Ping-pong gaze with “staircase” saccades in coma: An oculographic analysis using EEG
Objective:
We report two cases of comatose patients with saccadic ping-pong gaze and the oculographic analyses of these eye movements using EEG only.
Background:
Ping-pong gaze is a form of periodic alternating gaze deviation in coma with each cycle lasting for a few seconds. These eye movements are typically described as “smooth”. Saccadic ping-pong gaze has been rarely described. The underlying anatomy and physiology are poorly understood.
Design/Methods:
We analyzed the eye movement artifact in two comatose patients. The eye acts like an electrical dipole with the anterior being more positive than the posterior. This leads to a characteristic movement artifact on EEG, which can be used to study the type and direction of eye movements. In this study, we analyzed multiple pages of the eye movement artifact on EEG for both patients. We used a prolonged time constant (TC) of 10s to characterize eye position and direction. A prolonged TC gets as close as possible to DC oculographic analysis.
Results:
Both patients demonstrated ping-pong gaze on EEG analysis, alternating direction every 2-5 seconds. In the patient with anoxic brain injury, the eye movement was smooth and it acquired a saccadic character when patient was stimulated. In the patient with hyperammonemic encephalopathy, the transition between smooth and saccadic was spontaneous. When the saccadic phase was analyzed with TC=10s, the waveform appeared like a “staircase” of saccades, marching toward one extreme over 2-5 seconds and then immediately switching direction.
Conclusions:
We report a novel finding of ping-pong gaze with “staircase” saccades in coma. Staircase saccades have been elicited by prolonged electrical stimulation of superior colliculus. We suggest that the presence of such eye movements indicate a supra-collicular level of injury, whereas isolated smooth ping-pong gaze likely indicates an infra-collicular level of injury. This could reflect a “lighter” state of consciousness in coma.