Nystagmus alternans.

Nystagmus alternans is a seldom observed spontaneous nystagmus. Nevertheless, we would like to draw the attention to it and advice to be aware of its possibility when interpreting ENG recording for positional nystagmus. We draw the attention to some points eliciting suspicion. Continuous recording during several minutes, without changing any influencing factor, is required. Nystagmus alternans is certainly a central nystagmus and so it is important to differentiate it from a positional nystagmus type I. It may be congenital or a result of a lesion in brain stem or cerebellum. The exact localisation still remains an open question. Its interference with vestibular test results is discussed.