Visual Fixation of Skull-Vibration-Induced Nystagmus in Patients with Peripheral Vestibulopathy

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Abstract
Nystagmus induced by applying an intense vibratory stimulus to the skull (SVIN) indicates vestibular functional asymmetry. In unilateral vestibular loss, a 100 Hz bone-conducted vibration given to either mastoid immediately causes a primarily horizontal nystagmus. The test is performed in darkness to avoid visual fixation (VF) but there are no data about how much VF affects the often-intense SVIN. The aim is to analyze the amount of reduction in SVIN when VF is allowed during testing. Thus, all patients seen in a tertiary hospital for vertigo or dizziness with positive SVIN were included. SVIN was recorded for 10 s for each condition: without VF (aSVINwo) and with VF (aSVINw). We obtained an aSVINwo and an aSVINw as average slow-phase velocities (SPV) without and with VF. VF index (FISVIN) was calculated as the ratio of SPV. Among the 124 patients included, spontaneous nystagmus (SN) was found in 25% and the median slow phase velocity (mSPV) (without VF) of SN was 2.6 ± 2.4°/s. Mean FISVIN was 0.27 ± 0.29. FISVIN was 0 in 42 patients, and FISVIN between 0 and 1 was found in 82 (mean FISVIN 0.39 ± 0.02). Fixation suppression was found in all patients with SVIN in cases of peripheral vestibulopathy. FISVIN clearly delineates two populations of patients: with or without a complete visual reduction in nystagmus.

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