Intraoperative Visual Evoked Potential Monitoring Research Articles

How should visual function monitoring be performed and interpreted in surgery for suprasellar tumors?

Preservation of visual function is important in surgery for suprasellar tumors. Visual evoked potentials (VEPs) are expected to play an important role in monitoring visual function during surgery. Given the lack of information in this field, the authors aimed to investigate the effects of optic nerve compression caused by suprasellar tumors to understand the possible usefulness of VEP monitoring using off-response (OFR) VEP. Eleven healthy volunteers who underwent surgery for standard record confirmation and 32 patients with optic chiasm lesions who underwent surgery were examined. Preoperative, postoperative, and intraoperative VEPs were recorded. Propofol anesthesia was administered during intraoperative VEP monitoring. Patients who underwent surgery were monitored using the same stimulation method during surgery. Light stimulation was given from a luminant pad on the eyelids, and low-intensity stimulation with continuous 500-msec emission and 500 msec off was performed. The luminescence intensity of the stimulation was at a maximum of 8000 lx with three attenuation steps, each of which was recorded repeatedly. The OFR potentials and delay latencies decreased as stimulus intensity decreased. In the patient with temporal hemianopia, monocular stimulation produced the highest OFR in the contralateral occipital lobe of the stimulated eye. The authors recorded preoperative, intraoperative, and postoperative VEP in 32 patients and observed intraoperative changes in 23 patients. In the cases where VEP declined during intraoperative recording, it recovered when surgery was discontinued. Furthermore, 3 patients eventually achieved a higher VEP than that achieved at the beginning of the surgery, and rapid recovery was confirmed with visual field examination immediately after surgery. Of the 5 patients in whom VEP did not recover during surgery, 3 showed decreased visual field and acuity after surgery. In 15 cases, potential dropped temporarily but returned to the original potential, and their visual field recovered after surgery. OFR has a diagnostic element in the visual field, in which the maximal potential was recorded on the opposite side of the stimulus with monocular stimulation. Unambiguous determination required stimulation of different intensities in both eyes or 1 eye and multiple recording electrodes placed in the occiput. Monitoring the OFR provides real-time alerts, making it a valuable tool for visual function evaluation in suprasellar surgery.

Intraoperative Visual Evoked Potential

In neurosurgery, the intraoperative visual evoked potential(VEP)has recently been used for the management of anterior skull base and parasellar tumors related to the optic pathways to prevent postoperative visual complications. We used light emitting diode photo-stimulation thin pad and stimulator(Unique Medical, Japan). We also recorded the electroretinogram(ERG)simultaneously to exclude technical errors. VEP is defined as an amplitude between the maximum positive wave at 100 ms(P100)and the prior negative wave(N75). In intraoperative VEP monitoring, reproducibility of VEP should be ascertained, particularly in patients with preoperative advanced visual impairment and an intraoperative diminished amplitude. Furthermore, a 50% reduction in the amplitude is critical. In such cases, we should consider suspending or changing surgical manipulation. We have not clearly verified the relationship between the absolute intraoperative VEP value and postoperative visual function. Mild peripheral visual field defects cannot be detected in the present intraoperative VEP system. However, intraoperative VEP with ERG monitoring can serve as a real-time warning to guide surgeons to avoid postoperative visual impairment. We should comprehend the principles, characteristics, disadvantages, and limitations of intraoperative VEP monitoring for reliable and effective utilization.

The clinical application of intraoperative visual evoked potential in recurrent craniopharyngiomas resected by extended endoscopic endonasal surgery

ObjectiveThe protection of visual function is a major challenge for recurrent craniopharyngioma due to the interface between tumor and surrounding structure (esp. optic nerve) is more unclear. Intraoperative visual evoked potential (VEP) has been integrated into transsphenoidal surgery for pituitary adenomas and primary craniopharyngiomas in the prediction of postoperative visual outcome. In the present study, we firstly analyzed the clinical value of the VEP in the prediction of postoperative visual outcome for recurrent craniopharyngiomas by extended endoscopic endonasal surgery (EEES). MethodsThirty-six patients with recurrent craniopharyngiomas who underwent EEES with intraoperative VEP monitoring were retrospectively reviewed. The association between the changes in the VEP amplitude and postoperative visual outcome was determined. In addition, other potential prognostic factors with regard to postoperative visual outcomes were also included in the analysis. ResultsGross total resection (GTR) was achieved in 32 patients (88.9%). Reproducible and stable VEP was recorded in 72 eyes. We recorded that there were 29 eyes remained stable VEP amplitude intraoperation, 4 (13.8%) of which suffered new visual acuity (VA) and 5 (17.2%) suffered new visual field (VF) defects after surgery. A transient VEP decrease was recorded in 41 eyes, 8 (19.5%) of which suffered from visual deterioration. Of 2 eyes with a permanent VEP decrease, 2 (100%) experienced postoperative visual impairment. Clinical variable analyses revealed a significant correlation between recurrent craniopharyngioma and age, adhesion with optic nerve, postoperative VA, and intraoperative VEP amplitude. Unstable VEP amplitude during operation and tight adhesion were independent risk factors for postoperative VA. Tight adhesion and a larger tumor volume were independent risk factors for postoperative VF defects. ConclusionsEEES provides a unique view for the lesions under and behind the chiasm and achieve higher rates of GTR with improved visual outcomes. Tight adhesion and unstable VEP amplitude were strong predictors of postoperative visual impairment. Recurrent craniopharyngiomas are more likely to adhere to the optic nerve, VEP is more likely to be unstable during operation, and the improvement rate of postoperative visual function is lower than primary craniopharyngiomas. So intraoperative VEP is more necessary for recurrent craniopharyngiomas.

Prediction of Post-operative Visual Deterioration Using Visual-Evoked Potential Latency in Extended Endoscopic Endonasal Resection of Craniopharyngiomas

Background: The current study aimed to investigate the predictive value of visual-evoked potential (VEP) latency for post-operative visual deterioration in patients undergoing craniopharyngioma resection via extended endoscopic endonasal approach (EEEA).Methods: Data from 90 patients who underwent craniopharyngioma resection via EEEA with intraoperative VEP monitoring were retrospectively reviewed. P100 latency was compared between patients with and without post-operative visual deterioration, and the threshold value of P100 latency for predicting post-operative visual deterioration was calculated by the receiver operating characteristic curve analysis. In addition, other potential prognostic factors regarding post-operative visual outcomes were also analyzed by multivariate analysis.Results: Patients with post-operative visual deterioration showed a significantly longer VEP latency than those without (p < 0.001). An extension over 8.61% in VEP latency was identified as a predictor of post-operative visual deterioration (p < 0.001). By contrast, longer preoperative visual impairment duration and larger tumor volume were not significant predictors for post-operative visual deterioration.Conclusions: The current study revealed that intraoperative VEP monitoring in EEEA is effective for predicting post-operative visual deterioration, and an extension over 8.61% in VEP latency can be used as a critical cut-off value to predict post-operative visual deterioration.

뇌하수체 종양 수술에서 수술중신경계감시: 증례보고

Visual evoked potential (VEP) is a useful tool for monitoring the visual pathway during surgery. We report two cases with pituitary adenoma who had intraoperative VEP monitoring during the tumor removal surgery via the transsphenoidal approach. One case had transient delayed VEP latency during the tumor removal. The other case had transient decreased VEP amplitude with recovery to the baseline. Our two cases had no postoperative visual impairment. Intraoperative VEP monitoring could be applied during the surgical treatment of the lesion near the visual pathway such as the pituitary adenoma.

The predictive value of intraoperative visual evoked potential for visual outcome after extended endoscopic endonasal surgery for adult craniopharyngioma.

Due to the proximity of craniopharyngioma to the optic apparatus, one of the most common complications after surgery is visual deterioration. Intraoperative visual evoked potential (VEP), as a means of real-time visual function monitoring, has been integrated into transsphenoidal surgery for pituitary adenoma to predict postoperative visual outcome. Compared with pituitary tumor, craniopharyngioma often adheres to optic nerves, with increased risk of postoperative visual impairment. Furthermore, extended endoscopic endonasal surgery (EEES) can provide direct visualization of the surgical plane between the craniopharyngioma and the optic nerves, which contributes to analysis of the mechanism of real-time VEP changes during surgery. Therefore, VEP monitoring applied during EEES for craniopharyngioma may have more clinical value. However, only 9 patients who underwent EEES with VEP monitoring for craniopharyngioma have been sporadically reported to date. In this paper, the authors present the largest series to date analyzing the clinical value of VEP to predict postoperative visual outcome in adult patients with craniopharyngioma. Sixty-five adult patients who underwent EEES with intraoperative VEP monitoring for primary craniopharyngioma were retrospectively reviewed. The association between changes in VEP amplitude and postoperative visual outcome was determined. In addition, other potential prognostic factors with regard to postoperative visual outcomes were included in the analysis. Gross-total resection was achieved in 59 patients (90.8%). Reproducible and stable VEP was recorded in 128 of 130 eyes (98.5%). During surgery, VEP remained stable in 108 eyes, 10 (9.3%) of which had new visual acuity (VA) and/or visual field (VF) defects after surgery. Transient VEP decrease was recorded in 15 eyes, 4 (26.7%) of which had visual deterioration. Of the 5 eyes with permanent VEP decrease, 3 (60%) experienced postoperative visual impairment. Permanent VEP decrease (OR 19.868, p = 0.007) and tight adhesion (OR 6.104, p = 0.040) were independent adverse factors for postoperative VA deterioration. Tight adhesion (OR 7.150, p = 0.002) and larger tumor volume (OR 1.066, p = 0.001) were significant risk factors for postoperative VF defects. Intraoperative VEP monitoring can serve as a real-time warning to guide surgeons to avoid postoperative visual impairment. It effectively predicted VA changes in adult patients with craniopharyngioma after EEES. Tight adhesion and larger tumor volume were also strong predictors of postoperative visual impairment.

Comparison Between Conventional Flash and Off-Response Intraoperative Visual Evoked Potential Monitoring for Endoscopic Endonasal Surgery.

Intraoperative flash stimulation visual evoked potential (VEP) monitoring has been used for endoscopic endonasal approach (EEA). Recently, off-response VEP, which is recorded when the light stimulus is turned off, was introduced to monitor visual function intraoperatively. To evaluate off-response VEP monitoring in comparison with the conventional flash stimulation VEP monitoring for EEA. From March 2015 to March 2020, 70 EEA surgeries with intraoperative VEP monitoring (140 eyes) were performed. Light stimuli were delivered by a pair of goggle electrodes. Recording electrodes were placed on the scalp over the occipital region. The warning signal was prompted by a reduction of the peak-to-peak amplitude of the VEP by more than 50% compared to the initial amplitude. Visual function was assessed pre- and postoperatively. Results of flash and off-response VEP monitoring were compared. VEP was recorded in 134 eyes. Warning signal occurred in 23 eyes (transient in 17 eyes and permanent in 6 eyes). Two eyes showed permanent VEP attenuation for flash VEP monitoring, in which one patient had postoperative visual function deterioration. Four eyes showed permanent VEP attenuation for off-response VEP monitoring, where 2 patients had postoperative visual function deterioration. Sensitivity, specificity, positive predictive value, and negative predictive value were 100%, 98.9%, 50%, and 100%, respectively, for flash stimulation VEP, and 100%, 97.8%, 50%, and 100%, respectively, for off-response VEP. VEP monitoring was useful to monitor visual function in EEA surgery. Off-response VEP monitoring was not inferior to conventional flash stimulation VEP monitoring.

Intraoperative monitoring of visual evoked potentials in patients undergoing transsphenoidal surgery for pituitary adenoma: a systematic review

BackgroundTranssphenoidal surgery is the gold standard for pituitary adenoma resection. Although rare, a serious complication of surgery is worsened vision post-operatively.ObjectiveTo determine whether, in patients undergoing transsphenoidal surgery for pituitary adenoma, intraoperative monitoring of visual evoked potentials (VEP) is a safe, reproducible, and effective technological adjunct in predicting postoperative visual function.MethodsThe PubMed and OVID platforms were searched between January 1993 and December 2020 to identify publications that (1) featured patients undergoing transsphenoidal surgery for pituitary adenoma, (2) used intraoperative optic nerve monitoring with VEP and (3) reported on safety or effectiveness. Reference lists were cross-checked and expert opinion sought to identify further publications.ResultsEleven studies were included comprising ten case series and one prospective cohort study. All employed techniques to improve reliability. No safety issues were reported. The only comparative study included described a statistically significant improvement in post-operative visual field testing when VEP monitoring was used. The remaining case-series varied in conclusion. In nine studies, surgical manipulation was halted in the event of a VEP amplitude decrease suggesting a widespread consensus that this is a warning sign of injury to the anterior optic apparatus.ConclusionsDespite limited and low-quality published evidence regarding intra-operative VEP monitoring, our review suggests that it is a safe, reproducible, and increasingly effective technique of predicting postoperative visual deficits. Further studies specific to transsphenoidal surgery are required to determine its utility in protecting visual function in the resection of complex pituitary tumours.

WS5.6. Intraoperative Monitoring and Mapping of Visual Pathways

Background and Objective . Neurosurgical procedures along the visual pathways carry a risk of visual dysfunction. A reliable method for real time visual function monitoring assists in intraoperative decision making regarding radicality of excision and intermittent maneuvers near the optic apparatus. We present our experience of intraoperative flash visual evoked potential (VEP) monitoring. Methods 1) Anesthesia: Under total intravenous anesthesia (TIVA), the VEP shows larger amplitude and stable latency with less variability. Inhaled anesthetics, such as sevoflurane and isoflurane, markedly decrease the amplitude, and these should be avoided. 2) Stimulation: Flash VEP and ERG are recorded by flush stimulation on the closed eye lids. At least 1-minute preconditioning by flash stimulation should be done to obtain a steady VEP waveform before starting averaging. The stimulation intensity is decided by the supramaximal stimulation to the retina. As the evoked ERG is easy to record and the waveform is stable, it is utilized for checking the supramaximal value and inadequate stimulation to retina due to stimulator dislocation. 3) VEP Recording: Reference electrodes are set at A1 and A2, and these are electrically connected. Recording electrodes are set near Oz, O1, and O2. Simultaneous ERG recording is recommended as described above. Warning sign was defined as 50% decrease of baseline amplitude. 4) Optic nerve action potential: In cases of parasellar lesions such as craniopharyngiomas and giant aneyrysms, optic nerves, chiasm and tracts are sometimes streched around the lesion, and it is difficult to identify the location of the optic apparatus. In these situations, flash stimulation is deliverd as same manner as VEP, and action potentials recoeded from the operative fields facilitates to identifiy the optic apparatus. Results . VEP can be monitored in a patient with visual acuity greater than (0.03). Fifty percent decrease of VEP amplitude as warning sign detected postoperative visual aggravation in sensitivity of 99%. Reversible change in VEP means visual function to be preserved. Sudden VEP change correlates with visual acuity impairment and/or hemianopsia. The flat VEP indicates postoperative severe visual disturbance (nearly blindness). Conclusions . Intraoperative flash VEP monitoring is essential and reliable method for preserving visual function.

Transient Retinal Ischemia During Carotid Endarterectomy Estimated by Intraoperative Visual Evoked Potential Monitoring: Technical Note

Carotid revascularization surgery is known to carry a risk of postoperative visual deterioration associated with retinal ischemia. We checked intraoperative visual evoked potential (VEP) monitoring in carotid endarterectomy (CEA). Ten consecutive patients who underwent CEA in Shinshu University Hospital under total intravenous anesthesia were checked by intraoperative VEP and electroretinogram (ERG) recording in addition to somatosensory evoked potential monitoring. Two of 10 patients presented decreased amplitude of VEP and ERG on the ipsilateral affected side by clamping the common carotid artery and persistent attenuation of VEP and ERG during external carotid artery occlusion, using an internal carotid shunt. These findings disappeared immediately after releasing the cervical carotid artery clamping. In the other 8 patients, VEP and ERG did not change throughout the surgery. Transient retinal ischemia during even brief carotid artery occlusion in the CEA procedure could be estimated by intraoperative VEP and ERG monitoring.

Extended endoscopic endonasal resection of craniopharyngioma using intraoperative visual evoked potential monitoring: technical note.

To avoid deterioration of visual function, extended endoscopic endonasal transsphenoidal surgery (TSS) for craniopharyngioma was performed with visual evoked potential (VEP) monitoring using light-emitting diodes (LEDs). The position of the optic chiasm was carefully evaluated on the preoperative midsagittal magnetic resonance (MR) images. Intraoperatively, direct and sharp dissection of the tumor from the optic chiasm was performed under VEP monitoring with LEDs through extended endoscopic endonasal TSS. If the VEP finding changed and became unstable, the operator were informed and stopped the surgical manipulation for the optic chiasm to recover. After 5-10min, recovery of VEP findings was checked and the procedure resumed. Extended endoscopic endonasal TSS with VEP monitoring was performed in consecutive 7 adult patients with newly diagnosed suprasellar craniopharyngiomas with maximum diameters of 25-41mm (mean 33.7mm). VEPs were stable throughout the surgery in 5 cases, but showed temporary instability and amplitude decrease in 2 cases, although the VEPs had recovered at the end of the surgery. Visual function, evaluated using visual impairment score, was improved after surgery in all patients. Gross total removal was achieved in 5 cases, and subtotal removal (90%) in 2 cases. Intraoperative VEP monitoring is the only way to test visual function during surgery, and may be important and helpful in extended endoscopic endonasal TSS, which requires direct dissection between the optic nerve and craniopharyngioma under the endoscope.

The Predictive Role of Intraoperative Visual Evoked Potentials in Visual Improvement After Endoscopic Pituitary Tumor Resection in Large and Complex Tumors: Description and Validation of a Method

With the advent of extensive endoscopic approaches for pituitary tumors, there has also been an increase in surgery for larger and more complex tumors. Intraoperative manipulation during endoscopic resection of sellar tumors poses potential risk in postoperative visual function in this tumor population. This study proposes a method of accurate intraoperative monitoring of visual evoked potentials (VEPs) and its role in predicting visual function outcomes. Intraoperative VEPs were monitored for 42 resections from a single surgical team, with average tumor size of 2.84 cm. Changes in VEP amplitude and latency in excess of 50% were considered significant. Preoperative and postoperative visual information was obtained from ophthalmology and hospital records, along with patient demographics, comorbidities, and tumor characteristics. Patients were stratified as experiencing deteriorations in VEPs that did not restore to baseline (n= 4), deteriorations in VEPs that did restore to baseline (n= 6), no change in VEPs (n= 31), and improvement in VEPs (n=1). Correlation between VEP changes and postoperative visual fields was measured through univariate ordered logistic regression. Improved intraoperative VEP measurements were associated with odds ratio (OR) of visual field improvement of 3.15 (95% confidence interval, 1.15-8.59). Specifically, changes in VEP amplitude were positively associated with visual field improvement with OR of 4.35 (OR, 1.29-14.7). No association was observed between VEPs and other patient or tumor characteristics. Changes in VEP amplitude during endoscopic sellar tumor resection correlate with postoperative visual function. Intraoperative VEP monitoring can serve an important role in preventing postoperative visual field loss.

Intraoperative monitoring with visual evoked potentials for brain surgeries.

The goal of this study was to determine the performance of intraoperative visual evoked potentials (VEPs) in detecting visual field changes. Assessments of VEPs were performed with simultaneous retinal responses by using white light-emitting diodes protected from scialytic microscope lights. The alarm criterion was a reproducible decrease in amplitude of the VEP P100 wave of 20% or more. Visual fields were assessed preoperatively and 1 month postsurgery (Goldmann perimetry). The VEPs were analyzed for 29 patients undergoing resection of a brain lesion. In 89.7% of patients, steady VEP and retinal responses were obtained for monitoring. The absence of alarm was associated in 94.4% of cases with the absence of postoperative visual changes (specificity). The alarms correctly identified 66.7% of cases with any postoperative changes and 100% of cases with changes more severe than just a discrete quadrantanopia or deterioration of an existing quadrantanopia (sensitivity, new diffuse deterioration < 2 dB). In 11.5% of patients, a transitory VEP decrease with subsequent recovery was observed without postoperative defects. Intraoperative VEPs were performed with simultaneous recording of electroretinograms, with protection from lights of the operating room and with white light-emitting diodes. Intraoperative VEPs were shown to be reliable in predicting postoperative visual field changes. In this series of intraaxial brain procedures, reliable intraoperative VEP monitoring was achieved, allowing at minimum the detection of new quadrantanopia. The standardization of this technique appears to be a valuable effort in regard to the functional risks of homonymous hemianopia.

Comparison of visual evoked potential monitoring during spine surgeries under total intravenous anesthesia versus balanced general anesthesia

ObjectiveTo determine the comparison of its clinical utility and safety profile for visual evoked potential (VEP) monitoring during prone spine surgeries under total intravenous anesthesia (TIVA) versus balanced general anesthesia using the SightSaver™ visual stimulator. MethodsThe protocol was designed asa pilot, single center, prospective, randomized, and double-arm study. Subjects were randomized to receive either TIVA or balanced general anesthesia. Following induction and intubation, 8 electrodes were placed subcutaneously to collect VEP recordings. The SightSaver™ visual stimulator was placed on the subject’s scalp before prone positioning. VEP waveforms were recorded every 30min and assessed by a neurophysiologist throughout the length of surgery. ResultsA total of 19 subjects were evaluated and VEP waveforms were successfully collected. TIVA group showed higher amplitude and lower latency than balanced anesthesia. ConclusionsOur data suggested that TIVA is associated with higher VEP amplitude and shorter latencies than balanced general anesthesia; therefore, TIVA could be the most efficient anesthesia regimen for VEP monitoring. SignificanceThe findings help to better understand the effect of different anesthesia regimens on intra-operative VEP monitoring.

Intraoperative monitoring of flash visual evoked potential under general anesthesia.

In neurosurgical procedures that may cause visual impairment in the intraoperative period, the monitoring of flash visual evoked potential (VEP) is clinically used to evaluate visual function. Patients are unconscious during surgery under general anesthesia, making flash VEP monitoring useful as it can objectively evaluate visual function. The flash stimulus input to the retina is transmitted to the optic nerve, optic chiasm, optic tract, lateral geniculate body, optic radiation (geniculocalcarine tract), and visual cortical area, and the VEP waveform is recorded from the occipital region. Intraoperative flash VEP monitoring allows detection of dysfunction arising anywhere in the optic pathway, from the retina to the visual cortex. Particularly important steps to obtain reproducible intraoperative flash VEP waveforms under general anesthesia are total intravenous anesthesia with propofol, use of retinal flash stimulation devices using high-intensity light-emitting diodes, and a combination of electroretinography to confirm that the flash stimulus has reached the retina. Relatively major postoperative visual impairment can be detected by intraoperative decreases in the flash VEP amplitude.

Clinical utility and limitations of intraoperative monitoring of visual evoked potentials.

ObjectivesDuring surgeries that put the visual pathway at risk of injury, continuous monitoring of the visual function is desirable. However, the intraoperative monitoring of the visual evoked potential (VEP) is not yet widely used. We evaluate here the clinical utility of intraoperative VEP monitoring.MethodsWe analyzed retrospectively 46 consecutive surgeries in 2011-2013. High luminance stimulating devices delivered flash stimuli on the closed eyelid during intravenous anesthesia. We monitored VEP features N75 and P100 and took patients' preoperative and postoperative visual function from patient charts. Postoperative ophthalmologic workup was performed in 25 (54%) patients and preoperatively in 28 (61%) patients.ResultsVEP recordings were feasible in 62 of 85 eyes (73%) in 46 patients. All 23 eyes without VEP had impaired vision. During surgery, VEPs remained stable throughout surgery in 50 eyes. In 44 of these, visual function did not deteriorate and three patients (6 eyes) developed hemianopia. VEP decreased transiently in 10 eyes and visual function of all was preserved. VEPs were lost permanently in 2 eyes in two patients without new postoperative visual impairment.ConclusionsSatisfactory intraoperative VEP monitoring was feasible in all patients except in those with severe visual impairment. Preservation of VEPs predicted preserved visual function. During resection of lesions in the visual cortex, VEP monitoring could not detect new major visual field defects due to injury in the posterior visual pathway. Intraoperative VEPs were sensitive enough to detect vascular damage during aneurysm clipping and mechanical manipulation of the anterior visual pathway in an early reversible stage. Intraoperative VEP monitoring influenced surgical decisions in selected patients and proved to be a useful supplement to the toolbox of intraoperative neurophysiological monitoring.

33. Intraoperative visual evoked potential monitoring and its pitfalls

Introduction Visual evoked potentials (VEP) have been installed as a method for intraoperative visual function monitoring. However, there have been few reports regarding pitfalls of intraoperative VEP monitoring. To clarify this, relationships between intraoperative VEP waveform changes and postoperative visual function were analysed retrospectively. Material and methods Intraoperative VEP monitoring was performed in 123 surgeries, including three intraorbital, 96 parasellar and 24 cortical lesions in Shinshu University Hospital. Red flash light was provided to each eye independently. Supramaximal stimulation was found before starting surgical procedure and control VEP amplitude was measured. Decrease by 50% of control VEP amplitude was defined as a warning sign and reported to surgeons. The surgical procedure was ceased on the surgeon’s decisions. Intraoperative VEP monitoring and postoperative visual function were analysed. Results In 238 out of 246 eyes (97%), steady VEP monitoring was recorded. Transient VEP decrease was observed in 18 sides, but visual function was preserved. A permanent VEP decrease was seen in 18 sides, which resulted in visual impairment in 10 sides, and no visual aggravation in 8 sides postoperatively. The VEP amplitude was preserved greater than 50% in 200 of 202 sides, and visual function was preserved. In a patient with tuberculum sellae meningioma, visual acuity aggravated though VEP was maintained at 77% of control amplitude. In one side, visual acuity improved but minor visual field defect was encountered postoperatively, though VEP was unchanged throughout the surgery. Conclusions Intraoperative VEP monitoring predicts postoperative visual function: a 50% decrease of VEP amplitude can detect postoperative visual aggravation with a sensitivity of 99%. Pitfalls for intraoperative VEP monitoring are: (1) preoperative severe visual dysfunction, low amplitude of control VEP may interfere with intraoperative VEP monitoring in this method. (2) Visual field defect without decrease in the visual acuity may not be predicted by VEP monitoring. Attention should be paid to these pitfalls for reliable intraoperative VEP monitoring.

Usefulness of Intraoperative Monitoring of Visual Evoked Potentials in Transsphenoidal Surgery

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.

83. Intraoperative visual evoked potentials: A study in pituitary microadenoma surgery

Conflicting reports on the usefulness of intraoperative monitoring of visual function by means of visual evoked potentials (VEPs) are reported. In order to verify the real usefulness of this method in supporting pituitary surgery, we studied 10 patients with pituitary microadenoma without any visual problems to collect normative data. 10 patients (mean age 50 yrs), undergoing trans-sphenoidal surgery for pituitary microadenomas, were studied with intraoperative VEP monitoring. All patients had a normal pre- and postoperative visual function. ERG responses were recorded in 5 patients, mean latency = 54,11 ms (±3.98 DS). Intraoperative VEPs were recorded in 18 of 20 sides (90%). The variability of N2 latency did not exceed 10% of the baseline value (before surgical manoeuvres, mean = 140.95 ms ± 18.88 SD) and the P2–N2 amplitude variability was less than 50% in all but one patient. Our study confirms that generally VEPs remain stable during microsurgical procedures. Intraoperative VEP recording might be a potentially useful tool in the monitoring of the functional integrity of the posterior visual pathway.

Intraoperative visual evoked potential has no association with postoperative visual outcomes in transsphenoidal surgery

The aim of this study was to elucidate the relationship between changes in the intraoperative visual evoked potential (VEP) waveform and postoperative visual functional outcomes. Between February 2009 and December 2010, we performed endoscopic endonasal transsphenoidal surgery for sellar or perisellar lesions in 65 consecutive patients with intraoperative VEP monitoring using scalp electrodes under total venous anesthesia. Among the 65 patients, 53 patients were followed-up with postoperative visual function evaluation. VEP waveforms measured at baseline were compared with those obtained toward the end of surgery and the association between changes in VEP waveforms and visual outcomes measured preoperatively and postoperatively were assessed. Reproducible waveforms were obtained intraoperatively in 95 of 106 eyes (89.6%). Of the 95 eyes with reproducible VEP, 64 eyes had stable VEP during the surgery, 19 eyes showed VEP improvement, and 12 eyes had VEP deterioration. Of 64 eyes with a stable VEP, 42 showed no change in visual acuity postoperatively, 13 manifested improvement, and 9 worsened. Of 19 eyes with intraoperative VEP improvement, 13 exhibited no change, 4 improved, and 2 worsened postoperatively. Among 12 eyes with VEP deterioration, just 2 eyes showed visual worsening while the other 10 did not change or improved. Postoperative visual evaluation revealed no light perception in 2 eyes whose intraoperative VEP waveforms were stable throughout the surgery. Intraoperative monitoring of VEP with scalp electrodes under total venous anesthesia had a reproducibility of 89.6% during transsphenoidal surgery for sellar or perisellar lesions. However, the intraoperative VEP waveforms showed no association with postoperative visual outcomes.