Hatim Attar, MD
In recent years, a lot of interest has been generated on understanding plaque morphology and identifying plaques that are truly symptomatic. Various newer imaging modalities and techniques coupled with knowledge of basic histopathology of plaques has placed researchers in a position to answer these questions. Analyzing plaques, and being able to determine which ones are actually the problem, is a focus not only of vascular neurology, but also of cardiology. Thus, several joint efforts with pooling of carotid and coronary plaques have provided a rapid growth in literature in the last few years. This study has looked specifically into intracranial Artery to Artery (A to A) subtype of embolic strokes and used Whole Brain High Resolution MRI (WB- HRMRI) to investigate plaque characteristics.
74 patients were prospectively recruited. Inclusion criteria included first-time MCA strokes confirmed with MR diffusion sequences along with intracranial ICA or MCA stenosis. There were several exclusion criteria like concomitant high grade ipsilateral extracranial stenosis, history of ipsilateral vessel occlusion, coexisting vulnerable plaque or evidence of cardio-embolism, which would confound the etiology of A to A embolism. All patients were subject to a standardized contrasted WB HRMRI. Patients were then divided into two groups based on imaging. The first is the A to A group, of 36 patients, which was defined as 2 or more hyperintense areas on DW or internal border-zone MCA territory. The second group of 38 patients, representing non A to A infarcts, based on infarcts in deep structures corresponding to small perforator vessels. The imaging was reviewed by two experienced neuro-radiologists, and a culprit lesion was defined as a stenotic lesion in the vascular territory of the stroke or the most stenotic lesion when multiple plaques were visualized. Hyper-Intense Plaque (HIP) was defined as the brightest spot of a plaque when compared to the reference wall. HIP were categorized based on location and degree of plaque enhancement.
Of the 36 patients in the A to A embolism group, 30 culprit lesions were in the MCA and 6 in the intracranial ICA. In the non A to A group, all 38 were in the MCA. There was no difference in degree of stenosis between the two groups. However, the occurrence of HIP was significantly higher in the A to A group (75.0% versus 21.1%; P<0.001). 18 of the 27 HIPs (66.7%) showed the hyperintense area located adjacent to the lumen versus 9 HIPs (33.3%) within the plaque. A higher prevalence of plaque surface irregularity was also observed in A-to-A. These two markers were found to be statistically significant and independent factors for A to A embolism after multivariate analysis was completed. Other parameters like remodeling index, degree of plaque enhancement were not found to be statistically significant.
The authors point out that there have been some studies which associate HIP and plaque rupture. Yet, others have not used WB HRMRI to look specifically at intracranial plaque. They reckon that HIP can be used as a new imaging biomarker for prediction of recurrent stroke and assist in individualizing treatment plans. Next, two thirds of the HIPs noted in this study were located on the plaque surface as compared to the third located within the plaque. This suggests a probability of intraluminal thrombus, another feature strongly associated with thromboembolism.
This study does a good service to the literature on advanced imaging on vascular lesions and identifying vulnerable plaque. HIP is indicative of Intra-Plaque Hemorrhage (IPH) — a T1 hyperintensity on MRI is subacute blood or methemoglobin. And IPH is an established marker of plaque instability. In fact, the HR MRIs can also identify other features of plaques, such as neovascularization, lipid rich necrotic center, fibrous cap aside from IPH, which provide a histological understanding of the disease. All prior clinical trials have always focused only on degree of stenosis, but these recent studies introduce a paradigm shift where the focus is also on the plaque morphology, histology, progression rate. We need additional studies on advanced imaging to justify its clinical significance and routine use balancing our need to practice cost effective medicine.