In this study, we found that atherosclerotic plaques were observed on HR-MRI in 50.9% of RSSI patients in the LSA territory with no stenosis of ipsilateral MCA using contrast-enhanced 3D, despite the absence of ipsilateral MCA stenosis. The LS of RSSI lesions was independently associated with MCA plaques, with the optimal cut-off point of LS ≤ 1.
Yoon et al. reported the detection of MCA plaques by HR-MRI in 51.3% of patients with RSSI in 39 patients without MCA stenosis6. Another study from China also found that HR-MRI could identify approximately 46.9% of MCA plaques in a similar population19. The result in our study (50.9%) was consistent with the above studies. Our results, showing a 50.9% prevalence of MCA plaques, align with these earlier findings, reinforcing the utility of HR-MRI in diagnosing vascular pathology in patients with RSSI in the absence of visible MCA stenosis.
Previous studies had shown that there was prominent etiological heterogeneity in RSSI. The main pathological etiologies have been identified: one involves atherosclerotic plaques affecting the perforating artery at the proximal end of the parent artery or at the orifice of the perforating artery, and the other involves small vessel disease mechanisms such as fibrinoid degeneration or lipohyalinosis at the distal end of the perforating artery3,9. The pathomechanism of the former involves atherosclerotic plaques in the parent artery or isolated atherosclerotic plaques at the orifice of the penetrating artery, obstructing the orifice and resulting in orifice obstructive lesions, commonly presented as proximal RSSI (pRSSI) lesions. These pRSSI lesions are close to the orifice of the penetrating branch (e.g., the first layer of the lenticulostriate arteries or the ventral side of the pons), have a longer diameter, and typically involve more than three layers. In contrast, the latter etiology often appears as distal RSSI (dRSSI) lesions, which are located away from the opening of the penetrating branch, leading to smaller infarctions 10,14. Previous studies have shown that pRSSI is more commonly associated with any degree of PAD detectable by conventional vascular examinations compared to dRSSI20,21. Our previous work indicated that LS ≤ 2 and TNS ≥ 3 were the most likely lesion types for RSSI combined with PAD11. These results can be well explained by pathological findings. However, limited by traditional vascular imaging, previous studies have overlooked the potential non-stenotic atherosclerotic lesions in the parent artery proximal to the perforator opening. Even in RSSI patients with non-stenotic parent artery lesions, different etiologies of potential non-stenotic atherosclerotic lesions in the parent artery proximal to the perforator opening and distal small vessel disease may cause similar proximal and distal type lesions. With the clinical application of HR-MRI, more potential atherosclerotic plaques in non-stenotic large arteries that couldn’t be detected by traditional vascular imaging have been discovered. The result of this study was consistent with the conclusion of previous studies in RSSI patients with PAD, that is, the lower the involved level, the higher the probability of MCA plaques. Microscopic anatomy studies had shown that most perforating arteries arose vertically from the dorsal superior wall of MCA16. In patients with LSA occlusion caused by MCA plaques, lower LS was observed in a HR-MRI vessel wall imaging study22. Besides, Yang et al. combined LS and TNS to predict MCA plaques, and concluded that when LS = 1 or 2 and TNS ≥ 3, it was more likely to appear MCA plaques in RSSI patients23. According to the MRI serialized axial image template, the territory of LSA was divided into 6 representative levels from the proximal origin of the LSA branching from the MCA to the distal end. The lower the involved level of the lesion, the closer the lesion is to the orifice of the penetrating artery. MCA plaques involved the origin of the perforator, causing infarction of the entire supply area of the perforating artery, which indicated that pRSSI was probably caused by occlusion of the perforating artery by MCA plaques.
Furthermore, although most studies reported that TNS ≥ 3 was associated with PAD11,19, a study from Korea observed that the size of infarction focus (including TNS) had relatively lower sensitivity and specificity in predicting MCA plaques6. In our study, we also did not find an independent correlation between TNS and MCA plaques. MCA plaques involving the perforator opening lead to a lower level of involvement, but the number of involved levels is highly correlated with the course and variation of the perforating artery. If the perforator artery is longer, its blood supply area is relatively larger, and the number of involved levels is higher when the opening is blocked. Conversely, if the perforator artery is shorter, the number of involved levels may be smaller (Fig. 5). According to the results of this study, LS may be more specific than TNS in predicting MCA plaques.
The application of HR-MRI is highly significant for identifying the etiology of RSSI in clinical practice. However, due to its high cost, HR-MRI technology is not widely available in many hospitals, making it challenging to popularize. The lesion imaging pattern discovered in this study offers a simple and feasible method for etiological classification of RSSI patients with non-stenotic responsible MCA. This allows for the convenient selection of pRSSI patients who are more suitable for HR-MRI examination to identify MCA plaques, aiding physicians in making further clinical decisions.
There were some limitations in this study. First, although HR-MRI can sensitively detect non-stenotic MCA plaques, it is still unable to identify microaneurysms or atherosclerosis at the orifice or proximal site of perforating arteries. Therefore, HR-MRI cannot completely distinguish the pathogenesis between MCA plaque-related RSSI and those caused by lesions at the orifice of perforating arteries. Second, the outcome variable of this study was the presence of plaques on the dorsal superior wall of the MCA trunk. However, some perforating arteries originate not from the MCA trunk but from the upper and lower branches of the MCA, which may introduce some errors. Third, this was a retrospective study from a hospital-based prospective registry with a relatively small sample size, resulting in insufficient test efficiency. Larger scale and prospective studies are needed to further confirm the relevant conclusions.