Latest Revision of the McDonald Criteria for the Diagnosis of Multiple Sclerosis

Step 1: Understanding the Nature of the Disease
First, it is essential to understand what Multiple Sclerosis (MS) is. MS is a chronic disease where the immune system abnormally attacks the body's own central nervous system (CNS) – the brain and spinal cord. The target of this attack is the myelin sheath, the insulating layer surrounding nerve fibers. Myelin loss leads to slowed or blocked transmission of nerve signals, causing various neurological symptoms such as blurred vision, limb weakness, sensory abnormalities, and balance problems. Its core characteristic is dissemination of CNS lesions in time and space. This means:

• Dissemination in Space (DIS): Lesions occur in two or more distinct areas within the CNS.
• Dissemination in Time (DIT): Neurological symptom episodes or the appearance of new lesions are separated in time.

Step 2: Recognizing the Core Challenge and Goal of Diagnosis
There is no single "gold standard" test for diagnosing MS (like a specific blood marker). Diagnosis primarily relies on: 1) Clinical presentation (symptoms and signs found through history and neurological examination); and 2) Paraclinical tests, especially Magnetic Resonance Imaging (MRI), which can visually display demyelinating lesions within the CNS (appearing as bright "spots" or hyperintensities on MRI). The goal of diagnosis is to reliably demonstrate the presence of both DIS and DIT, after excluding other diseases that can cause similar presentations (such as neuromyelitis optica spectrum disorder, vasculitis, infections, etc.).

Step 3: Introducing the Key Tool – The McDonald Criteria
To standardize the above diagnostic principles, an international panel of experts developed the McDonald Criteria. First published in 2001, they have been revised in 2005, 2010, 2017, and most recently (typically referring to the 2017 revision with minor clarifications in 2021) as MRI technology advanced and clinical evidence accumulated. Its core value lies in: allowing MRI evidence to substitute for some clinical evidence when specific conditions are met. This potentially enables a definitive diagnosis of MS early on, after only a single clinical event (Clinically Isolated Syndrome), facilitating the early initiation of disease-modifying therapy.

Step 4: Detailed Explanation of Core Provisions of the Diagnostic Criteria (Using the 2017/2021 Revision as an Example)
The diagnosis requires meeting the following two points, after other diagnoses have been excluded:

1. Demonstrating Dissemination in Space (DIS): Can be demonstrated by one of two ways:

   • Clinical Evidence: Objective clinical evidence of lesions in two or more areas of the CNS (e.g., optic nerve, spinal cord, brainstem, cerebellum, cerebral hemispheres).
   • MRI Evidence: MRI shows one or more T2-hyperintense lesions that are characteristic of MS in two or more of the following four CNS areas: periventricular, cortical/juxtacortical, infratentorial (brainstem/cerebellum), spinal cord. This specifies the requirement for lesion "typicality" in location.

2. Demonstrating Dissemination in Time (DIT): Can also be demonstrated by one of two ways:

   • Clinical Evidence: Two or more clinical attacks (typically separated by at least 30 days).
   • MRI Evidence: Either of the following scenarios meets the criteria:
     a) The simultaneous presence of gadolinium-enhancing (active) and non-enhancing (chronic) lesions at any time. This indicates coexistence of new and old lesions.
     b) A new T2-hyperintense or gadolinium-enhancing lesion on follow-up MRI compared to a baseline scan. This holds true even if the patient did not experience new clinical symptoms during that period (i.e., "asymptomatic disease activity").

Step 5: Special Populations and Important Revision Highlights

• Clinically Isolated Syndrome (CIS): Refers to a patient's first episode of symptoms suggestive of a demyelinating event (e.g., unilateral optic neuritis). If their MRI simultaneously meets the MRI criteria for both DIS and DIT as described above, a diagnosis of MS can be made without waiting for a second clinical attack.
• Primary Progressive MS (PPMS): In this form, symptoms show continuous, slow progression from onset without clear relapses. Its diagnostic criteria are more stringent, requiring at least one year of disease progression, plus meeting at least two of the following three criteria: ① Evidence of DIS on MRI (≥1 lesion in a typical CNS region, or ≥2 lesions in the spinal cord); ② Evidence of DIT on MRI (new T2 or enhancing lesions on follow-up); ③ Positive cerebrospinal fluid (CSF) oligoclonal bands (OCB) (evidence of specific antibody production within the CNS by the immune system, an important supporting finding).

Summary: The McDonald Criteria are a highly structured diagnostic framework based on clinical and MRI evidence. Its successive revisions (especially the 2017 version), by more precisely defining "typical locations" for lesions and simplifying MRI determination of DIT, have achieved faster diagnostic speed and higher sensitivity while maintaining high specificity. Its fundamental purpose is to promote early diagnosis and early treatment to improve the long-term prognosis for people with MS.