Parameters of skull vibration-induced nystagmus in normal subjects

The knowledge of vibration-induced nystagmus test (SVINT) values in the normal population is highly relevant to provide a rapid orientation on the diagnosis attitude in a patient with vertigo.

Although mastoid bone vibration should only induce nystagmus in the presence of vestibular asymmetry, it has also been reported in normal individuals raising doubts as to how to interpret the SVINT. To date, no population studies involving the use of the SVINT and that establish normative values have been published.

This study was carried out at two tertiary healthcare centres on a total of 122 subjects. We stimulated at three frequencies (30, 60 and 100 Hz), in increasing order, first stimulating the right mastoid and then the left mastoid, and waiting for 30 s between each stimulus. The response was recorded with a videonystagmography system. The following variables were evaluated in each subject: the mean and maximum speed of the slow phase of nystagmus, the frequency of the nystagmatic response (NR) and the component and direction of the rapid phase of nystagmus.

Only 26 subjects (20.5%) of the subjects studied here (122 subjects) developed any kind of nystagmatic response and 96 subjects (79.5%) did not display any response. Stimulation at 100 Hz provoked the largest number of responses (p = 0.04), while there was no difference in the number of responses induced by 30 and 60 Hz stimulations (p = 0.85). The frequency of nystagmus was ≤ 0.7 n/s in 80.8% of the positive responses. The mean velocity of the horizontal component of the NR was 2.2°/s (SD 1.6) and that of the vertical component was 1.3°/s (SD 1.2).

Healthy subjects do not generally develop to NR upon vibratory stimulation and only 20% of the subjects studied here developed any kind of NR, this being a slow and inconsistent response of low frequency. The establishment of normal values contributes to improve the orientation in clinical practice in the pathological population and this opens possibilities for tackling more reliable studies in this population.