Predictors and Impact of Sulcal SAH after Mechanical Thrombectomy in Patients with Isolated M2 Occlusion

DISCUSSION

In the current study, SS after MT for M2 occlusion was not uncommon, and two-thirds of SS cases were covert SS, which were angiographically occult but revealed by postprocedural imaging. In addition, several procedural factors including distal location, superior division, acute vessel angulation, and multiple passes (≥3) were independently associated with the occurrence of SS. Although common, SS has a non-notable impact and is not associated with unfavorable clinical outcome.

To the best of our knowledge, this is the first study to show the prevalence of SAH as well as related procedural factors and clinical impact in patients undergoing M2 thrombectomy. Although there have been several studies on MT for M2 occlusion, they merely reported either the rate of symptomatic ICH or safety outcomes that focused on intraprocedural SAH or perforation.^{2,4,5,12,18-21} However, the present study attempted to assess not only overt SS but also covert SS and investigated procedural predictors of SS. The current study indicates that SS following M2 thrombectomy is relatively common, and distally located, superior division, acute-angle M2 occlusion with multiple thrombectomies is likely to result in SS.

The incidence of SAH after MT ranges from 0.5% to 24% according to previous reports,^{9-11,15,22-25} and in our cohort, the observed incidence was 15.8%. In previous M2 studies, SAH after M2 thrombectomy ranged from 0.9% to 7.7%,^{1-5,8,18,19,23} which was still lower than our data (15.8%). However, if the definition of SAH is restricted to the procedural angiographic findings, intraprocedural SAH was 4.8% in this study, showing similar or lower rates of SAH than in other studies. In addition, the incidence of SAH in previous studies might have been under-reported because of the lack of MR imaging or follow-up CT. According to the protocol at our center, most patients with AIS who underwent MT also underwent MR imaging within 3 days after EVT. Therefore, postprocedural MR imaging was available for 184 (88.0%) of the enrolled patients. As described in Table 2, in addition, 71% of angiographically occult SAH cases were found by MR imaging; thus, a higher proportion of SAH might be attributable to the application of the GRE sequence, which offers higher sensitivity than CT alone in diagnosing SAH.

The clinical consequences of SAH differ according to the type of SAH and the co-occurrence of ICH.^24,25 While isolated SAH after MT does not influence long-term clinical outcomes, the presence of diffuse SAH, including that associated with concomitant parenchymal hematoma, reduces the chances of clinical recovery. Qureshi et al²⁵ showed that the rates of independent functional outcome were lower among subjects with SAH with intraparenchymal hemorrhage or other ICH but not in subjects with isolated SAH. Renú et al²⁴ recently reported that the post-MT diffuse subarachnoid hyperattenuation pattern was mostly composed of blood extravasation and emerged as a biomarker of an increased risk of poor outcome and death. In line with these studies, covert SS was not associated with unfavorable outcome, whereas overt SS showed a trend toward unfavorable outcome.

The elevated SAH risk due to distal location and acute M2 angulation in the current study might be explained by several possible mechanisms: First, a more curved, distal location with thinner vessel walls causes difficulties in procedural accessibility and a higher risk of microguidewire perforation and tearing of tiny arterioles. Second, in the distally located M2 segment, because these vessels have a small normal diameter, any reduction in vessel caliber would result in greater friction and traction forces during MT than in larger proximal vessels. Third, acute M2 angulation increases friction between the vessel wall and the device, with increased vessel stretching during stent retrieval.²⁶ As noted in our study, Ng et al¹⁵ found a trend toward postprocedural SAH occlusion in patients with extreme MCA angulation and distal device positioning. Lee et al¹¹ also reported that the distal location of vessel occlusion was associated with a higher probability of SAH.

A higher number of MT passes was associated with SS in this study. Previous studies have shown that a greater number of thrombectomy device passes increases the prevalence of SAH, supporting our results.^10,11,15 This outcome may occur because additional passes of the SR cause repeat traction injury to small perforating branches or venules. In addition, multiple passes of MT necessitate repeat guidewire probing of the occluded vessel, which increases the potential for unrecognized microguidewire perforation or dissection. Microperforation may not result in overt contrast extravasation on DSA or immediate bleeding if the vessel remains occluded or if the vessel immediately develops vasospasm. Subsequent recanalization of an occluded vessel with occult microperforation or dissection may lead to delayed occult SAH.¹⁵

Most interesting, there was no occurrence of SS in patients treated with CA alone as the primary MT method for M2 occlusion. In contrast, 78.8% of patients in the SS group were treated with an SR alone. Similar to our findings, Renieri et al²¹ recently reported that SR and combined techniques were associated with higher rates of intraprocedural SAH. Theoretically, SRs result in more extensive endothelial injury than aspiration catheters during stent retrieval because they exert a continuous radial action against the vessel wall.^27,28 Additionally, it has been suggested that an SR exerts traction forces applied to the vessel, and deformation of the adjacent anatomy causes stretching and rupture of small arterioles or venules.²⁹ Because distal M2 branches typically have a small diameter (≤2.0 mm), a higher traction force might be required during SR thrombectomy, which increases the likelihood of stretch injury to M2 perforating arterioles, and this phenomenon might be exacerbated by the greater M2 angulation compared with a proximal and smaller M2 angulation.^15,27

In our study, although clinical consequences were different between overt and covert SS, distal M2 occlusion and acute M2 angulation were associated with both overt and covert SS. Therefore, when planning a procedural strategy, identifying these angiographic findings is important, and when the performing M2 thrombectomy under these anatomic conditions, a tailored thrombectomy technique as well as delicate microguidewire navigation is required to reduce SS occurrence.

This study has several limitations. First, inherent selection bias was inevitable because of the nonrandomized and retrospective study design. In particular, it is challenging to deliver a thromboaspiration catheter to the face proximal to the clot, especially in the distally located or curved M2, and this issue may influence the selection of first-line MT devices. Thus, careful interpretation of the results is required. Second, the sample size may not have been large enough to show statistical differences between the subgroups of this cohort from a single center. Most important, the sample size of the SS subgroups was very small, thus hampering appropriate analysis of their association with clinical outcome variables. Additionally, the application rate of CA was low (10.0%), and the differential effect derived from the use of the device on SS was not sufficient for assessment. Third, the wide range of the study period, including the potential effect of advancements in endovascular techniques and operator skill on treatment outcomes, is another potential limitation of this study.