Clinical meaning
The McDonald criteria (revised 2017) provide a structured framework for diagnosing multiple sclerosis based on demonstrating dissemination in space (DIS) and dissemination in time (DIT) of demyelinating lesions in the central nervous system. DIS requires lesions in at least 2 of 4 characteristic CNS locations: periventricular (≥3 lesions), juxtacortical/cortical (≥1), infratentorial (≥1), and spinal cord (≥1). DIT requires evidence that lesions developed at different time points: simultaneous presence of gadolinium-enhancing (active/new) and non-enhancing (older) lesions on a single MRI, or a new T2/enhancing lesion on follow-up MRI compared to baseline. Importantly, the 2017 revision allows CSF-specific oligoclonal bands (OCBs) to substitute for DIT, enabling earlier diagnosis: a patient with a clinically isolated syndrome (CIS) meeting DIS criteria with positive CSF OCBs can now be diagnosed with MS without waiting for a second clinical attack or follow-up MRI. This is clinically significant because early treatment with disease-modifying therapy (DMT) delays disability progression. The NP must distinguish MS from mimics: neuromyelitis optica spectrum disorder (NMOSD — anti-AQP4 antibodies, longitudinally extensive transverse myelitis spanning ≥3 vertebral segments), acute disseminated encephalomyelitis (ADEM — monophasic,...