Anesthetic Challenges in the Management of Distal Posterior Cerebral Artery Aneurysm for Surgical Clipping: A Report of 2 Cases
Aneurysms of the distal posterior cerebral artery (PCA) are rare among intracranial aneurysms (1% to 2 % overall incidence) and occur in young patients.1 Options for management include surgical clipping, parent vessel occlusion and coil embolization, or occlusion of the parent artery with or without bypass.2,3 Limited literature exists on its anesthetic management and intraoperative monitoring for ischemia. Anticipated intraoperative problems include brain ischemia and monitoring for this ischemia as PCA supplies important areas of the brain. We discuss the role of intraoperative evoked potential monitoring in 2 cases of such aneurysms presenting for surgical clipping.
A 45-year-old man presented with sudden onset of occipital headache. He was evaluated as World Federation of Neurosurgical Society grade 1. A computed tomography angiogram showed a partially thrombosed wide-necked saccular aneurysm from the P2-P3 segment of the right PCA. The posterior communicating artery was hypoplastic bilaterally. After uneventful anesthesia, the patient was monitored with visual evoked potentials (VEP) and upper-limb somatosensory evoked potentials. The surgeon applied intermittent temporary clip electively in the P2 segment, which lasted for a total of 8 minutes. There was prolongation of VEP (P100 wave) intraoperatively after 4 minutes of temporary clip, which returned to normal after temporary clip release. (Fig. 1) Permanent clip application was uneventful, and the patient had an uneventful postoperative course.
A 44-year-old woman presented with subarachnoid hemorrhage. A computed tomography angiogram showed the P2 segment of the PCA aneurysm. The patient was scheduled for surgical clipping. Intraoperative VEP and motor evoked potential (MEP) were monitored. There were temporary clip applications (3 times) in P1, which were associated with a reduction in the amplitude (>50%) of MEP, which improved after the removal of the temporary clip. (Fig. 2) There was no change in the VEP. The permanent clipping was uneventful. Postoperatively, the patient did not have neurological deficits.
The PCA supplies important structures, namely the thalamus, the occipital lobe, and the midbrain and can be subjected to ischemia during the above procedures. The PCA arises from the basilar artery, and the short segment from origin to posterior communicating anastomosis is named as P1 and P2 segments. Thereafter, it branches into medial (supplies the parieto occipital lobe) and lateral arteries (supplies temporal lobe), where it forms the P3 segment, and the terminal portion is called the P4 segment. Perforators to the thalamus, the para-hippocampus, and the midbrain arise from the P1 segment. Variable and unpredictable anastomoses are seen between the distal branches of PCA, middle cerebral artery, and anterior cerebral artery segments.
The majority of the distal posterior cerebral artery aneurysms arise from the P2 segment.2 In case 1, the surgeon applied the clip in the P2 segment. The midbrain and the thalamus may be spared, but the occipital lobe can be ischemic, and VEP monitoring will be most helpful in detecting ischemia. In case 2, occlusion of the P1 segment by a temporary clip in the operation room or balloon occlusion in the radiology suite may cause ischemia in the midbrain and the thalamus, especially in the corticospinal fibers, the occulomotor nerve nucleus. Both MEP and VEP monitoring will be very useful in detecting ischemia resulting from the P1 segment. Patients with PCA infarcts usually have hemianopia, hemiparesis, and sensory disturbances.4 As literature does not exist on this subject, we believe that in patients undergoing PCA aneurysm endovascular coiling or surgical clipping, the intraoperative evoked potential monitoring modality must take into consideration the location of the temporary clip/balloon occlusion; monitoring with both MEP and VEP was more effective in detecting ischemia.