Optokinetic nystagmus: six practical uses.

Optokinetic nystagmus (OKN) is a reflexive eye movement in response to movement of the viewer's visual environment that consists of a slow phase eye movement in the direction of the stimulus followed by a quick phase in the opposite direction. When tested at the bedside, the slow phases represent smooth pursuit, while the quick phases represent saccades. Normally, OKN is conjugate and symmetric (horizontally and vertically). Abnormalities in the optokinetic response can provide diagnostic and localising value. We describe six clinical scenarios where OKN testing is most useful for the practising neurologist.