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NI FEATURE: THE EDITORIAL DEBATE III-- PROS AND CONS
Year : 2018  |  Volume : 66  |  Issue : 4  |  Page : 960-961

Visual evoked potentials during surgery of the sellar-suprasellar region: The unaddressed issues and the vital caveats


Department of Neurosurgery, Central India Institute of Medical Sciences, Nagpur, Maharashtra, India

Date of Web Publication18-Jul-2018

Correspondence Address:
Dr. Lokendra Singh
Department of Neurosurgery, Central India Institute of Medical Sciences, Nagpur, Maharashtra
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0028-3886.237009

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How to cite this article:
Singh L. Visual evoked potentials during surgery of the sellar-suprasellar region: The unaddressed issues and the vital caveats. Neurol India 2018;66:960-1

How to cite this URL:
Singh L. Visual evoked potentials during surgery of the sellar-suprasellar region: The unaddressed issues and the vital caveats. Neurol India [serial online] 2018 [cited 2018 Aug 17];66:960-1. Available from: http://www.neurologyindia.com/text.asp?2018/66/4/960/237009




Surgery around the sellar region carries an inherent risk to the visual pathways. This risk is more apparent while approaching sellar-suprasellar tumors utilizing the trans-nasal trans-sphenoid approaches as the optic fibers come into the operative view much later on during the course of surgery, as compared to their early visualisation in the transcranial approaches.[1] The risks involved in endoscopic sinus surgeries to the visual pathways is a well- recognized complication.[2]

Many workers have tried to evaluate the usefulness of visual evoked potentials (VEPs) during surgery since the last many decades with an aim to safeguard vision. The results have been very confusing and oscillating from being useful to being not-so-useful. A few workers have recommended the intra-operative use of VEPs with a lot of confidence.[3]

VEPs provide information regarding abnormal conduction in the visual pathway. An increase in the retino-striate conduction time due to manipulation of the optic pathway that may represent the phenomenon of demylination can be detected by measuring the latency of this cortical response. Abnormalities in the amplitude and waveform of the VEPs may also be caused by the loss of axons in the pathway. The sensitivity of VEPs depends upon the technique used to evoke them. The waverform is also fairly non-specific and any prexisting condition that might impair conduction in the retino-striate pathway is likely to give rise to abnormalities in the latency, amplitude, or waveform of the VEPs. Thus, a number of studies have also shown that the changes in waveform may not be specific to the operative manipulation, may be influenced by any pre-existing abnormality in the optic pathway and may also be influenced by the type of stimulation given to elicit the waveform.[4]

Actually, the use of intra-operative VEPs has been plagued with many problems since the beginning. These have been related from hardware to software issues, the type of anaesthesia, the type of visual evoked potentials used, i.e., the steady-state or the transient ones. Intra-operative conditions like extreme ranges of blood pressure, raised intra-cranial pressure, presence of haematoma in the operative field, and even the use of a bipolar cautery, may influence results.[5] The use of TIVA (total intravenous anaesthesia) has ensured that there is not much difference in the pre-anaesthetic and post-anaesthetic latency and amplitude of visual evoked potentials. The steady-state visual evoked potential recordings are much more stable as compared to the transient ones.[5] The hardware used for generating a visual stimulus for VEPs has also undergone significant changes over time.

With the utilization of intraoperative VEP over a prolonged period of time, several caveats have emerged. The first of them is that VEP is useful even in patients with a mildly decreased vision although it is not useful in situ ations where the patient is suffering from severe visual loss. VEP is more sensitive in predicting loss of visual acuity as compared to loss of visual field. It is more insensitive in predicting quadrantanopias as compared to hemianopsias. The damage to visual pathways can be due to: (a) vascular compromise; and (b) mechanical damage during dissection of the tumor. The immediate but transient absence of VEPs is very common while dissecting a tumor or during the temporary clipping while performing aneurysm surgery. Thus, it is very useful to the surgeon as a safety guide as the VEPs become abnormal during the recoverable time window. The mechanical manipulation of visual fibres during sellar-suprasellar tumour surgeries is invariably seen in most of the cases but VEPs usually return to a normal pattern within 5 minutes of stopping the dissection. If the wave forms do not return to a normal form within this period then the event may be associated with post-operative deterioration of vision. The absence of any decrease in the amplitude of VEPs ensures that any significant deterioration in the post-operative visual functions usually is not anticipated in the postoperative period.

The present study by Nishimura et al., from Nara Medical University, Nara promises to establish the usefulness of intra-operative VEPs.[6] In a total of 82 patients studied (that included164 eyes), the authors have used total intravenous anaesthesia (TIVA), thus avoiding the lack of, or the abnormalities of signals associated with halothane and nitrous oxide. They have also taken care of many of the problems commonly associated with the visual stimulation devices by designing very light-weight goggles that used red light flash independent stimulation for each eye. However, they have not considered other intra-operative factors which can affect the VEP waveforms like the occurrence of severe hypo- and hypertension; the sudden increases in intracranial pressure due to intrathecal injection of saline to facilitate the tumour resection; the presence of blood clots in the operative field; and, the use of a bipolar cautery while simultaneously eliciting VEPs. It is possible that they have taken care of these factors while recording the waveform but have not mentioned them in the text. It is, therefore, essential for the personal recording intraoperative VEPs to be aware of all these factors, which may influence VEP recordings.

The pathogenesis of how operative manipulation of the visual tract affects the VEP waveforms that are being recorded during surgery in real-time, as well as the temporal sequence of changes that take place, are not equivocally known. However, there is a saying that 'the highest form of ignorance is to reject something you know nothing about.' As more of the workers try to test the efficacy of intra-operative VEP waveform recordings, with improved hardware and software, and with a better understanding of the subject, it seems that in not- so-distant future, intraoperative VEP recordings will emerge as a very well accepted technique. At the present stage of knowledge, however, the technique still needs to establish its usefulness beyond any reasonable doubt.



 
  References Top

1.
Bhatoe HS. Evolution of concepts in the management of craniopharyngiomas: Lessons learnt from Prof. S.N. Bhagwati's article published in 1993. Neurol India 2018;66:613-36.  Back to cited text no. 1
    
2.
Sharma BS, Sawarkar DP. Changing trends in surgery for suprasellar lesions. Neurol India 2018;66:4-8.  Back to cited text no. 2
[PUBMED]  [Full text]  
3.
Kamio Y, Sakai N, Sameshima T, Takahashi G, Koizumi S, Sugiyama K, et al. Usefulness of Intraoperative Monitoring of Visual Evoked Potentials in Transsphenoidal Surgery. Neurologia Medico-Chirurgica 2014;54:606-11.  Back to cited text no. 3
    
4.
Walsh P, Kane N, Butler S. The clinical role of evoked potentials. J Neurol, Neurosurg, Psychiatry 2005;76:ii16-ii22.  Back to cited text no. 4
    
5.
Sharika R, Mirela VS, Dinesh GN. Intraoperative visual evoked potentials: There is more to it than meets the eye. J Neurol Neurosci 2016;7:2-11.  Back to cited text no. 5
    
6.
Nishimura 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;66:1075-80.  Back to cited text no. 6
  [Full text]  




 

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