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NI FEATURE: THE EDITORIAL DEBATE III-- PROS AND CONS
Year : 2018  |  Volume : 66  |  Issue : 4  |  Page : 958--959

Visual evoked potentials for visual function monitoring during endoscopic sphenoidal surgery: Advancement and challenges

Parthasarathy D Thirumala 
 Department of Neurological Surgery and Neurology, Center for Clinical Neurophysiology University Presbyterian Medical Centre, Pittsburgh PA, USA

Correspondence Address:
Dr. Parthasarathy D Thirumala
Department of Neurological Surgery and Neurology, Center for Clinical Neurophysiology, University Presbyterian Medical Centre, Presbyterian Suite B-400, 200 Lothrop Street, Pittsburgh PA 15213
USA




How to cite this article:
Thirumala PD. Visual evoked potentials for visual function monitoring during endoscopic sphenoidal surgery: Advancement and challenges.Neurol India 2018;66:958-959


How to cite this URL:
Thirumala PD. Visual evoked potentials for visual function monitoring during endoscopic sphenoidal surgery: Advancement and challenges. Neurol India [serial online] 2018 [cited 2020 Aug 14 ];66:958-959
Available from: http://www.neurologyindia.com/text.asp?2018/66/4/958/237010


Full Text



Nishimura et al., describe the utility of visual evoked potential (VEP) monitoring during endoscopic transnasal transphenoidal surgery. In their series, transient decreases in VEP responses were noted but without any change in visual function. Further, permanent gradual decrease in VEP responses were not associated with any change in visual function. The authors concluded that intraoperative VEP should be performed in all skull base surgeries.[1]

Worsening of visual function after endoscopic sphenoidal surgery is an undesirable outcome in the surgical removal of skull base tumors. New perioperative worsening of visual function can be secondary to direct damage of the optic nerves and/or injury to the vascular supply to the superior hypophyseal arteries supplying the chiasm and pituitary stalk.[2] Minimally invasive approaches like endoscopic approaches to skull base surgery can balance the goal between gross total resection and sparing the neurovascular structures.

VEPs are electrophysiological responses recorded in the visual cortex secondary to photic stimulation of the retina. They can provide an assessment of visual pathways from the retina, the optic tracts, the optic radiation and finally to the primary visual cortex. Multiple reports have indeed suggested that VEP monitoring can be used to preserve visual function during surgical removal of tumors near the optic tracts, pituitary tumors, chiasmal gliomas, tumors of visual cortex and clipping of ophthalmic aneurysm.[3] However, VEP monitoring during the surgical procedures is not widespread due to various challenges described below.

Conventional VEPs in the diagnostic laboratory utilize pattern shift evoked light stimulation, which provides maximal activation of the retinal fibers, resulting in a robust response. Intraoperatively, this cannot be achieved as the eyes are closed. Technological advancements have resulted in the use of flash stimuli by light emitting diodes (LEDs),[4] as utilized by the current team. Another challenge during the surgical procedure is the use of anesthetic medication. Inhalational anesthetic agents, due to their action on the synaptic connections, cause a significant decrease in the amplitude of the VEP response. Utilization of total intravenous anesthesia (TIVA) has overcome the limitation in obtaining reliable VEP responses during the entire procedure.

The criteria of significant change in VEPs in the article was “a decrease in the amplitude of VEP response over 50%”. Unfortunately, it is unclear if this is the correct alarm criteria that might be utilized to indicate a significant change in neurological status. The American Clinical Neurophysiological Society (ACNS) suggest the use of a 50% decrease in amplitude and/or 10% increase in response of an evoked potential as a potential alarm for a significant change.[5] They further suggest that this is an empirical value and new perioperative neurological deficits can occur in patients without any changes being discernable in the VEPs. To be clear, the alarm criteria suggested by ACNS does have some support in the literature in evoked potential studies done in primates.[6] In our personal experience of monitoring over 30,000 procedures involving evoked potentials, we rarely see changes in the latency of a response. Hence, it is very important to be cautions in utilizing the alarm criteria that may be relevant for other modalities of electrophysiological monitoring, for use in VEPs.[6],[7]

I applaud Dr Nishimura and his team for evaluating the use of VEPs during endoscopic sphenoidal surgery. It represents a significant advancement and will encourage more and more surgical teams to utilize VEPs to monitor visual pathways during surgical procedures. The advances in anesthetic medications and in the delivery of the visual stimuli have improved the collection of consistent VEP responses during surgery. We should proceed with caution, however, as the criteria for determining significant changes in VEP response which might be indicative of damage to the visual pathways have never been validated. The current publication is faced with a similar challenge, there were no patients with worsening of visual function. More importantly, no patients with permanent decrease in VEPs had any change in visual function. In summary, before advocating the widespread use of VEPs, further research needs to be performed. Perhaps an international consortium of skull base surgeons can attempt to evaluate this comprehensively by performing independent visual testing before and after the procedure, using a standard anesthesia regimen, a uniform protocol of obtaining VEPs, and finally, correlating the changes to post-operative outcomes.

References

1Nishimura F, Wajima D, Park YS, Motoyama Y, Nakagawa I, Yamada S, et al. Efficacy of the visual evoked potential monitoring in endoscopic transnasal transsphenoidal surgery as a real-time visual function. Neurol India 2018;2018;66:1075-80.
2Stefko ST, Snyderman C, Fernandez-Miranda J, Tyler-Kabara E, Wang E, Bodily L, et al. Visual outcomes after endoscopic endonasal approach for craniopharyngioma: The Pittsburgh experience. J Neurol Surg B Skull Base 2016;77:326-32.
3Thirumala PD, Habeych ME, Crammond DJ, Balzer JR. Neurophysiologic intraoperative monitoring of olfactory and optic nerves. J Clin Neurophysiol 2011;28:538-42.
4Sasaki T, Itakura T, Suzuki K, Kasuya H, Munakata R, Muramatsu H, et al. Intraoperative monitoring of visual evoked potential: introduction of a clinically useful method. J Neurosurg 2010;112:273-84.
5American Clinical Neurophysiology Society. Guideline 9D: Guidelines on short-latency somatosensory evoked potentials. J Clin Neurophysio 2006;23:168-79.
6Symon L. The relationship between CBF, evoked potentials and the clinical features in cerebral ischaemia. Acta Neurol Scand Suppl 1980;78:175-90.
7Thirumala PD, Huang J, Brahme IS, Thiagarajan K, Cheng H, Crammond DJ, Balzer J. Alarm criteria for motor evoked potentials. Neurol India 2017;65:708-15.