Low intensity galvanic vestibulo-ocular reflex in normal subjects

An electrical stimulation in man applied between the two mastoids could facilitate the distinction between labyrinthine and retrolabyrinthine lesions by stimulating directly the primary vestibular afferences. However, for this test to be really effective in current medical practice, the results obtained in normal subjects must be symmetrical and reproducible one day to another. The ocular responses induced by a constant electrical stimulation of 2.5 mA, applied between the two mastoids for 30 s (electrically evoked vestibulo-ocular reflex [EVOR]), in one direction and the other, were quantified in ten healthy subjects. Each subject was studied in two different sessions separated by 1 week. Horizontal eye movements were recorded in darkness by an infrared light reflection eye-tracking system. Slow-phase velocity and nystagmus frequency were about 20% higher when the cathode was on the right mastoid than when it was on the left mastoid. This directional preponderance (DP) displayed large individual differences between the two sessions. The reproducibility of the reflectivity (mean of right and left EVOR) was high (r about 0.8). The weak reproducibility of the DP makes the EVOR at weak intensity inadequate to evaluate unilateral vestibular hypofunction. On the other hand, because of the high reproducibility of reflectivity, the EVOR should be effective in detecting bilateral vestibular hypofunction. Moreover, because of the weak intensity of stimulation, no local anaesthesia is needed so the manoeuvre is easy to repeat in case of chronic diseases.