Situation: A 30-year-old woman with relapsing-remitting multiple sclerosis (MS) comes to the neurology clinic. She was diagnosed 2 years ago and now takes interferon beta injections and oral baclofen for leg spasticity. Which diagnostic finding supported her diagnosis of multiple sclerosis (MS)?

MS is diagnosed by showing demyelinating plaques in different regions of the central nervous system, seen on MRI as scattered white matter lesions, typically periventricular, juxtacortical, infratentorial, and in the spinal cord. Slowed peripheral nerve conduction points to a peripheral neuropathy such as Guillain-Barré syndrome, which MS does not involve. A decremental response to repeated nerve stimulation is typical of myasthenia gravis, and a single wedge-shaped lesion suggests an infarct.

Core diagnostic concept Multiple sclerosis is a central nervous system demyelinating disease, not a peripheral nerve disorder. The hallmark finding on MRI is multiple demyelinating plaques scattered in different regions of the CNS white matter, which supports the requirement for dissemination in space ^[1][3].

Why scattered white matter lesions are diagnostic MS lesions appear as T2-hyperintense foci in characteristic locations: periventricular, juxtacortical, infratentorial, and spinal cord ^[1][4]. The 2017 and 2024 McDonald criteria require evidence of lesions in at least two of these four CNS regions to establish dissemination in space ^[1][2]. A single lesion is insufficient unless accompanied by other clinical or paraclinical evidence, which is why a single wedge-shaped lesion suggests an ischemic infarct rather than MS ^[3].

Differential diagnosis of the distractors

Finding	Mechanism	Typical condition
Falling response on repeated nerve stimulation	Postsynaptic acetylcholine receptor fatigue at neuromuscular junction	Myasthenia gravis
Single wedge-shaped lesion	Vascular territory occlusion causing localized ischemia	Cerebral infarction
Slowed nerve conduction in both legs	Peripheral nerve demyelination or axonal loss	Guillain-Barré syndrome, peripheral neuropathy

Why peripheral findings exclude MS MS affects only central myelin produced by oligodendrocytes. Peripheral nerve conduction studies evaluate Schwann cell–myelinated axons, which are not involved in MS. Therefore, slowed nerve conduction in the legs points to a peripheral neuropathy, not MS ^[3]. This distinction is frequently tested because both MS and Guillain-Barré syndrome are demyelinating conditions, but they affect completely different parts of the nervous system.

Clinical correlation with the patient This patient has relapsing-remitting MS with leg spasticity managed by baclofen, a GABA-B agonist that reduces spasticity by acting on the spinal cord. Interferon beta is an immunomodulatory therapy that reduces relapse frequency. The diagnosis was established two years ago, meaning the initial MRI already demonstrated dissemination in space and time through multiple lesions in distinct CNS regions ^[1][2]. The 2024 McDonald criteria emphasize that MRI remains the cornerstone imaging modality for detecting these demyelinating lesions ^[3].