The existence of optokinetic after-nystagmus (OKAN) has long been known, as far back as the age of Barany.
The term OKAN means nystagmus appearing after first inducing optokinetic nystagmus, and then the optokinetic stimulation is removed. It is easy to appear with the eyes open in a dark place.
There have been various theories about the mechanism of the onset of OKAN.
Sakata et al. previously classified the types of OKAN into the following 7 types : 1) The normal type, 2) The DP-type, 3) The disinhibitory type, 4) The inversive type, 5) The inhibitory type, 6) The dysmetric type, and 7) The clonic type.
In the present study, the authors performed a vestibular equilibrium function inspection, including an OKAN inspection on about 1, 300 patients who visited the Department of Neuro-Otology with complaints of vertigo and equilibrial disturbance.
The results of the inspection were classified in accordance with Sakata's method, and the diagnostic contribution of the OKAN inspection was examined.
The diagnostic significance of the OKAN inspection is considered as follows : 1) This inspection can detect a very small difference between the left and the right of nystagmus in the vestibular-optokinetic system, which difference cannot be detected with OKP inspection giving a rather strong stimulation or with the caloric test giving a non-physiological strong stimulation. 2) This can be a focal localization diagnostic method by the classification by type.
Thus. the diagnostic contribution of OKAN inspection was examined, and the importance of OKAN inspection was recognized again as one the equilibrium function inspections. It is considered that this inspection method will spread henceforth for general use.
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