Intracranial Aneurysms and Intraoperative Neuromonitoring

By Marie Liang and Faisal R. Jahangiri Treatment for intracranial aneurysms is increasing with the global aging popula…

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Oct 11, 2022
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Treatment for intracranial aneurysms is increasing with the global aging population. Statistics for the United States estimate that 1 in 50 people have an unruptured aneurysm, with the risk for rupture higher in older women above 55. The delicate anatomy of the cerebrovascular system follows that any disruption is closely linked to neurological function. While the risk of rupture is relatively low, ruptured aneurysms have a mortality rate of 40 to 50 percent, and permanent neurological deficits are present in 66 percent of the non-fatal cases. Surgical treatment such as aneurysm clipping and interventional treatment such as endovascular coiling for unruptured aneurysms are lower-risk and lower-cost preventative measures compared to emergency treatment of ruptured aneurysms. Overall, intraoperative neuromonitoring (IONM) for ruptured and unruptured intracranial aneurysm treatment provides additional protection in navigating and ensuring the integrity of neural structures and the vascular structures connected to them.
 
As aneurysms represent a weakening in the arterial walls, the goal of treatment is to seal off the neck of the aneurysm to prevent further blood pooling into the aneurysm and rupturing the vascular walls. Surgical aneurysm clipping involves the skull opening to place a metal clip on the neck of the aneurysm, which will block any further blood flow to the aneurysm and cause the eventual shrinking of the balloon. However, clipping may cause ischemic changes to surrounding structures intraoperatively. Endovascular coiling is a less invasive alternative to aneurysm clipping by inserting a catheter into the femoral or radial artery to reach the area of the aneurysm. A coil is detached from the catheter through the aneurysm neck to fill the widened area. While generally lower-risk, interventional coiling can sometimes result in intraoperative rupture of the arterial walls or thromboembolic complications if the coil release is placed inaccurately or displaced.
 
IONM modalities provide real-time detection of any ischemic injury or neural pressure that may occur during clipping or coiling the aneurysm to prevent postoperative neurologic deficits. Particularly for aneurysm clipping, modalities such as somatosensory evoked potentials (SSEPs), motor evoked potentials (MEPs), electroencephalography (EEG), and brainstem evoked potentials (BAEP) can monitor any changes in cerebral blood flow after clip positioning and evaluate their effect on neural function. Continuous monitoring can be run during critical periods such as temporary clipping and can be used to detect unexpected bleeding or changes in vital signs. Reviews of the literature have also found that aneurysm clipping cases with IONM saw lower incidence rates of postoperative neurologic deficits than those without IONM. While endovascular coiling may be less invasive and lower risk than aneurysm clipping, the risk of thromboembolic complications and intraprocedural rupture remains. In addition to SSEPs, MEPs, BAEP, EEG, and additional IONM modality transcranial Doppler (TCD) can also supplement the monitoring of regional cerebral blood flow. Using multiple IONM modalities has shown higher specificity and sensitivity rates for detecting any intraoperative changes and predicting postoperative deficit, and thus, it is highly recommended. Early detection of changes in any of the IONM signals can allow the appropriate intervention to improve postoperative outcomes for the patient during intracranial aneurysm clipping and coiling.
 
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