Intraoperative monitoring of the visual function using cortical potentials after electrical epidural stimulation of the optic nerve.

BACKGROUND

Central skull base meningiomas commonly present with visual deficit, and their removal often leads to improvement of visual function. However, the incidence of postoperative visual deterioration has been reported to be up to 10%. Intraoperative monitoring using flash visual evoked potential has only recently been used with success. Cortical potentials (CP) after electrical epidural stimulation of the optic nerve (ON) were correlated with ON manipulation due to central skull base tumor removal to contribute to improvement of the intraoperative monitoring of the visual function.

METHODS

Blunt needle stimulating electrodes were attached epidurally alongside ON in an unroofed optic canal and used for delivering a rectangular current pulse (intensity 0.2-5.0 mA; duration 0.1-0.3 ms; rate 2 Hz). CPs after electrical epidural stimulation of ON were recorded with corkscrew electrodes at O(z) with the reference electrode at F(z).

RESULTS

P20 and N30 amplitudes were significantly lower (p < 0.05) during tumor removal associated with ON manipulation than in other phases of surgery; the amplitude reductions were reversible in all cases. There were no significant changes in P20, N30 and P40 latencies during the surgery. Immediate postoperative visual function was unchanged in all patients.

CONCLUSIONS

P20 and N30 amplitude changes seem to reliably correspond with the manipulation of ON during anterior skull base tumor removal. Reversible reduction of P20 and N30 amplitude was associated with unchanged immediate postoperative visual function. No correlation between intraoperative variation of CP and newly acquired postoperative visual deficit can presently be made.