Iron-Rim Lesions in the Optic Chiasm in Multiple Sclerosis

OBJECTIVES: The aim of this study was to demonstrate the presence of iron-rim lesions (IRLs) in the anterior visual pathway in multiple sclerosis (MS). METHODS: Postmortem formalin-fixed paraffin-embedded anterior visual pathway tissues (optic chiasms) from 3 donors with MS were examined for iron using Turnbull Blue histochemistry. Demyelinated lesions were identified and staged with immunohistochemistry for myelin proteolipid protein and CD68. RESULTS: A total of 9 lesions were identified, all of which were mixed active/inactive. Iron-positive microglia and macrophages were identified at the inflammatory borders in all 3 cases, in 8 of the 9 lesions. Within the inactive core, iron-positive astrocytes and, rarely axons, could be seen. Iron-positive macrophages were also seen in nonlesional white matter. DISCUSSION: IRLs are common in mixed active/inactive MS lesions in the anterior visual pathway. IRLs are the pathologic correlate of paramagnetic rim lesions (PRLs) on MRI, the number of which in the brain correlates with the extent of disability. PRLs have, heretofore, not been described in the anterior visual pathway. Future work aimed at overcoming the challenges in detecting these lesions in the anterior visual pathway is warranted, to ensure that extent of chronic inflammation is not underestimated.