[Nystagmus and vibratory test: evidence for mechanism. Material conditions and methods in the fast detection of unilateral vestibular lesions].

OBJECTIVE: The aim of the present study was to examine the characteristics of vibratory nystagmus (VN), optimise the topography and the frequency of the stimulus, determine the origin of the VN and analyze its clinical implications.

MATERIAL AND METHOD: Fifty-two severe unilateral vestibular lesions (SUVL) (post-surgical vestibular areflexy) were studied. The vibratory nystagmus was measured by 2D and 3D videonystagmography (Synapsis, France). The stimulus was applied with a 3S vibrator at the vertex, both mastoïds, and posterior cervical muscles, at frequencies ranging from 20 to 150 Hz. For topographic optimization, stimulation was given with S vibrator (Synapsis, France) at 100 Hz. The interferences between the vibratory test (VT) and the caloric test (CT) were studied on the normal ear (in 11 subjects). The same study was carried out with the optokinetic test.

RESULTS: The VT revealed a defective nystagmus at all frequencies of stimulation. Optimal response was obtained in a band frequency of 80-120 Hz. Stimulations at 100 Hz showed optimal responses for mastoid topography (responses in 96% of the cases). Stimulation at the posterior cervical muscles and at the vertex indicated respectively a response in 90% and 60% of the cases. The efficiency of the mastoid stimulation was not correlated with the side of stimulation (p=0.9). The interference between the VT and caloric test (CT) with cold water (30 degrees C and 20 degrees C) exhibited the inversion of the caloric nystagmus during the vibrator stimulation. The resulting nystagmus was respectively an algebraic subtraction or addition between the pre-existing caloric nystagmus and the value of the vibratory nystagmus obtained before the caloric test. Adaptation of the VN was moderate. The interference between the VT and the optokinetic test resulted in a subtraction or addition effect, according to the side of the lesion and the direction of the optokinetic stimulus.

CONCLUSION: The VT is an efficient stimulation in mastoid topography. The vestibular contribution is bilateral by bony conduction of the vibration, it explores frequencies ranging from 30 to 120 Hz, with a maximum of response at 100 Hz. The VT interacts with the caloric test and the optokinetic test. The stimulation is very strong, and is able to inverse the caloric nystagmus at cold water stimulation (20 degrees C) In labyrinthine-defective subjects, the VN is always defective at all frequencies, whatever the topographic location of the stimulus, and the position of the head.