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H. J. Simonsz, R. J. Florijn, A. A. B. Bergen, M. Kamermans; What Causes Nightblindness-Associated Transient Tonic Downgaze (NATTD) and Horizontal Nystagmus in Infant Boys With CSNB ?. Invest. Ophthalmol. Vis. Sci. 2010;51(13):1466.
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© ARVO (1962-2015); The Authors (2016-present)
In congenital stationary nightblindness (CSNB), apart from a rapid horizontal nystagmus, a transient tonic downgaze occurs. The coexistence of horizontal and vertical nystagmus has been previously described in dark-reared cats. We explored its causes.
Eleven infant boys presented with chin-up head posture, tonic downgaze and, on attempted upgaze, large-amplitude upward saccades with deceleration during the slow phase downward. The gaze-evoked upward saccades disappeared at the age of two or three years. They also had high-frequency, small-amplitude horizontal pendular nystagmus that remained. Among the infant boys were two pairs of maternally related half-brothers, two cousins, and two siblings. We sought for underlying mutations in CACNA1F and NYX.
Visual acuity ranged from 0.1 to 0.6. ERG-amplitudes (both A- and B-wave) were reduced. Severe myopia was found in five cases. Eight boys had CACNA1F mutations, and one boy had a NYX mutation, compatible with iCSNB or cCSNB, respectively.
CACNA1F is located on the rod side of the rod - ON-bipolar-cell synapse, whereas NYX is located on the side of the ON-bipolar cell. Under scotopic conditions the rod signals travel via the rod ON-bipolar cell via AII amacrine cells to cone ON- and OFF-bipolar cells. Under photopic circumstances these AII amacrine cells may play a role in the detection of approaching objects. Under conditions where the rod ON-bipolar cells are not receiving a proper synaptic input, the receptive field structure of ganglion cells may be different or ganglion cells may spontaneously oscillate. It is conceivable that a resulting bias in motion detection and / or oscillation of retinal ganglion cells is related to the emergence of nystagmus or tonic downgaze.
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