Tetsuro Takao

Facial nerve motor-evoked potential monitoring during skull base surgery predicts facial nerve outcome

Objective: To determine whether monitoring facial nerve motor-evoked potentials (FNMEPs) elicited by transcranial electrical stimulation during skull base tumour surgery is useful for predicting facial nerve outcome. Methods: This study analysed FNMEP findings in 26 patients with skull base tumours. Corkscrew electrodes positioned at C3 or C4 and Cz were used to deliver supramaximal stimuli (180–550 V). FNMEPs were recorded from the orbicularis oculi and oris muscles. The correlation between the final-to-baseline FNMEP ratio and postoperative facial nerve function was examined. Results: Postoperative facial nerve function correlated significantly with the FNMEP ratios in the orbicularis oculi (p = 0.004) and orbicularis oris (p<0.001) muscles. An FNMEP ratio of <50% consistently predicted immediate postoperative facial palsy, although the degree of palsy differed among patients. All patients had satisfactory facial nerve function (House and Brackmann grades I and II) postoperatively if the FNMEP ratio remained at >50%. Conclusions: Intraoperative FNMEP monitoring can be useful for predicting facial nerve function after skull base surgery. This new method is a valuable adjunct to conventional facial nerve monitoring.

Short-term plasticity visualized with flavoprotein autofluorescence in the somatosensory cortex of anaesthetized rats

In the present study, short‐term plasticity of somatosensory neural responses was investigated using flavoprotein autofluorescence imaging in rats anaesthetized with urethane (1.5 g/kg, i.p.) Somatosensory neural activity was elicited by vibratory skin stimulation (50 Hz for 1 s) applied on the surface of the left plantar hindpaw. Changes in green autofluorescence (λ = 500–550 nm) in blue light (λ = 450–490 nm) were elicited in the right somatosensory cortex. The normalised maximal fluorescence responses (ΔF/F) was 2.0 ± 0.1% (n = 40). After tetanic cortical stimulation (TS), applied at a depth of 1.5–2.0 mm from the cortical surface, the responses elicited by peripheral stimulation were significantly potentiated in both peak amplitude and size of the responsive area (both P < 0.02; Wilcoxon signed rank test). This potentiation was clearly observed in the recording session started 5 min after the cessation of TS, and returned to the control level within 30 min. However, depression of the responses was observed after TS applied at a depth of 0.5 mm. TS‐induced changes in supragranular field potentials in cortical slices showed a similar dependence on the depth of the stimulated sites. When TS was applied on the ipsilateral somatosensory cortex, marked potentiation of the ipsilateral responses and slight potentiation of the contralateral responses to peripheral stimulation were observed after TS, suggesting the involvement of commissural fibers in the changes in the somatosensory brain maps. The present study clearly demonstrates that functional brain imaging using flavoprotein autofluorescence is a useful technique for investigating neural plasticity in vivo.

Ruptured intracranial aneurysm following gamma knife surgery for acoustic neuroma

SummaryA-63-year-old woman underwent gamma knife surgery (GKS) for acoustic neuroma. Six years later, she suffered sudden onset of severe headache followed by a disturbance of consciousness and subarachnoid haemorrhage due to a ruptured aneurysm originating from the distal anterior inferior cerebellar artery. The aneurysm was not located at a branching site and was included within the radiation field. The aneurysm was treated by endovascular embolization, and now, 15 months later, the patient has recovered satisfactorily. This is the first report of aneurysm formation following GKS for acoustic neuroma.

Clinical factors predicting outcomes after surgical resection for sporadic cerebellar hemangioblastomas.

OBJECTIVE To determine whether various clinical factors are related to long-term outcomes of patients with sporadic cerebellar hemangioblastomas. METHODS Subjects included 36 patients (19 men and 17 women) who underwent resection of sporadic hemangioblastoma in the cerebellum. Age at surgery ranged from 17-79 years (mean, 49.7 years). The tumor size, which was defined as the largest diameter of the lesion including the extratumoral cyst, ranged from 10-67 mm (mean, 36.4 mm). Obstructive hydrocephalus secondary to mass effect on the fourth ventricle was present in 21 (58.3%) patients preoperatively. RESULTS Total tumor removal was achieved in 31 of 36 patients (86%). In 4 (11%) patients with solid tumors, postoperative hematoma occurred in the removal cavity, and hematoma removal was required immediately after surgery. We followed 30 patients for >12 months after the initial surgery (mean, 72.9 months; range, 12-274 months). Recurrence of hemangioblastoma developed in 4 of 30 patients (13%) at 6 months, 17 months, 6 years, and 22 years after surgery. At the final follow-up examination, 9 (30%) of 30 patients showed some residual neurologic symptoms (poor group), whereas the remaining 21 patients showed no deficits (good group). Using univariate analysis, both age at surgery and tumor characteristics (cystic or solid) were significantly related to long-term patient outcomes (P < 0.05). However, in a multiple logistic regression analysis, only tumor characteristics were correlated with outcomes (P = 0.017). At the final follow-up examination, patients with solid tumors more frequently showed poor outcomes than patients with cystic tumors. CONCLUSIONS The solid configuration observed on preoperative images of sporadic cerebellar hemangioblastomas is one of the most important clinical factors related to both immediate and long-term outcomes after surgery.

Three-dimensional visualization of neurovascular compression: presurgical use of virtual endoscopy created from magnetic resonance imaging.

OBJECTIVE To assess the usefulness of presurgical simulation of microvascular decompression (MVD) by virtual endoscopy (VE), a new tool to analyze three-dimensionally reconstructed magnetic resonance data sets in patients with trigeminal neuralgia or hemifacial spasm (HFS). METHODS In 17 patients (10 with trigeminal neuralgia and seven with HFS) determined to be candidates for MVD, we performed presurgical simulation of MVD using VE. We used constructive interference in steady-state imaging and magnetic resonance angiography to obtain the original images. VE findings were compared with surgical findings. RESULTS The three-dimensional relations between visible structures seen on VE were consistent with intraoperative findings in all patients. In total, 20 (91%) of 22 neurovascular compression sites in all 17 patients were correctly delineated on VE, with the exception of two small branches identified as offending vessel in two patients with HFS. Perforators that were not apparent on VE limited our ability to accomplish transpositioning of the offending vessels as simulated. The positions of structures that can affect individual surgical approaches, such as the petrosal vein, cerebellar flocculus, and vertebral artery, were also adequately predicted on VE. All patients had excellent surgical outcomes. CONCLUSION Presurgical VE in patients with trigeminal neuralgia or HFS is a novel technique that provides excellent visualization of the three-dimensional relations between neurovascular structures and allows simulation of MVD.

Etiopathological factors related to hydrocephalus associated with vestibular schwannoma.

OBJECTIVEWe retrospectively analyzed various clinical factors to determine whether or not these factors are etiopathologically related to the development of hydrocephalus in patients with vestibular schwannomas. METHODSThere were 68 patients (29 men, 39 women) in this study who underwent resection of a vestibular schwannoma. The age at the time of surgery ranged from 19 to 76 years (mean age, 51.4 yr). The maximum diameter of the tumor in the cerebellopontine cistern ranged from 0 (localized within the internal auditory canal) to 56 mm (mean, 32.0 ± 12.9 mm). Cerebrospinal fluid (CSF) protein concentration in the cerebellomedullary cistern was measured intraoperatively in all patients. RESULTSSixteen (23.5%) of the 68 patients exhibited radiographic evidence of hydrocephalus. Univariate analysis of various factors revealed that both tumor size and CSF protein concentration were positively related to development of hydrocephalus (P < 0.05 and P < 0.01, respectively). However, in multiple logistic regression analysis, only the CSF protein concentration was predictive for development of hydrocephalus (P = 0.022). There was a trend toward increased CSF protein concentration in patients with a large tumor (≥40 mm) compared with those with a small tumor (<40 mm) (P = 0.06). CONCLUSIONA high CSF protein concentration in fluid from the cerebellomedullary cistern is one of the most important factors contributing to hydrocephalus associated with vestibular schwannoma. It is important to judge whether or not any further treatment is required for hydrocephalus, in addition to tumor resection, especially in patients with communicating hydrocephalus.

Monitoring of abnormal muscle response and facial motor evoked potential during microvascular decompression for hemifacial spasm.

Background: To determine whether the monitoring of abnormal muscle response (AMR) and facial motor evoked potential (FMEP) during microvascular decompression (MVD) for hemifacial spasm (HFS) might be useful for predicting the postoperative clinical course and final outcomes. Methods: We analyzed 45 HFS patients who underwent both AMR and FMEP monitoring during MVD. Patients were divided into two groups on the basis of post-MVD disappearance (group AMR-A) or persistence (group AMR-B) of AMR. With regard to FMEPs, patients were classified into one of the two groups according to the ratio of the final to baseline FMEP amplitudes recorded for the orbicularis oculi muscle: one group with a ratio of <50% (group FMEP-A), and the other with a ratio of ≥50% (group FMEP-B). Results: Twenty-one of the 26 (81%) patients in group AMR-A were assigned to group FMEP-A, whereas 9 of the 17 (53%) patients in group AMR-B were assigned to FMEP-B (P < 0.05). In 38 of the 40 (95%) patients in whom the AMRs disappeared or persisted at amplitudes <50% that at the baseline, HFS had subsided at the final follow-up. Forty of the 42 (95%) patients whose FMEP amplitude ratios indicated reduction in the amplitudes from the baseline, had complete relief of the symptoms. Nineteen of the 20 (95%) patients whose AMRs disappeared after MVD experienced immediate relief of their symptoms after the operation. With regard to 14 of the 20 (70%) patients whose AMRs persisted at the final recordings, the symptoms of HFS improved over time and eventually subsided (P < 0.001). Conclusions: Intraoperative monitoring of both AMR and FMEP during MVD may be useful in predicting the postoperative outcomes in HFS patients. The AMR-related findings may help to predict whether HFS disappears immediately after surgery or some time later.

Importance of awareness of the rhomboid lip in microvascular decompression surgery for hemifacial spasm

OBJECT The authors adopted the infrafloccular approach for microvascular decompression (MVD) surgery to treat hemifacial spasm (HFS). The inferior portion of the flocculus is retracted to observe the root exit zone of cranial nerve (CN) VII between CN IX and the flocculus. During the procedure, the rhomboid lip, a sheetlike layer of neural tissue forming the lateral recess of the fourth ventricle, is sometimes encountered. The existence of the rhomboid lip in cases of HFS was reviewed to determine the importance of the structure during MVD surgery. METHODS Preoperative imaging and intraoperative observations in 34 consecutive cases of HFS treated in the period from October 2008 through September 2011 were used to assess the frequency of encountering the rhomboid lip. RESULTS The rhomboid lip was observed during MVD surgery in 9 (26.5%) of the 34 cases but had been demonstrated on preoperative MR images in only 3 cases (8.8%). On T2-weighted images, it appeared as a high-intensity nonstructural area on the ventral side of the flocculus and continued into the fourth ventricle via the foramen of Luschka. CONCLUSIONS A large rhomboid lip presents an impediment to MVD surgery in a significant minority of patients with HFS. It is seldom observed on preoperative MR images. Proper dissection of the rhomboid lip away from the arachnoid membrane and/or the lower CNs during MVD surgery provides good visualization of the root exit zone of CN VII and reduces injury of CNs IX and X, avoiding postoperative deficits like dysphagia.

Thrombosed giant aneurysm of the distal anterior cerebral artery treated with aneurysm resection and proximal pericallosal artery-callosomarginal artery end-to-end anastomosis: Case report and review of the literature.

Background: Giant distal anterior cerebral artery (DACA) aneurysms are extremely rare, with only 32 cases reported in the literature. Most giant DACA aneurysms have features that make standard neck clipping difficult, and bypass surgery is sometimes required, although this surgery was performed in only three reported cases. This report presents the fourth case treated with bypass surgery. Case Description: A 69-year-old female presented with an unruptured thrombosed giant DACA aneurysm. She underwent wrapping operation 7 years before, but radiological imaging revealed enlargement of the aneurysm at the left pericallosal artery (PerA)–callosomarginal artery (CMA) junction. Before operation, three different strategies were considered for bypass surgery in case the neck could not be clipped. Aneurysm resection and left proximal PerA–CMA end-to-end anastomosis were successfully performed under intraoperative digital subtraction angiography (DSA) and motor-evoked potential (MEP) monitoring. Conclusion: Most DACA aneurysms are located at the PerA–CMA junction. In some cases, adequate retrograde flow to the distal PerA from the posterior or middle cerebral artery can be expected, making distal PerA reconstruction unnecessary. Moreover, when the distal PerA is cut, proximal PerA–CMA end-to-end anastomosis can be easily performed because of reduced tension in both vessels. We therefore conclude that this strategy should be utilized for treating such patients. We also presented here the effectiveness of intraoperative modalities, such as intraoperative DSA and MEP monitoring, for performing a safe operation.

Novel devices for intraoperative monitoring of glossopharyngeal and vagus nerves during skull base surgery

Background: Swallowing disturbance is among the most burdensome complications suffered by patients with glossopharyngeal and vagus nerve involvement in lesions adjacent the jugular foramen. For monitoring these nerves, we have developed new devices that comprised four contacts adhering to the surface of the cuff of an endotracheal tube, with attachment the posterior pharyngeal wall. To determine whether these devices are useful for monitoring the glossopharyngeal and vagus nerves and predicting postoperative swallowing dysfunction in patients undergoing removal of skull base tumors involving these nerves. Methods: We studied 10 patients. Compound muscle action potentials (CMAPs) were recorded from the posterior pharyngeal wall by stimulating the glossopharyngeal or vagus nerve in order to identify the nerve course, especially in patients showing severe nerve distortion due to the tumor. Pharyngeal motor evoked potential (PhMEP) elicited by transcranial electrical stimulation were recorded in all patients. The correlation between the final to baseline PhMEP ratio and postoperative swallowing function was examined. Results: Responses were obtained in six of the seven patients in whom CMAP monitoring was performed. Deterioration of swallowing function postoperatively was demonstrated in six of seven (86%) patients with intraoperative PhMEP ratios <50%. None of the three patients with intraoperative PhMEP ratios >50% showed deterioration of swallowing function after surgery, although the one patient already had severe swallowing dysfunction requiring preoperative tracheostomy. Conclusions: Our novel devices were useful for monitoring the glossopharyngeal and vagus nerves in patients undergoing removal of skull base tumors involving these nerves.