Efficacy of the visual evoked potential monitoring in endoscopic transnasal transsphenoidal surgery as a real-time visual function
Keywords: Intraoperative electrophysiological monitoring, visual evoked potential, visual acuity, visual field
Neurosurgical procedures for treatment of tumors or vascular lesions along the visual pathway carry a risk of visual dysfunction. This applies not only to parasellar tumors and aneurysms but also to temporal and occipital lobe tumors and intraorbital lesions. An already impaired visual pathway is particularly at risk in patients with parasellar lesions, such as pituitary tumors, tuberculum sellae meningiomas, craniopharyngiomas, and internal carotid paraclinoid aneurysms. Postoperative visual impairment is a larger concern in patients with preserved visual function. A reliable method for real-time visual function monitoring would assist in intraoperative decision-making regarding radical excision and intermittent maneuvers near the optic apparatus. It is difficult to obtain a stable visual evoked potential (VEP) in real-time intraoperative monitoring because of interruption by anesthesia and due to insufficient and unstable stimuli delivery., However, VEP has recently been implemented in intraoperative visual function monitoring.,,,,,,,,,,,,,,, Due to contradictory reports on the usefulness of intraoperative monitoring of visual function by VEP, a consensus has yet to be reached, and it remains unclear whether intraoperative VEP monitoring can be used in real-time visual function monitoring with satisfactory results and sensitivity. In the present study, VEP was examined in surgeries associated with a high risk of optic apparatus damage. The relationships between VEP waveform changes and postoperative visual function were examined.
Between 2009 and 2016, intraoperative VEP monitoring was performed in 164 eyes of 82 patients (40 males and 42 females) at our institute. The clinical application of intraoperative VEP monitoring was approved by the Ethical Committee of Nara Medical University, Nara, Japan and informed consent was obtained from the patients and their families. Flash VEP was recorded with a MEE-1232 evoked potential measuring system (Nihon Kohden Corporation, Tokyo, Japan). All patients were maintained under total intravenous anesthesia (TIVA). Silicon disks with 16 red light (100 mcd) emitting diodes (LEDs) [Unique Medical Co. Ltd, Tokyo, Japan] were placed over the patients' closed eyelids bilaterally [Figure 1]. The flashing light intensity of the diodes was adjusted to a light intensity between 500 and 20,000 lx in the silicon disk. The averaged VEP waveform was obtained after each eye was stimulated separately. The stimulus average was one flash (10–20 ms) per se cond, and 50–100 flashes were recorded to obtain each averaged VEP waveform. Recording was performed in a montage of five channels, with electrodes placed on the earlobes bilaterally (A+), on the left occiput (LT and LO), the occipital midline (Oz), and the right occiput (RO and RT). The bandpass filter was set from 1 to 100 Hz. A small negative potential and a large positive potential around 200 ms were recorded, and the amplitude of the VEP was defined as the voltage difference between these two potentials. Needle electrodes were inserted subcutaneously at the lateral canthi for electroretinogram (ERG) recording. The ERG was recorded simultaneously with VEP to guarantee delivery of adequate flash stimuli to the retina. The VEP was recorded first after completion of patient setup. At least two consecutive ERG and VEP recordings were performed to confirm the reproducibility of the ERG and VEP waveforms after setup prior to commencement of the surgical procedure. The VEP was then recorded at each step during the surgical procedure. The maximum negative wave of VEP around 75 ms was defined as N75, and the maximum positive wave around 100 ms was defined as P100. The VEP amplitude was defined as the distance between N75 and P100. The amplitude that decreased over 50% from the control level was defined as a significant decrease. When a significant VEP amplitude decrease was recorded twice, the VEP amplitude decrease was considered to be persistent and not transient. The examiner promptly advised the operator if a significant VEP change was observed, and the surgical procedure was ceased until the VEP recovered based upon the operator's decisions. Intraoperative VEP changes were assessed and evaluated in comparison to postoperative visual function changes. Visual function was evaluated via preoperative and postoperative visual acuity and visual field examination, as diagnosed by the ophthalmologists. Changes in preoperative and postoperative visual acuity and visual fields were also evaluated. When either or both were improved, unchanged, or worse, the postoperative visual function outcome was considered improved, unchanged, or worse, respectively.
Intraoperative VEP was recorded in 160 of 164 eyes (98%). Preoperative visual acuity ranged from 0.01 to 1.5. VEP was not recorded in four eyes of two patients who had a visual acuity of less than and equal to 0.05. The minimum visual acuity in the patients for whom a stable VEP was recorded was 0.4. Stable VEPs were acquired in 160 eyes throughout the surgeries, and no complications related to VEP monitoring were encountered. The results of VEP changes and visual outcome are shown in [Table 1].
In the unchanged VEP amplitude group, 62 (50%) eyes showed improved postoperative visual function and 62 (50%) eyes had an unchanged visual function in the postoperative state. When the VEP amplitude decreased significantly, the operator paused the tumor removal procedure for several minutes. After confirming restoration of the VEP amplitude or maintenance of VEP decline of 50% or less, the operator restarted the tumor removal procedure. Among patients with decreased and restored VEP amplitude, eight (31%) eyes had improved postoperative visual function and 18 (69%) eyes had unchanged visual function in the postoperative state. In the group where VEP amplitude decreased and was maintained at a decline of 50% or less, all the examinerd eyes had unchanged visual function in the postoperative state.
Patient 1 was a 54-year old woman with sudden-onset bitemporal hemianopsia. Her head plain computed tomography (CT) showed a suprasellar mass lesion with bleeding [Figure 2]a, and the mass lesion had a high signal intensity on a brain magnetic resonance (MRI) T1-weighted image (T1WI) [Figure 2]b and a low signal intensity on T2-weighted image (T2WI) [Figure 2]c. The lesion was homogenously enhanced on the gadolinium-enhanced MRI (Gd-MRI) [Figure 2]d,[Figure 2]e,[Figure 2]f. Endoscopic transnasal transsphenoidal surgery was performed. During the tumor removal maneuver near the left optic nerve, the VEP decreased temporarily [Figure 3]a. However, the VEP recovered 5 min after stopping of the maneuver near the left optic nerve [Figure 3]b. The VEP amplitude was maintained after the maneuver. The pituitary tumor was completely removed [Figure 4]a,[Figure 4]b,[Figure 4]c, and visual acuity and disturbance improved after surgery [Figure 4]d and [Figure 4]e.
Patient 2 was a 53-year old woman with bitemporal hemianopsia. A Gd-MRI showed a large pituitary tumor compressing the chiasma and the right temporal lobe [Figure 5]a,[Figure 5]b,[Figure 5]c. Endoscopic transnasal transsphenoidal surgery was performed. During the tumor removal maneuver, the VEP amplitude decreased gradually, and was maintained at a level of 50% reduction or less. The sudden VEP change recovered after release of the occlusion, at which point, removal of the tumor ceased. The patient did not show any visual impairment, and improved postoperatively [Figure 5]d,[Figure 5]e,[Figure 5]f.
Feasibility of VEP for intraoperative real-time monitoring
Intraoperative flash VEP monitoring has been considered useful.,,,,,,,,,,,,, However, it can sometimes be unreliable.,,,,,, VEP has been criticized for not being “stable.” Some authors have reported considerable intra- and interindividual variability in the interpretation of VEP recordings, thus concluding that intraoperative VEP monitoring in surgically anesthetized patients is not reliable., Conversely, under total intravenous anesthesia (TIVA), the amplitude of the VEP is larger and less variable in amplitude and latency.,, In our study, some degree of VEP changes were observed in patients under TIVA, but stable waveforms were recorded even with longer operating times. In comparing waveform changes before and after a given surgical procedure, it is reasonable and acceptable that VEP changes during the procedure may be due to the procedure itself. It is necessary to suspend the procedure to observe chronological VEP changes. In our study, VEP amplitude gradually decreased in eight eyes during surgery, and all patients showed unchanged visual acuity postoperatively. In contrast, in 26 eyes with transient VEP decrease and restoration during surgery, eight (31%) showed visual acuity improvement after surgery, and 18 (69%) were unchanged. No patients showed postoperative worsening of visual acuity in our study. With our method, one VEP recording required only 20 s of time (interstimulation interval 200 ms × 100 times). A sudden VEP change during a particular surgical step seems to indicate visual impairment, and intraoperative flash VEP monitoring is feasible for real-time visual function monitoring. A VEP recording method with greater temporal resolution may be able to detect reversible VEP changes, even in patients with visual impairment.
Stable stimulation method
Stimulation delivery is another critical point for stable intraoperative VEP monitoring. In this study, we used goggles that incorporated red light emitting diodes (LEDs) capable of withstanding sterilization. Supramaximal stimulation intensity on VEP was measured initially and stably maintained in all patients using these goggles. Development of lighter and more durable goggles and a careful goggle setup resulted in a stable delivery of stimuli, which had been a major obstacle for intraoperative VEP monitoring.
We performed ERG recording simultaneously with VEP to guarantee delivery of adequate flash stimuli to the retina. This method was superior, in the point that this method decreases false negative data, to that performed in a study that concluded that VEP is not useful in endonasal surgery.
Mechanism of VEP change
Damage to the visual pathway during surgical procedures can be divided into three types: (1) ischemic injury due to occlusion of the arteries supplying the optic apparatus, such as transient artery occlusion in aneurysm clipping surgery; (2) mechanical damage to the optic apparatus by dissection and/or retraction of the visual pathway, such as the optic nerve, chiasm, and tract; and, (3) a combination of (1) and (2).
The temporal VEP decrease that occurred while operating on Patient 1 was considered to be due to temporary mechanical damage to the optic nerve and/or chiasm. Intraoperative VEP monitoring detected changes within a few minutes after the onset of mechanical damage. In Patient 2, the VEP decrease seemed to reflect temporal mechanical damage of the optic radiation and/or the optic nerve and/or chiasm.
It is difficult to determine which type of damage in the visual pathway is related to changes in VEP. Therefore, it is important to evaluate and confirm the presence of VEP changes at each step in the surgical procedure to prevent postoperative visual complications.
We hypothesized that a transient VEP decrease may be indicative of optic nerve injury and that recovery or stability of the VEP amplitude may be indicative of an intact optic nerve. However, the outcomes of the patients whose VEP amplitudes recovered after an initial decrease but showed no disappearance, were unchanged, not worse.
Limitation of intraoperative VEP for real-time monitoring
Postoperative hemianopsia could be detected as sudden VEP changes, but quadrantanopia could not be detected on intraoperative VEP monitoring in this study. In addition, there is no index for the improvement of visual acuity observed postoperatively. Therefore, VEP can disclose visual acuity impairment with greater sensitivity than partial visual field defects, which do not affect acuity. Sudden VEP changes can disclose hemianopsia caused by ischemia, but it may be difficult to detect minor visual field defects resulting from gradual manipulation of the optic radiation.
VEP can be steadily monitored in patients with corrected visual acuity greater than 0.1. Permanent VEP loss may indicate severe visual dysfunctions postoperatively. Transient VEP changes do not indicate postoperative visual disturbance. Visual field defects without decreases in visual acuity may not be predicted by VEP monitoring.
Declaration of patient consent
The authors certify that they have obtained all appropriate patient consent forms. In the form the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]