A newly-published postmortem study has reported that paramagnetic rim lesions (PRL) can occur in the optic chiasm in MS patients (Lockhart et al. Neurology 2026;106:e214515). This is the first report of PRLs detected in the anterior visual pathway. PRLs are most commonly seen in periventricular and deep white-matter lesions in MS patients using 3T or 7T MRI.

The postmortem study identified nine demyelinated lesions; all were mixed active/inactive chronic lesions. In eight of nine lesions, an iron rim composed of iron-laden macrophages and microglia was identifiable at the lesion edge. Iron-positive macrophages were also detected in normal-appearing white matter.

PRLs are emerging as an important biomarker of chronic active (smouldering) inflammation in MS. PRLs are highly specific for MS and rarely occur in MS mimics. The estimated diagnostic specificity is 98% (Martire et al. Expert Rev Neurother 2022;22:829-837). As a result, PRLs were incorporated into the most recent iteration of the McDonald criteria (Montalban et al. Lancet Neurol 2025;24:850-865) to increase the specificity of an MS diagnosis. For example, MS can be diagnosed in CIS patients with lesions in only one topography if there is >1 PRL with dissemination in time or a positive CSF. However, PRL+ is not required for a diagnosis since PRLs are not observable in a majority of MS patients.

A recent systematic review of five studies reported that PRLs were prognostic of a higher risk (odds ratio 2.24) of progression independent of relapse activity (PIRA) (Zarco et al. Mult Scler Relat Disord 2025:104:106804). PRL number is correlated with EDSS score, T2 lesion volume, MS severity score (MSSS) and brain atrophy (Misin et al. Mult Scler Relat Disord 2026:107:107012. Motamedgorji et al. Neuroradiology 2025;67:2249-2265).

Conversely, resolution of PRLs – an emerging treatment goal for DMTs – has been shown to be associated with slower rates of PIRA and disability outcomes (Reeves et al. Neurology 2024;103:e210004).