From combined testing to bedside inference: Linking ASC/PSC gain ratio with nystagmus phenotypes in acute unilateral vestibulopathy

To characterize semicircular canal involvement and nystagmus phenotypes in acute unilateral vestibulopathy (AUVP) under the 2022 Bárány criteria, focusing on the anterior-to-posterior canal (ASC/PSC) gain ratio and its link to horizontal-only (hN) versus horizontal-upbeat (huN) patterns. Cross-sectional study of 121 patients AUVP ( n = 121), comparing AUVP-acute (spontaneous nystagmus positive, SN+, n = 102) versus AUVP-convalescent (SN−, n = 19); within SN+, patients were stratified as hN or huN. Caloric indices, six-canal video head impulse test (vHIT) gains, number of affected semicircular canals, the ASC/PSC ratio, and exploratory multivariable logistic regression results for huN. Compared with SN−, SN+ showed earlier testing, higher canal paresis/directional preponderance (CP/DP), lower HSC/ASC gains, more affected semicircular canals, and higher gain-defined vHIT positivity. Within SN+, huN exhibited greater horizontal slow-phase velocity, higher CP/DP, more affected semicircular canals, and lower HSC/ASC gains than hN. The ASC/PSC ratio differentiated phenotypes: hN clustered near unity (median ≈ 0.97), whereas huN showed a reduced ratio (median ≈ 0.76). In the exploratory multivariable model, onset time (OR 0.843, 95% CI 0.753–0.944) and the ASC/PSC ratio (OR 0.079, 95% CI 0.011–0.543) were independently associated with huN, whereas HSC gain lost independent predictive value; the combined model achieved an apparent AUC of 0.79. Caloric testing and six-canal vHIT are frequency-complementary across stages: The ASC/PSC ratio anchors nystagmus vector interpretation: values near 1.0 align with hN, whereas lower ratios indicate huN with relatively greater ASC involvement. The combined model serves as a hypothesis-generating tool for exploratory early phenotyping, rather than a definitive clinical classifier. • SN+ AUVP showed higher CP/DP and lower HSC/ASC gains than SN−. • Within SN+, huN showed stronger nystagmus and more affected canals. • The ASC/PSC ratio was lower in huN, indicating greater ASC involvement. • Shorter onset time and lower ASC/PSC ratio independently favored huN. • The combined exploratory model achieved an apparent AUC of 0.79.