Epileptic monocular nystagmus

To the Editor:

Grant et al.1 reported a patient with monocular epileptic nystagmus. The essential evidence documenting the unusual occurrence of monocular epileptic nystagmus was an EEG recording (figure 3).1 A critical analysis of this EEG sample suggests that deflections identified by the authors as a reflection of monocular nystagmus most probably represent “pop” artifacts of the electrode Fp1.

The eyeball can be regarded as a direct current dipole, with the positive pole facing the cornea and the negative pole the retina. Movement of this dipole creates a large alternating current potential field detectable by EEG recording amplifiers.2 Left-beating nystagmus of the left eye only (as described in this patient) would move the positive pole of the eyeball in the direction of the lateral left frontal electrode F7, resulting in a maximal positivity at that electrode. Since the field of a potential falls exponentially with increasing distance to the generator, the adjacent left temporal electrode T3 would show some, but significantly less, involvement. The frontopolar and frontocentral electrodes Fp1 and F3 would remain relatively unaffected by lateral eye movements. Therefore, in a bipolar montage, the channel Fp1-F7 would show a substantial upward deflection, whereas channel F7-T3 would show a downward deflection of slightly lesser amplitude. On the other hand, channel Fp1-F3 should be only minimally involved (figure 1). 3