6. 先天性眼振―その特徴と闇に隠れた本態

抄録

　Congenital nystagmus (CN) is known as an involuntary to-and-fro movement of the eyes characterized by a wide variation in waveforms ranging from jerk to pendular types. CN is presumed to be present at birth. In antiquity, midwives or shaman occasionally noticed CN. The first description of CN, popularly known in china as “shiji,” was made around 2000 years ago. Despite its various waveforms, people with CN do not develop visual symptoms and rarely complain of oscillopsia. CN can be idiopathic but is most likely caused by gaze stability, although some reports have suggested familial cases. No detailed mechanisms have been proposed to explain the generation of CN waveforms in people with CN in whom the vestibulo-ocular reflex (VOR) cannot be established. The vestibular time constant showed a good correlation between VOR and perception in normal subjects when assessed using perceptual measures. The time constant (TC) of decay of vestibular sensation in individuals with CN was half the duration of the TC found in normal subjects. Thus, individuals with CN have short vestibular time constants, probably due to changes induced in velocity storage processing by the persistent retinal image motion present in individuals with CN. There is a fascinating paper showing that FRMD7, also known as the CN gene, is necessary for optokinetic nystagmus (OKN) in humans and mice. In the retina of FRMD7-mutant mice, horizontal direction selectivity and asymmetry of inhibitory inputs to horizontal direction-selective retinal ganglion cells (HDSCs) are both lost. This could be a hidden essential characteristic of CN. HDSCs are responsible for horizontal OKN and project to the nucleus of the optic tract (NOT), and NOT, in turn, is related to the velocity storage mechanism, vestibular adaptation, and habituation. Inversion of OKN and suppression of nystagmus with eye closure are characteristics of CN. Both of these features can be explained by abnormalities in HDSCs. So, HDSCs can only play the role in the light. In the future, innovative ideas could help to elucidate the nature of CN.