Otolithic (counter-rolling) & Cervico-ocular Nystagmus
- Searching for an Otolithic Nystagmus (Evaluation)
The search for a “counter-rolling” nystagmus is the stage immediately following examination. It is the only sign, in healthy subjects, of the utricular and saccular functions.
It is searched for using 30° to 40° inclination, strictly front-facing, of the head towards the right shoulder then left. One observes, during inclination and while maintaining the position, the movement of the eye-balls. We advise the practitioner to identify any anatomical anomaly of the iris, focus on it and follow its movement during the test. It is traditional to describe two components of the compensatory counter-rolling eye movements:
- one kinetic, which takes place during movement and which corresponds to the stimulation of the vertical canals and the otoliths,
- the other, static, which is observed when inclination is maintained (during at least ten seconds). The eye remains "counter rolling", this reflects the state of the otolithic function only.
The gain of this reflex is weak in men (0.1). This test could currently be correlated against the subjective vertical test and especially the otolithic evoked response test. The latter, objectively studies the sacculocolic reflex. It records myogenic potentials at the level of the sternocleidomstoid muscle (SCM) during stimulation by brief, high intensity clicking sounds. This test enables us to objectively detect a potential saccular dysfunction (C. de Waele).
- Search for the Cervico-ocular nystagmus (Evaluation)
This is a supplementary stage to this protocol. The test consists of maintaining the subject’s head perfectly still while rotating the chair first towards the right and then towards the left. With a normal subject, there is no nystagmic response. Nystagmus can only be observed in subjects with bilateral areflexia (Sémont et al.). The response is always in the opposite direction to compensation.