Different pathological processes for acute white matter lesions in multiple sclerosis

Multiple sclerosis (MS) has variable clinical presentations, prognoses, pathogeneses, and pathological patterns. We conducted a pathological review of acute MS-associated lesions that focused on the degree of axonal injury, myelin loss, and glial reaction to determine whether the observed demyelination was of the primary or secondary type.

After searching the records for a 15-year period at the London Health Sciences Centre Pathology Department, we identified 8 cases of surgical acute lesion biopsies in which clinical MS diagnoses were made before or after the biopsy.

The white matter pathologies in these cases could be sorted into 3 morphological patterns. The first pattern, which represents typical demyelinated plaques, was observed in 4 cases and was characterised by nearly complete demyelination accompanied by variable degrees of axon preservation and axonal swelling. The second pattern was observed in 3 cases and was characterised by demyelinating lesions containing variable numbers of myelinated axons mixed with a few demyelinated axons and variable numbers of axonal swellings. The myelinated axons ranged from scattered fibres to bands of variable thickness, and the demyelination was a mixture of primary and secondary demyelination. The third pattern was observed in 1 case and was characterised by well-demarcated areas of reduced myelin staining and numerous apoptotic nuclei. Axonal staining revealed many fragmented axons with reduced myelin staining but no definitely demyelinated axons.

This report shows that the predominant pathology underlying acute MS-related lesions is not limited to demyelination but can include axonal degeneration alone or in combination with primary demyelination which reflect different pathogenesis for these acute lesions.