A 28-year-old woman presents with acute vision loss in the left eye and pain on eye movement (optic neuritis). MRI brain with contrast is performed to evaluate for demyelinating disease. The scan reveals multiple T2-hyperintense lesions perpendicular to the lateral ventricles, oriented along perivenular spaces, as marked **A** in the diagram. Which of the following best describes the significance of these lesions in the context of the McDonald Criteria for Multiple Sclerosis diagnosis?

Explanation

## Why Option 1 is correct The structure marked **A** — "Dawson Fingers" (periventricular T2-hyperintense plaques oriented perpendicular to the lateral ventricles along perivenular spaces) — is one of the four typical CNS locations recognized by the 2017 McDonald Criteria for dissemination in space (DIS). The criterion requires ≥1 T2 lesion in ≥2 of 4 areas: periventricular, cortical/juxtacortical, infratentorial, or spinal cord. Periventricular lesions are the most common and characteristic finding in MS. The presence of these lesions in the periventricular location, combined with lesions in another typical area (or clinical evidence of dissemination in time), supports the diagnosis of MS. [Robbins 10e Ch 28; Harrison 21e Ch 444] ## Why each distractor is wrong - **Option 2**: While periventricular lesions are highly suggestive of MS, clinical symptoms + MRI alone are insufficient for definitive diagnosis. The McDonald Criteria also require either dissemination in time (new lesions on follow-up or simultaneous enhancing/non-enhancing lesions) OR positive CSF oligoclonal bands. CSF analysis remains a key diagnostic tool. - **Option 3**: Although Dawson Fingers are characteristic of MS, they are not pathognomonic. Other demyelinating conditions (ADEM, NMO, PRES) can occasionally present with periventricular lesions. CSF oligoclonal bands and follow-up imaging are essential to confirm MS diagnosis and exclude mimics. - **Option 4**: The McDonald Criteria do not require gadolinium enhancement for lesions to count toward dissemination in space. Both enhancing and non-enhancing T2 lesions in typical locations (including periventricular) satisfy the DIS criterion. Gadolinium enhancement is used to demonstrate dissemination in time, not to validate the location criterion. **High-Yield:** Periventricular "Dawson Fingers" are the hallmark of MS on MRI and directly satisfy the dissemination-in-space criterion of the McDonald Criteria when present with lesions in ≥1 other typical CNS location. [cite: Robbins 10e Ch 28; Harrison 21e Ch 444]