Usefulness of intraoperative monitoring of visual evoked potentials in transsphenoidal surgery

Abstract

Postoperative visual outcome is a major concern in transsphenoidal surgery (TSS). Intraoperative visual evoked potential (VEP) monitoring has been reported to have little usefulness in predicting postoperative visual outcome. To re-evaluate its usefulness, we adapted a high-power light-stimulating device with electroretinography (ERG) to ascertain retinal light stimulation. Intraoperative VEP monitoring was conducted in TSSs in 33 consecutive patients with sellar and parasellar tumors under total venous anesthesia. The detectability rates of N75, P100, and N135 were 94.0%, 85.0%, and 79.0%, respectively. The mean latencies and amplitudes of N75, P100, and N135 were 76.8 ± 6.4 msec and 4.6 ± 1.8 μV, 98.0 ± 8.6 msec and 5.0 ± 3.4 μV, and 122.1 ± 16.3 msec and 5.7 ± 2.8 μV, respectively. The amplitude was defined as the voltage difference from N75 to P100 or P100 to N135. The criterion for amplitude changes was defined as a > 50% increase or 50% decrease in amplitude compared to the control level. The surgeon was immediately alerted when the VEP changed beyond these thresholds, and the surgical manipulations were stopped until the VEP recovered. Among the 28 cases with evaluable VEP recordings, the VEP amplitudes were stable in 23 cases and transiently decreased in 4 cases. In these 4 cases, no postoperative vision deterioration was observed. One patient, whose VEP amplitude decreased without subsequent recovery, developed vision deterioration. Intraoperative VEP monitoring with ERG to ascertain retinal light stimulation by the new stimulus device was reliable and feasible in preserving visual function in patients undergoing TSS.

Fig. 1.

Preoperative Gd-enhanced MR images: coronal (A), sagittal (B), and visual fields: left (C), right (D) postoperative Gd-enhanced MR images: coronal (E), sagittal (F), and visual fields: left (G), right (H). The tumor was removed subtotally, but visual field demonstrated complete bitemporal hemoanopsia postoperatively. Gd: gadolinium, MR: magnetic resonance.

Fig. 2.

Intraoperative VEP findings at the beginning of surgery as control (A), at the stage of tumor removal (B), and at the end of surgery (C). Negatively is shown as an upward deflection. The VEP amplitude was defined as the voltage difference from P100 to N75. During tumor removal, the VEP amplitude decreased and did not recover to the control level. Bil: bilateral, Lt: left, Rt: right, VEP: visual evoked potential.

Fig. 3.

Preoperative Gd-enhanced MR images: coronal (A), sagittal (B), and visual fields: left (C), right (D) postoperative Gd-enhanced MR images: coronal (E), sagittal (F), and visual fields: left (G), right (H). Although, residual tumor was observed under chiasm, visual field recovered postoperativeply. Gd: gadolinium, MR: magnetic resonance.

Fig. 4.

Intraoperative VEP findings at the beginning of surgery as control and the end of surgery (A) and the stage of tumor removal (B). Negatively is shown as an upward deflection. The VEP amplitude was defined as the voltage difference from P100 to N75. During tumor removal, the VEP amplitude decreased transiently (B: arrow) and recovered to the control level during suspended surgical manipulation for 5 min. VEP: visual evoked potential.