Every revision of the McDonald criteria has admitted more kinds of evidence while trying to hold specificity steady. This Brain review asks a different question about the 2024 version: not whether the criteria are valid in expert hands, but what happens when they are applied where imaging quality, laboratory infrastructure and subspecialist reporting are variable.
The 2024 revision brings in the optic nerve as a topography, and admits the central vein sign and paramagnetic rim lesions as supporting evidence. Each of these depends on sequences acquired and interpreted a particular way. A central vein sign read off a scan that was not optimised for it is not the same finding, and the criteria as written do not always make the acquisition requirements explicit.
The authors' concern is that more routes to a diagnosis produce heterogeneity in how the diagnosis is reached, and heterogeneity is where misdiagnosis lives - with the consequence, in multiple sclerosis, of committing a patient to years of disease-modifying therapy they do not need. They propose safeguards and firmer definitions for the new biomarkers.
For a neurologist outside a specialist MS centre the usable message is conservative. If a diagnosis rests on one of the new biomarkers, be explicit in the letter about which sequence was used and who reported it, and be readier to review the diagnosis than to defend it. In India, where MRI protocols vary widely between centres and radiology subspecialisation is uneven, that caution applies with more force, not less.
- State in the clinic letter which criterion was met and on what imaging - not just 'meets 2024 McDonald'
- Confirm the MRI sequence was appropriate before accepting a central vein sign or rim lesion
- Treat a diagnosis resting on a single new biomarker as provisional and re-review it
- Keep the differential open in atypical presentations; wider criteria make anchoring easier
- Ask who reported the scan - these signs need a reader who looks for them routinely
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