Detection of slowly expanding lesions in people with multiple sclerosis using MRI: A scoping review

Slowly expanding lesions (SELs) have recently been described in people with multiple sclerosis (pwMS). This pattern of slow lesion expansion, often too subtle to be identified visually, can be detected on conventional brain MRI scans and represents a candidate imaging biomarker of chronic inflammation. The present study undertakes a scoping review of articles reporting the detection of SELs in pwMS using MRI. Our main objectives were to summarize the different methods used to detect SELs, describe their frequency, examine their relationship with other markers of chronic inflammation, and assess their potential prognostic role. We also discuss the limitations of this biomarker and potential methodological improvements that should be explored before considering its translation into clinical routine. We identified 33 studies that detected SELs using longitudinal MRI acquisitions. Two distinct methods for detecting SELs were identified, each implemented with various parameter settings. SELs were detected in most pwMS (60-99%) across all disease phenotypes. SELs were more frequent than paramagnetic rim lesions (PRLs), and these two markers showed only modest co-localization. Several studies suggest that SELs may be a relevant marker for predicting disease progression and evaluating the effects of treatments on chronic inflammation. While SELs appear to be a promising biomarker in pwMS, current methods require further improvement and standardization, which limits inter-study comparability and hinders translation into clinical practice. Moreover, evidence regarding the sensitivity and specificity of SELs in identifying chronic active lesions in vivo remains limited.