Risk factors for refractory BPPV: subtype-dependent association of serum 25-hydroxyvitamin D

Benign paroxysmal positional vertigo (BPPV) generally responds well to canalith repositioning procedures (CRPs), although some patients require repeated interventions. In this study, we evaluated refractory BPPV based on the number of CRP attempts required in 171 patients with acute BPPV and investigated clinical factors associated with treatment resistance, with particular emphasis on serum 25-hydroxyvitamin D [25(OH)D] levels and BPPV subtype. Kaplan-Meier analysis showed that cumulative resolution increased substantially during the first three CRP attempts but plateaued thereafter, indicating limited additional benefit beyond three sessions. Refractory BPPV was therefore defined as requiring three or more CRP attempts, and 47 patients (27.5%) met this criterion. In the multivariable model without interaction terms, BPPV subtype and prior vestibular neuritis were the strongest predictors of treatment resistance, and lower serum 25(OH)D levels were independently associated with refractoriness. However, inclusion of an interaction term between 25(OH)D and subtype revealed a significant subtype-dependent effect, attenuating the main effect of 25(OH)D. Subgroup analysis demonstrated that lower 25(OH)D levels were associated with treatment resistance only in typical BPPV. These findings suggest that 25(OH)D may serve as a supplementary prognostic marker reflecting metabolic vulnerability of the otolith system, particularly in typical BPPV.