In 14 ventilated, normocapnic baboons anaesthetised with alpha-chloralose, local CBF (hydrogen clearance) and the amplitude and latency of local components of the somatosensory evoked potential (SEP, median nerve stimulation) were measured bilaterally in ventrobasal thalamus (VPL), medial lemniscus (ML), and cerebral cortex before and during progressive ischaemia, produced by occlusion of the right middle cerebral artery and subsequent controlled reductions in mean systemic blood pressure (MSBP). The first significant reduction from control of the left cortical SEP amplitude occurred in the range of 30-40 mm Hg MSBP, but those of the VPL and ML responses only below 30 mm Hg; in the range of 20-30 mm Hg, the average SEP amplitudes in cortex, VPL, and ML were 8.6, 72.6, and 90.7% of control, respectively. In terms of local CBF, the cortical SEP threshold was in the range of 15-20 ml/100 g/min (as in previous work), that of VPL in the range of 10-15 ml/100 g/min, but the ML response was only markedly reduced below 10 ml/100 g/min. Thus, the differential ischaemic sensitivity of the SEP between the three regions was clearly demonstrated. These results indicate that as one descends the neuraxis, there is an increasing resistance of electrophysiological function to systemic hypotension, together with a decreasing threshold for local ischaemia.
Somatosensory evoked potentials have been recorded during 34 operations for intracranial aneurysm. The central conduction time (CCT), the time between the N14 peak (recorded at C-2) and the N20 peak (recorded at the cortex) in response to median nerve stimulation, has been found to be increased by administration of halothane, by brain retraction, and by temporary vascular occlusion in some instances. Increase of CCT to more than 10 msec, or disappearance of the response was associated in two cases with postoperative neurological deficit, neither permanent. In a further case, prolongation of CCT was used as a guide to the repositioning of an imperfectly placed clip on a middle cerebral artery aneurysm. The value of the technique as a perioperative monitoring system is discussed.
Somatosensory evoked responses to median nerve stimulation have been recorded during the management of subarachnoid hemorrhage from intracranial aneurysms. The type of measurement was central conduction time (CCT), the time interval between the N14 peak (at C-2) and the N20 peak (at the cortex). Significant differences were found between patients with aneurysms in clinical Grade 4 and normal individuals, although there was no significant difference between most aneurysm patients in Grades 1 through 3. The prolongation of CCT was found of prognostic value. Significant prolongation (mean plus two standard deviations) from the normal period of 5.4 +/- 0.4 msec was found significantly useful to predict a less than satisfactory outcome (a patient with neurological signs or disabled) at 2 months, and these differences were detectable as early as 48 hours postoperatively. Differences between conduction time in the two hemispheres could also be used prognostically, although the significance of the differences did not appear until 48 to 72 hours after surgery, and was in any event less than that of CCT.
A 55-year-old male patient presented with a giant olfactory groove meningioma supplied by both ophthalmic arteries. The tumor was debulked through a bifrontal craniotomy, but the surgery was discontinued because of extensive blood loss. During a second operation, the dura of the frontal base was dissected extradurally before the tumor was removed. The meningeal blood supply was occluded with bipolar coagulation of the epidural meningeal vessels around the crista galli. The highly vascular tumor then was totally removed with considerably less blood loss. The patient's postoperative course was uneventftil.Because the ophthalmic artery cannot be embolized preoperatively, the direct extradural approach to the skull base to devascularize the blood supply around the olfactory groove may reduce blood loss and facilitate resection. KEYWORDS: Giant olfactory groove meningioma, ophthalmic artery, orbitofrontal craniotomy, extradural devascularization Among skull base meningiomas, olfactory was recommended by Hassler and Zentner.4 From groove meningiomas are relatively easy to access. Cushing and Eisenhardt5 to Samii and Ammirati,6 Most surgeons, like Ojemann and Swann' and the surgical mortality rate has decreased from 19 to Symon2, favor a bifrontal craniotomy. Al-Mefty3 0%. Occasionally, an olfactory groove meningioma added an orbital osteotomy to gain generous access is giant and highly vascular, and surgery becomes to the anterior cranial fossa. The pterional approach difficult. We report such an experience.
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