Interpretation of Nystagmogram

I. Slow and Fast Phase of Nystagmus

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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 fast-phase velocity in caloric, optokinetic, and optovestibular testing is always a sign of central nervous system disease and frequently indicates a posterior fossa lesion.