Downbeat nystagmus caused by a paramedian ponto-medullary lesion

Background: Downbeat nystagmus (DBN) is commonly caused by cerebellar disorders involving the (para-)floccular lobe. Cases of DBN caused by isolated lesions in sites other than the caudal cerebellum are rare. We describe a patient who developed DBN secondary to a hemorrhaged venous cavernoma located at the ponto-medullary junction. Patient and Methods: A 45-year-old man, who had suffered a hemorrhage from a venous cavernoma located at the ponto-medullary junction two years previously, presented at our dizziness unit with blurred vision and unsteadiness of gait. On clinical examination, he had downbeat nystagmus and a defect of fixation suppression of the VOR. Further findings included paresis of the right hypoglossal nerve, slight hyperreflexia of the left leg, impairment of discrimination of touch, pressure, vibration particularly in the left leg, slight dysmetria of the left leg and severe gait ataxia. We recorded three-dimensional eye and head movements with dual search coils placed on the left eye and the forehead. Three-dimensional slow-phase velocity (SPV) was evaluated in different eye positions as described previously (Glasauer et al, J Neurophysiol 2003). Horizontal and vertical smooth pursuit eye movements were tested by having the patient follow a laser dot. Gains were determined by linear regression of SPV to target velocity for positive and negative target velocities separately. Results: Search coil recording revealed downbeat nystagmus which was gaze-dependent and associated with impaired gaze holding when the laser dot was flashed, but not when it was displayed continuously. Vertical smooth pursuit was asymmetrically impaired, being profoundly saccadic with downward and relatively normal with upward movements. Discussion: We describe a case of downbeat nystagmus secondary to a paramedian pontomedullary lesion. It was associated with impairment of the oculomotor neural integrator, which could be suppressed by fixation. These findings support the model of a brainstem-cerebellar feedback loop which augments the neural integrator time constant and adjusts the position and orientation of Listing's plane (Glasauer et al, J Neurophysiol 2003). Based on experimental data obtained in cats, caudal pontine paramedian tract (PMT) cells may be an important component of this neural integration system (Nakamagoe et al, Science 2000).