Smooth pursuit mechanisms in congenital nystagmus

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Abstract
Patients with congenital nystagmus (CN) generally have nystagmus of irregular waveform in the horizontal plane. The eye position of minimal nystagmus intensity (neutral zone, NZ) often does not coincide with the midposition of the eye. Eye movements are not only disturbed at different angles of gaze but particularly also during smooth pursuit eye movements (SPEM), optokinetic nystagmus (OKN) and suppression of the vestibulo-ocular reflex by visual fixation (VORS). From experimental evidence it is known that under all three stimulus conditions (SPEM, OKN and VORS) neuronal smooth pursuit mechanisms are activated. There are conflicting conclusions about the cause of the deficit for these eye movements in CN. Pathological smooth pursuit mechanisms have been suggested by some authors. Others have attributed the deficits to the dynamic shift of the NZ (DNZ), which can be most clearly seen during SPEM. In the present study the eye movements of 30 CN-patients have been studied under static conditions, SPEM, OKN and VORS. The analysis permits the conclusion that eye movement deficits of patients with CN can be explained on the basis of an NZ-shift, without assuming a major smooth pursuit deficit.