IN-vision
Furthering Research into Infantile Nystagmus (IN)
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Transient
Epileptic nystagmus
Epileptic nystagmus is seen only during epileptic seizures and is brought on by flashing lights, image blurring, stress, fatigue, and changes in light intensity. Epileptic nystagmus can also occur spontaneously and can be accompanied by symptoms of oscillopsia, blurred vision, amaurosis and hallucinations. The most likely type of oscillations that will be seen in epileptic nystagmus are conjugate (the same in both eyes), horizontal, jerk waveform oscillations which beat in opposition to the direction of gaze.
There are two forms of epileptic nystagmus. In the first, the nystagmus oscillations cross the primary position with a large amplitude slow-phase. The second type of epileptic nystagmus has oscillations which do not cross the midline and always beat in opposition to the direction of gaze.
Voluntary nystagmus
A small percentage of people (~8%) can induce a horizontal nystagmus oscillation of the eyes at will, often by forcing convergence (Zahn 1978). This movement is very small (~2-5° (Fisher, Davies and Wallis 1979)) and fast (15-30Hz (Ramat et al. 2008)), and is believed to be caused by inhibition of omnidirectional pause neurons, which are located in the raphe interpositus nucleus and serve to tonically inhibit excitatory burst neurons prior to saccade generation. The release of tonic inhibition at the onset of a saccade is known as ‘post-inhibitory rebound’ and is what affords saccades their great speed. When the omnidirectional pause neurons are inhibited for long periods of time however, a feedback loop is created, causing oscillations to occur.
Voluntary nystagmus is not usually considered to be a pathological condition due to its voluntary nature, however cases of ‘involuntary’ voluntary nystagmus have been documented (Neppert and Rambold 2006).