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The McDonald criteria, widely used for establishing a diagnosis of multiple sclerosis (MS), were updated in 2024 and published in late 2025. Although the complexities of these criteria are beyond the scope of a typical NEJM Clinician summary, several key points will be useful for non-neurologists encountering patients with suspected MS.
The presence of typical MS lesions in more than one anatomic location — known as dissemination in space — has long been required for diagnosis. The new criteria add the optic nerve as a fifth location that can fulfill this requirement, alongside cortical, periventricular, infratentorial, and spinal cord locations. Optic nerve involvement can be assessed via MRI, optical coherence tomography, …