A 26-year-old presenting male had developed TLE seizure 10 years previously. His seizure started with an epigastric rising sensation, followed by unresponsiveness, and rarely evolving to generalization. Despite taking antiepileptic medications, focal-onset seizure with impaired awareness continued a few times per week. The findings of a neurological examination were normal. Brain magnetic resonance imaging (MRI) revealed a right medial temporal tumor at the parahippocampal gyrus (Fig. 1A, B). During a 7-day stay at an epilepsy monitoring unit, frequent interictal spikes and 12 automotor seizures were documented, and ictal subtraction single photon emission computerized tomography demonstrated that the seizures originated from the right mesial TLE lesion. Right AMTR along with tumor removal was decided after performing functional evaluations including 18F-FDG PET, neuropsychological tests, and the Wada test.

Under total intravenous anesthesia (TIVA) induced using propofol, remifentanil, and vecuronium, multiple modalities of INM were performed during AMTR, including transcranial electrical MEPs, median and posterior tibial nerve SSEPs, and flash VEPs. For MEPs, five repetitive pulses of transhemispheric constant-voltage stimulation at 400 volts were applied at the C3 and C4 positions, which evoked polyphasic compound muscle action potentials in the contralateral arm and leg muscles (abductor pollicis brevis, abductor digiti quinti, tibialis anterior, and abductor hallucis). For SSEPs, electrical stimulation applied to the median nerve at the wrist and to the posterior tibial nerve at the ankle evoked P14 waves in the C5S area, and N20 waves at the C3’, Cz’, and C4’ electrodes when using the following parameters: 20 mA (upper extremities) or 30 mA (lower extremities), 2.9 Hz, and 50 averages. For VEPs, light-emitting diode (LED) stimulation evoked P100 waves in the left occipital (LO), middle occipital (MO), and right occipital (RO) regions according to the Queen square system.

The patient underwent right AMTR and total removal of the tumor after frontotemporal craniotomy (Fig. 1C), during which the baseline MEPs, SSEPs, and VEPs could be reliably measured (Fig. 2A-C). Electrocautery was applied to bleeding that occurred during the dissection of the mesial temporal area around the tumor. After applying the electrocoagulation there was a -75% amplitude reduction of N20 waves in the SSEPs of the left median nerve, and the P100 waves became poorly formed in RO recordings of the left and right VEPs (Fig. 3). The anesthesia level and MEPs remained stable. Interventions including increasing the blood pressure and removing a surgical retractor did not improve these changes, which persisted until monitoring was completed (Fig. 2D-F). The pathology was confirmed to be anaplastic epithelioid pleomorphic xanthoastrocytoma.

A postoperative examination revealed left temporal upper quadrantanopia and left hemisensory deficits. Brain MRI showed two ischemic lesions in the lateral part of the right thalamus and the mesial area of the RO lobe, especially in the lingual gyrus. Brain magnetic resonance angiography (MRA) demonstrated occlusion of the P1 segment of the right posterior cerebral artery (PCA), indicating right PCA infarction (Fig. 1D-F).

AMTR has been acknowledged as a very effective and safe treatment for intractable TLE, but serious neurological complications are still possible.3 These complications can be directly related to the surgery (traction, vasospasm, edema, hemorrhage, coagulation, direct trauma, and infection) or be long-term neuropsychological effects. Even though the rate of serious complications is less than 3%, the conditions are very harmful and disabling.4 However, INM has been rarely reported in epilepsy surgery. The decreased amplitude of N20 waves with normal P14 waves in SSEPs found in the present case suggest a lesion in the right somatosensory pathway above the cervical cord level. Considering that electrocautery was applied in the right mesial temporal areas, a new lesion in the ventroposterolateral (VPL) nucleus of the right thalamus could be suspected, since this is supplied by the thalamogeniculate artery from the P2 portion of the PCA. The lesion indicated infarction, because the SSEPs changes were strongly correlated with the regional cerebral blood flow and ischemia, predicting neurological deficits.5,6 Meanwhile, the poor formation of P100 waves in LO recordings of the left and right VEPs suggested the occurrence of a new retrochiasmal lesion along the visual pathway including the lateral geniculate body and occipital cortex, which are supplied by the lateral branch of the posterior choroidal artery and the PCA. Postoperative brain MRI demonstrated two ischemic lesions: 1) a right lingual gyral lesion with polar sparing, indicating a calcarine arterial infarction originating from the P4 portion of the PCA, and 2) a right lateral thalamic lesion, indicating thalamogeniculate artery infarction. Considering the location of bleeding and the use of cautery, the mechanism of the infarction could have been injury-related thromboembolism.

VEPs monitoring has recently overcome the limitations of near-field potentials in INM with the introduction of TIVA and LEDs, which has improved the reproducibility of the formation of P100 waves during aneurysmal or parasellar surgery around the anterior visual pathway.7,8 In the present case we monitored VEPs during AMTR of TLE around the posterior visual pathway, which detected the calcarine artery infarction. Therefore, this case provides evidence of the usefulness of VEPs monitoring for lesions in the posterior visual pathway and the anterior visual pathway.

The hippocampal region has long been considered a risky area for surgery due to its highly vascularized structure. The hippocampus is supplied mainly from the PCA via anterior, middle, and posterior hippocampal arteries, which are highly vascularized through dense connections in lateral terminal branches and anastomosis of the uncal branch of the anterior choroidal artery (AChoA) in the uncal sulcus.9 Moreover, a tumorous lesion that is tightly attached to the surrounding structure can easily bleed during surgery. To prevent such damage, MEPs monitoring can be used to detect the AChoA suppling the internal capsule in the pyramidal pathway. SSEPs monitoring can be applied to the PCA supplying the lateral thalamus and the VPL nucleus in the somatosensory pathway, and VEPs monitoring can be applied to the posterior choroidal artery supplying the lateral geniculate body, the calcarine artery supplying the lingual gyrus, and the parieto-occipital artery supplying the cuneus. Therefore, INM for AMTR should include SSEPs, VEPs, and MEPs.

This is the first reported case of thalamic and medial occipital infarction being detected by monitoring SSEPs and VEPs during AMTR. This case supports the usefulness of SSEPs and VEPs for detecting thalamic and occipital damage during AMTR.