Background/Objectives: Hyperventilation-induced nystagmus (HVIN) has been described as a sensitive clinical sign in vestibular schwannoma (VS), potentially reflecting a reversible conduction block in partially demyelinated vestibular nerve fibers. However, its relationship with tumor size and instrumental vestibular deficits remains controversial. This study aimed to investigate the association between HVIN, tumor size, clinical presentation, and vestibular function assessed by the video Head Impulse Test (vHIT) and functional Head Impulse Test (fHIT). Methods: Eighty consecutive patients with unilateral VS were retrospectively evaluated. All underwent bedside vestibular examination, the vHIT, the fHIT, and the Hyperventilation Test (HVT). Tumors were classified according to the modified Koos–Sanna grading system. Associations between HVIN (presence and direction), demographic and clinical variables, vestibular deficits, and tumor size were analyzed using binary logistic regression, Fisher’s exact test, Welch’s independent samples t-test, and the Mann–Whitney U test. Results: HVIN was observed in 73% of patients. Among the patients with HVIN, ipsilesional horizontal nystagmus occurred in 57% of cases, 41% of the subjects showed contralesional nystagmus and one patient had downbeat nystagmus. vHIT abnormalities were identified in 54% of patients, while 30% of these also demonstrated fHIT deficits. No patient presented isolated fHIT abnormality. HVIN was detected even in patients with intracanalicular or small tumors and in some asymptomatic individuals. No statistically significant correlations were found between tumor size and HVIN presence, HVIN direction, or vestibular deficits in the vHIT/fHIT (all p > 0.05). Conclusions: HVIN is frequently observed in patients with VS, including those with small or asymptomatic lesions. However, neither its presence nor direction correlates with tumor size or objective vestibular deficit. The Hyperventilation Test should not be considered a prognostic tool but may serve as a simple and valuable adjunctive bedside examination in the early diagnostic suspicion of vestibular schwannoma.
Santopietro et al. (Sun,) studied this question.