Atypical impairment of fixation suppression in a patient with unilateral flocculus lesion.

Fixation suppression of caloric nystagmus was examined in a patient with a unilateral flocculus lesion demonstrated by NMR-CT. Visual suppression (Takemori & Cohen, 1974) of nystagmus to the side of the lesion was less than that to the contralateral side. However, this value may not be accurate because of mixed spontaneous pathological eye movements other than caloric nystagmus. The degree of fixation suppression (FS%=(1-F/D)×100) was calculated from the mean slow-phase velocity (F) of caloric nystagmus during fixation and the maximum slow-phase velocity (D) with eyes open in darkness. FS% of nystagmus to the contralateral side of the lesion was lower than that to the ipsilateral side; that is the reverse of the results of experimental flocculus destruction. The difference between FS% of nystagmus to the ipsilateral side of the lesion and that to the contralateral side is due mainly to the asymmetry of nystagmus during fixation. This may be in compensation for unilateral vestibular system lesions.