Factors causing variability of the slow and fast phases of nystagmus in caloric testing are reviewed. The range of normal values is wide. This can arise from three different sources: technical variation in conducting the test, anatomical differences among temporal bones, and involvement of the central nervous system during caloric responses. A model for organization of the eye movement in vestibular testing is presented. Parameters based on the slow phase of nystagmus do not discriminate a peripheral lesion from a central lesion except for the following: (1) lack of visual suppression in light during caloric testing indicates a posterior fossa lesion and (2) decrease of slow-phase velocity in unilateral or bilateral optovestibular testing is suggestive of brain stem disorder. However, examination of the fast phase of nystagmus can provide additional information for differentiating a peripheral from a central lesion. A decrease of fastphase velocity in caloric, optokinetic, and optovestibular testing is always a sign of central nervous system disease and frequently indicates a posterior fossa lesion.
INTRODUCTIONEye movements can be divided into voluntary and involuntary types. The voluntary eye movements consist of saccades and smooth pursuits. The involuntary eye movements consist of fast and slow phase of nystagmus (although saccades released from peripheral retina may also be included). Valuable information concerning these four different forms of eye movement can be derived from electronystagmography. Until recently only factors related to the slow component of vestibular and optokinetic nystagmus have been analyzed in depth. In this paper the value of examining the slow component of optovestibular nystagmus and the fast component of nystagmus is discussed.