Infantile Nystagmus (Congenital Nystagmus)

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  • 乳児眼振（先天性眼振）
  • ニュウジガンシン(センテンセイガンシン)

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<p>Since nystagmus, in which the eyeballs move rhythmically from side to side, is not observed immediately after birth, the term congenital nystagmus has been replaced by the term infantile nystagmus in recent years. Patients with infantile nystagmus exhibit impaired gaze fixation, horizontal smooth pursuit and optokinetic nystagmus (OKN), and use saccadic eye movements for these underlying impairments. In order to induce saccades, they make a sufficient angle between gaze and target by precedent exponential slow eye movements. Infantile nystagmus consists of the alternate appearance of saccades and slow eye movements, so that it is therefore considered as a necessary strategy for obtaining better vision and not an obstacle to clear vision. Because patients follow the moving target during the fast and not the slow phase of nystagmus, the lines obtained by connection of the end eye positions during the fast phase of nystagmus coincide with the trajectories of the targets. By setting the direction of the slow phase of nystagmus in a direction opposite to the direction of the OKN stimulus, a sufficient angle can be effectively obtained between the gaze and target for the induction of saccade. This is the mechanism of the reversed OKN response.</p><p>A negative feedback mechanism exists for controlling the eye movements in healthy people. In the case of infantile nystagmus, if the line of sight deviates slightly from a stationary target, it is impossible to induce a small saccadic eye movement to correct for the small deviation with saccadic eye movement. Therefore, the deviation must be increased, and control to move the eye movement away from the target is necessary, so that a positive feedback mechanism is used. The output under control by the positive feedback mechanism changes exponentially. Because the slow phase of infantile nystagmus is controlled by a positive feedback mechanism, the position and velocity data of the slow phase change exponentially.</p>

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