Multiple sclerosis now appears less like a single disease and more like two main damage trajectories sharing one label. New work pairing advanced brain MRI with serum neurofilament light chain maps how tissue injury accumulates and shows that patients tend to cluster into inflammation‑first or neurodegeneration‑first courses.

In the inflammation‑first pattern, inflammatory lesions dominate early, with frequent clinical relapses and new gadolinium‑enhancing spots on MRI. Serum neurofilament light chain rises sharply in step with these bursts of demyelination and axonal injury. Many current disease‑modifying therapies directly target this immune‑driven phase and substantially reduce relapse burden, yet they only partly slow long‑term axon loss.

In the neurodegeneration‑first pattern, global or regional brain atrophy precedes strong biomarker surges. Patients may show subtle symptoms and few obvious relapses while neurons and axons quietly disappear. This trajectory highlights the limits of a purely anti‑inflammatory playbook and underscores the need for neuroprotection, remyelination strategies and more quantitative MRI, including volumetric analysis, to detect change early.

The two‑trajectory framework invites earlier stratification, more targeted trial design and counseling that distinguishes visible relapses from silent structural loss. It also reframes the central puzzle of multiple sclerosis: one autoimmune and neurodegenerative process, yet two diverging paths through the same nervous system.