A 28-year-old woman presents with acute optic neuritis and is referred for brain MRI. The scan reveals multiple T2/FLAIR hyperintense lesions. The structure marked **A** in the diagram shows ovoid lesions perpendicular to the lateral ventricles. Which of the following best describes the pathophysiological basis and diagnostic significance of this finding?
A. Vasculitic inflammation of cortical penetrating vessels with secondary gliosis
B. Acute axonal transection in the subcortical white matter unrelated to venous anatomy
C. Demyelination along medullary veins satisfying dissemination in space (DIS) criterion for MS diagnosis
D. Osmotic demyelination from hyponatremia affecting periventricular water content
Explanation
Why "Demyelination along medullary veins satisfying dissemination in space (DIS) criterion for MS diagnosis" is right
The ovoid periventricular T2 lesions perpendicular to the lateral ventricles are the hallmark of Dawson's fingers — a highly specific finding in multiple sclerosis. These lesions reflect demyelination along medullary veins, which drain the periventricular white matter. This orientation perpendicular to the ventricles is pathognomonic for MS and satisfies the dissemination in space (DIS) criterion when present alongside lesions in other typical locations (juxtacortical, infratentorial, spinal cord). The McDonald 2017 criteria explicitly require ≥1 T2 lesion in ≥2 of 4 typical locations to establish DIS, and periventricular lesions are one of the four cardinal locations. The immune-mediated demyelination is driven by T-cell and B-cell attack on CNS myelin.
Why each distractor is wrong
Acute axonal transection in the subcortical white matter unrelated to venous anatomy: While axonal loss does occur in MS as a secondary phenomenon, the primary pathology of Dawson's fingers is demyelination along medullary veins, not primary axonal injury. The perpendicular orientation to ventricles is specifically related to venous drainage anatomy, not random subcortical pathology.
Vasculitic inflammation of cortical penetrating vessels with secondary gliosis: This describes vasculitis (as seen in CNS vasculitis or ANCA-associated disease), not MS. MS is an immune-mediated demyelinating disease centered on myelin attack, not vasculitis. Cortical vasculitis would not produce the characteristic perpendicular periventricular pattern.
Osmotic demyelination from hyponatremia affecting periventricular water content: Central pontine myelinolysis (osmotic demyelination) is a metabolic complication of rapid sodium correction, not an immune-mediated process. It does not produce Dawson's fingers or satisfy DIS criteria for MS diagnosis. The periventricular lesions in MS are immune-inflammatory, not osmotic.
High-YieldNEET PG
Dawson's fingers = ovoid periventricular T2 lesions perpendicular to ventricles = demyelination along medullary veins = hallmark of MS and key DIS criterion.
