We demonstrated in our previous study that advancing imaging techniques have made it possible to select a target treatment for patients with acute ischemic stroke 11. Since our previous study, a series of studies have been published to discuss the effect of EC-IC bypass. Horiuchi et al. suggested that EC-IC bypass improves neurological function in approximately 70% of patients 10. Gunawardena et al. argued that hemodynamic insufficiency could be a rationale for EC-IC bypass 8. Very recently, Kim et al. integrated the results of recent EC-IC bypass papers through pooled analysis and demonstrated that EC-IC bypass may be helpful in IAT-ineligible patients 7,11,14−21. Unlike earlier studies, recent papers that performed surgical reperfusion in patients with acute ischemic stroke present promising results.
In our opinion, the difference between previous and recent studies lies in patient selection and operation criteria; there have been studies criticizing this point of the EIBT and COSS studies 22–26. Moreover, surgical tools have evolved and surgical skills have advanced since these two studies. Therefore, it is reasonable to re-evaluate the role of EC-IC bypass in the treatment of acute cerebral infarction with large vessel occlusion.
In particular, in recent years, several studies have been conducted on perfusion examination using CT and MRI as well as the importance of each parameter. American Heart Association (AHA) analyzed existing studies to identify how to comprehend perfusion studies in clinical practice in 2020 13. They reviewed the literature on how it is reasonable to judge salvageable tissue or penumbra. They also argued that as perfusion imaging techniques develop, it is possible to identify patients who will benefit from reperfusion from among patients who are beyond the conventional timeframe. Furthermore, as the collateral channel can be different for each patient, they also insisted that individualized treatment is possible when using a perfusion examination rather than simply taking the time from symptom onset.
In our study, some areas of hypoperfusion due to large vessel occlusion had already become ischemic cores, but there are cases where a significant area still had salvageable tissue, even after 24 hours, which is the IAT timeframe. The reason for symptom fluctuation despite the treatment for induced hypertension when IAT fails is probably because the perfusion state in the penumbra region repeats wax and wane. There have been studies showing that the patients’ neurologic status is improved after EC-IC bypass, but this is the first study to quantitatively prove that the perfusion state improves when EC-IC bypass is performed in these cases.
In our study, immediately after EC-IC bypass, the Tmax was significantly improved in all sections. Assuming that the part of Tmax > 6 s minus CBF < 30% is the penumbra, the size of the penumbra also significantly decreased pre- and postoperatively. These changes occurred between an examination performed at 24 hours preoperatively and 48 hours postoperatively; therefore, it is reasonable to assume that these changes were due to surgery rather than natural course of collateral vessel formation after cerebral infarction. Considering that there was no case of spontaneous reopening of occlusion or stenosis in TFCA performed 1 week postoperatively, it is more reasonable to argue that this change is due to surgery. Notably, CBF < 30%, known to represent an ischemic core in several studies, have been shown to decrease slightly immediately postoperatively in our study. This is considered to be because in the acute ischemic state, cerebral edema occurs around the ischemic core; due to this, CBF < 30% appeared in a wider range than the actual ischemic core.
Only the preoperative neurological condition of the patient was found as the preoperative factor predicting good outcome. Among the CTP parameters, there was no factor that could predict good postoperative outcomes. This could be due to selection bias as this study includes patients who have been selected using strict operation criteria. Instead, in the postoperative comparison between the good and bad outcome groups, preoperative mismatch volume, immediate postoperative Tmax > 6 s, Tmax > 4 s, and mismatch volume were significantly different. This means that the patient with a preoperative mismatch volume of < 58.5 ml should undergo surgery, and the patient's mismatch volume must be clearly improved to < 22.5 ml immediately postoperatively to achieve a good outcome. In other words, it is important that an experienced surgeon performs the surgery skillfully to obtain good prognosis. In all patients in our study, STA patency was well maintained in TFCA at postoperative 1 week. The high patency may be attributed to the difference in characteristics of the disease itself partly in light of the many experiences of moyamoya disease treatment at our hospital, but basically, the patient's perfusion status improves a lot when the bypass is well done.
Among the patients in this study, despite three silent infarctions, one infarction progression, and one postoperative hematoma, none of the patients showed postoperative deterioration. The infarction progression rate was 2.56%, which is similar to recent studies 7,10 16. The proportion of patients with good postoperative outcomes was 61.5%, which was also similar to other studies 7,15 8. Notably, there was no patient with hyperperfusional hemorrhage in this study. Compared to the previous studies where the incidence of intracerebral hemorrhage (ICH) after IAT was approximately 5%, it is estimated that the incidence of hyperperfusional hemorrhage is not high even though the operation was performed beyond the timeframe from the symptom onset. However, as the target groups are completely different, it is not reasonable to make a simple comparison. However, compared to antegrade recanalization, which enables a high flow to the area where the ischemic core has already been formed, the occurrence of hyperperfusional ICH may be relatively low if retrograde recanalization with lower flow by STA-MCA bypass is performed in blood vessels avoiding the ischemic core.
This study was conducted with larger number of patients than previous studies; however, there are still a few limitations. The number of patients is not absolutely large. In addition, it has a disadvantage of being a retrospective study without a control group. Furthermore, we failed to predict what factors should be preoperatively considered to make prognosis good. Further randomized controlled studies with a larger number of patients are warranted to judge whether urgent EC-IC bypass is actually effective in IAT-ineligible acute ischemic stroke with large vessel occlusion patients.