Abstract
INTRODUCTION
Optic nerve/chiasmal injury is a devastating outcome that may happen during endoscopic surgery. A key goal of endoscopic skull-base surgery is visual improvement. Currently, however, there is limited intraoperative visual pathway monitoring. We examine a novel technique that uses continuous flash visual evoked potentials (FVEPs).
METHODS
Eyes were stimulated by light stimulators (3 LEDs on each side, 640 nm peak wavelength, 10 ms pulse width, 3000 mCd of luminous intensity). Uniform illumination was placed over eyelids. Recording electrodes were placed at Oz-Fz. The filter cuts were = 5 Hz and 100 Hz with amplifier gain 20,000 or 50,000. EEG was recorded. Recordings were correlated to pre and post operative VFs and acuity. Dropping in the FVEP was examined in relation to intraoperative events. A drop of 50% from the base line was considered positive.
RESULTS
>101 patients had FVEPs in addition to other neurophysiologic monitoring. Patients demographic data, co-morbidities, diagnosis, surgical approach, length of surgery, MAP, and blood loss during surgery were recorded. All patients' visual acuity and field deficits were evaluated by neuro-ophthalmologist before their surgery and within 30 days after surgery. In our cohort, one patient had true positive pre-chiasmatic while another patient had false negative test result. However, the latter patient's deficit was post chiasmatic with no optic nerve or chiasmal injury. Another patient had false positive test (drop of 60%). Eight patients had transient changes related to traction of the chiasm or optic nerves. For predicting optic nerve or chiasmal injury, our study showed sensitivity of 100% (CI2.5-100), specificity of 99% (CI94.5-99.97) and negative predicted value of 100%.
CONCLUSION
FVEP is reproducible throughout surgery and can predict the post surgical outcome. Additionally, we found that FVEP is transiently affected by different stages of surgery. Further validation is required given the small number of optic/chiasmal injuries in our study.
Optic Nerve Monitoring