On-line monitoring of visual pathway function by VEP
K. Ruether1, M. Bohne2, J. Finger3
Introduction: The intraoperative monitoring of visual pathway function has been hindered not only by the small amplitudes of the visual evoked potential (VEP), but also by the delay between the current function of the visual pathway and its evaluation. To give instant information, a new method of VEP analysis has been developed.
Methods: 16 Hz flickering light emitting diodes (LED) mounted in goggles served as stimulus. VEP recording was done using gold cup electrodes located at Oz and FPz. The ground electrode was attached to the ear lobe. The digitised and amplified potentials could be observed at the monitor. At the same time, the 16 Hz component of the last 20 cycles (duration of cycle 1 second) was calculated by Fast Fourier Transform (FFT). In a first step, the method was performed in 10 individuals with normal vision.
Results: Generally, a reliable potential could be observed after 20 cycles. Interruption of the stimulation e.g. by lid closure lead to an instant reduction of the potential. Returning to normal stimulation, i.e. opening the eyelids, the original potential could be reached in short time. A substantial problem of the method consisted in an insufficient reproducibility. VEP responses of bad quality may potentially be due to muscle or EEG artefacts.
Discussion: Intraoperative monitoring during neurosurgical interventions close to the visual pathway can only be effective if simultaneous information about the visual function can be delivered. This aim can be achieved by the simultaneous analysis of the VEP by FFT. A better compromise between immediate information and reproducibility has to be found by optimising the number of cycles analysed. However, pilot investigations indicate that the reproducibility of the method described here seems to be much better in the anaesthetised patient.
(1) Augenklinik UKE Hamburg, Martinistraße 52, 20246 Hamburg,
(2) Charité-Augenklinik, Berlin
(3) Roland Consult, Wiesbaden