Intraoperative monitoring of visual function

Abstract

Due to the excellent functional outcomes reported in thestudy published in this issue of Acta Neurochirurgica byBenedicic et al. [1], Intraoperative monitoring of the visualfunction using cortical potentials after electrical epiduralstimulation of the optic nerve, the authors have first of allto be congratulated.Indeed, they recorded the intraoperative visualevoked potential after electrical epidural stimulationof the optic nerve in 11 patients who underwentsurgical resection for a skull base meningioma. Theyobserved that P20 and N30 amplitude changes wererelated to the manipulation of ON during anterior skullbase tumor removal. Interestingly, there was noimmediate postoperative visual worsening, althoughthe rate of postsurgical visual deterioration is usuallyhigher in the literature, up to 10% in patients with nopresurgical deficit. It is nonetheless worth noting thatBenedicic et al. have already reported the stability andreproducibility of cortical potentials after electricalepidural stimulation of the ON. In addition, severalother authors have previously performed intraoperativemonitoring of visual function in patients with tumorsalong the visual pathways (from ON to optic radia-tion), using different techniques such as flash visualevoked potentials [2] or direct subcortical electrostimu-lation of the optic radiation in awake patients [3].Therefore, from a methodological point of view, no newdata are described here. The results presented byBenedicic et al. using cortical potentials should now bereproduced and validated by other teams.Furthermore, it is not really surprising to observe P20and N30 amplitude changes during manipulation of ON.Using the recording of ON-evoked potentials, Kikuchi et al.already noted attenuation of amplitude as a response to ONmanipulation [4]. Thus, the actual contribution of thepresent series would have been to demonstrate that thenew technique introduced by the authors was able to predictpostoperative deterioration. Unfortunately, the authorsfailed to identify any correlation between intrasurgicalchanges of cortical potentials and newly acquired postop-erative visual worsening. It would have been helpful tofocus more on this point. Indeed, the aim of oncologicalsurgery is to optimize the benefit-to-risk ratio of the surgicalprocedure, that is, to increase the extent of resection whilepreserving the quality of life. In the present study, nooncological considerations have been detailed: the extent oftumor removal and the long-term follow-up were notdescribed. Thus, because P20 and N30 amplitude changeswere related to the manipulation of ON or chiasma duringanterior skull base tumor removal, one could think that theauthors stopped the resection earlier. As a consequence, thequestion is to know whether it was justified to interrupt themeningiomal removal according to the results provided bythese online cortical potentials or whether the patientswould have been able to recover even if the authors hadpursued the resection in spite of the electrophysiologicalchanges.In summary, despite the rigorous methodology andthe favorable functional outcomes here described, on thebasis of this small series reporting a negative result (i.e.,no correlation between intrasurgical modifications ofcortical potentials and newly acquired postoperative