular reflex stabilizes vision during high-acceleration head motion. Low 25-hydroxyvitamin D [25(OH)D] has been linked to otolith dysfunction, but its relation to functional performance is unknown.Objectives To compare otolith, canal and functional measures across serum 25(OH)D categories in adults without overt vestibular disease.Materials and methods Thirty-nine adults aged 18–65 years with normal hearing were stratified by serum 25(OH)D into three groups of 13 (<20, 20–29.9, ≥30 ng/mL). All underwent audiometry, videonystagmography, cVEMP, oVEMP, vHIT and fHIT. Kruskal–Wallis and Mann–Whitney U tests were used.Results fHIT correct response rates differed for posterior (p = 0.001) and anterior canals (p = 0.019), being lowest at <20 ng/mL; lateral performance was preserved (p = 0.234). All vHIT gains remained within acceptable limits and only anterior canal gain differed (p = 0.001). VEMP latencies were comparable; cVEMP amplitude asymmetry differed (p = 0.023) but opposite to expectation and within normal range.Conclusions and significance Low 25(OH)D was accompanied by reduced functional vestibulo-ocular performance in the vertical canal planes despite normal gains. fHIT may capture subclinical change that vHIT gain does not.