2019 年 78 巻 3 号 p. 164-177
A-31-year-old woman presented with a history of experiencing oscillopsia every time she looked at the clock on the night table from a lateral lying posture or at a red traffic light 2 blocks away from her car while driving at night. She had never experienced vertigo, dizziness or unsteadiness before in her life. She visited the outpatient clinic of our university hospital, worried about the nystagmus that she had already noticed. She exhibited lateral gaze nystagmus in both lateral gaze positions. No induced nystagmus was recognized in any specific position or in the positional change tests. Brain MRI revealed no remarkable findings. She was diagnosed as having congenital nystagmus (a manifest latent nystagmus; fusional maldevelopment nystagmus syndrome) based on the absence of any remarkable neurological or neuroradiological findings, with the following characteristic findings on electronystagmography (ENG): 1) In the light, lateral gaze nystagmus in both lateral gazes, and also leftward nystagmus at the primary position 2) In the dark also, lateral gaze nystagmus in both lateral gazes, with rightward nystagmus being predominant. The slow phase showed an approximately exponential decreasing velocity. 3) Rightward nystagmus was still recognized with both eyes closed, even though congenital nystagmus is usually suppressed by eye closing 4) Rightward nystagmus while her head was tilted leftward, while leftward nystagmus was scarcely induced when her head was tilted rightward 5) When her head was tilted rightward, leftward nystagmus was noticed when she closed her right eye and rightward nystagmus when she closed her left eye. However, when her head was tilted leftward, only rightward nystagmus was induced, regardless of whether she kept either eye closed or kept both eyes open. 6) On testing with one eye covered, latent nystagmus was recorded on both the side of the covered eye side and on the contralateral side. 7) Both horizontal and vertical ETTs were preserved 8) Caloric nystagmus was induced bilaterally. Visual suppression (VS) was also recognized bilaterally, with “augmentation” of caloric nystagmus during the VS test. 9) The peak slow phase velocities of optokinetic nystagmus (OKN) were remarkably reduced and gradually showed an “inversion pattern” as the OK-stimulation velocities increased, while no OKN responses were noticed intermingled with the square wave jerks (SWJs) as the OK-stimulation velocities decreased. 10) The optokinetic after-nystagmus (OKAN) on the left showed an “inversion pattern,” while OKAN was scarcely evoked on the right side. These findings are mainly discussed for comparison, from the pathophysiological point of view, with the usual findings of congenital nystagmus. In conclusion, it is speculated that such unique ENG findings are derived from asymmetry of the bilateral velocity storage mechanisms receiving asymmetrical visual inputs from the bilateral nuclei of the optic tracts (NOTs) since her birth.