gms | German Medical Science

Objective: Visual evoked potentials are routinely employed for monitoring the function of the visual pathways during surgical removal of pathologies involving the sella, parasellar and temporo-occipital areas, thus providing information about the integrity of the visual pathways on the timescale the system operates on. Historically, visual evoked potentials are measured in awake subjects using a reversing-pattern checkerboard of standard contrast, whereas during surgical interventions under general anesthesia the visual stimulus consists of light flashes delivered at a given intensity through closed eyelids. Although there exists a consensus about the usefulness and reliability of the intraoperative monitoring of the visual evoked potentials, it remains unclear how the responses of the visual neurons elicited by flashes of light at various wavelengths and intensities compare to the reversing-pattern checkerboard.

Methods: 20 healthy subjects are included in the protocol. First we measure the visual evoked potentials using the reversing pattern checkerboard in a room with dimmed light, as to bring the photoreceptor elements in a state of adaptation. Recordings are done with skin needle electrodes positioned in a standard fashion over O1 (left), O2 (right), Oz and Cz, while the visual stimulus is delivered from a calibrated monitor placed 1m in front of the subject. The frequency of the reversal of the checkerboard pattern is chosen at 1 Hz. The light intensity at the 1m distance is measured using a photometric diode, and the contrast of the stimulus adjusted as to match the light intensity delivered by the eye pads and goggles. Next we measure the visual evoked potentials using the flashing white light LED goggles (Medtronic) as well as red light (660 nm) LED flash goggles (Inomed). The pulse length is set al 10ms with a stimulation frequency of 1-4 Hz, delivering a light intensity of 3000-7000 lux. Each eye is examined separately and the result of 100 trials averaged for each of the techniques.

Results: Measurement of N75 and P100 is reproducible and reliable with reversing-pattern checkerboard stimuli. We will report about the difference in response amplitude and latency using visual stimuli delivered with flashing goggles of different types.

Conclusion: The measurement of VEP using three different techniques (pattern reversal, LED goggles, and LED eye pads) in normal volunteers allows the decision whether goggles or eye pads are more reliable to recognizes evoked potentials over the occipital lobe. The most reliable technique will be introduced in the operative setting.