Acute disseminated encephalomyelitis and demyelinating disease — branching viva

Opening branch — the encephalopathic six-year-old

A previously well six-year-old presents four days after a viral illness with fever, drowsiness, irritability, a right hemiparesis, and slurred speech, and the brain MRI shows diffuse, bilateral, poorly demarcated white-matter lesions with thalamic involvement. The candidate must first recognise this as a first central nervous system demyelinating event and apply the International Pediatric Multiple Sclerosis Study Group definition: the polysymomatic deficit plus encephalopathy - an altered conscious level not explained by fever alone - establishes the acute disseminated encephalomyelitis phenotype, distinguished from a non-encephalopathic clinically isolated syndrome by that one feature. The teaching point is that encephalopathy is the mandatory discriminator and that the post-infectious timing and the diffuse bilateral white and deep grey matter lesion pattern are supportive but not the defining feature. [1]

The examiner probes the acute treatment. The candidate states that the first-line therapy for every demyelinating event is high-dose intravenous methylprednisolone at 20 to 30 mg/kg per day to a maximum of 1 g per day for three to five days, begun as soon as infection is reasonably excluded, with the workup in parallel. The escalation rule is stated precisely: if the deficit has not improved after forty-eight to seventy-two hours, the event is steroid-refractory and the response is intravenous immunoglobulin (2 g/kg total) or plasma exchange, not a repeat steroid course. The antibody triad - serum MOG-IgG and AQP4-IgG by cell-based assay, and cerebrospinal-fluid oligoclonal bands - resolves the diagnosis and governs the long-term treatment. [1] [4]

Second branch — the bilateral optic neuritis and the area-postrema syndrome

The examiner switches to a thirteen-year-old of South Asian ancestry with five days of intractable hiccups, nausea, and vomiting, then bilateral visual loss and a flaccid paraplegia, with a longitudinally extensive transverse myelitis spanning six segments on spinal MRI. The candidate identifies the area-postrema syndrome (a lesion at the floor of the fourth ventricle), the severe bilateral optic neuritis, and the longitudinally extensive myelitis as the core clinical characteristics of neuromyelitis optica spectrum disorder, and states that the positive aquaporin-4 IgG confirms the diagnosis under the IPND 2015 criteria. [3]

The examiner tests the most important safety point. The candidate explains why a multiple-sclerosis disease-modifying therapy must never be started: NMOSD is an antibody-complement astrocytopathy, biologically distinct from the T-cell-driven disease that the multiple-sclerosis drugs target, and several of those drugs worsen AQP4-IgG-positive disease. The maintenance therapy is NMOSD-specific - rituximab or another anti-CD20 agent, satralizumab, a complement blocker, mycophenolate, or azathioprine - and the acute attack is treated with methylprednisolone then early plasma exchange for the severe AQP4-IgG-positive presentation. The safeguard is the antibody status before any long-term drug. [3] [5]

Third branch — the first optic neuritis and the McDonald criteria

The examiner closes with a fourteen-year-old with a single right-eye optic neuritis that recovered, six periventricular and juxtacortical white-matter lesions on brain MRI, and cerebrospinal-fluid oligoclonal bands unique to the CSF, with negative MOG-IgG and AQP4-IgG. The candidate applies the 2017 McDonald criteria - which apply to children aged eleven and older with a non-ADEM onset - and explains that the MRI dissemination in space combined with the CSF oligoclonal bands (which substitute for dissemination in time under the 2017 revision) allows a diagnosis of multiple sclerosis at the first attack. The management is early high-efficacy disease-modifying therapy because the paediatric disease is more inflammatory and early control of relapses preserves long-term function. [1] [5]

The examiner revisits the unifying principle across all three branches. The candidate closes with the two governing rules: treat the acute attack immediately with high-dose methylprednisolone and escalate when steroid-refractory, because the acute mechanism is inflammatory across all four entities; and never start a multiple-sclerosis disease-modifying therapy before the aquaporin-4 and MOG antibody status is known, because the wrong drug can precipitate a devastating relapse in NMOSD. The diagnostic rate-limiting step is the antibody answer, and the long-term treatment is matched to the confirmed diagnosis. [1] [2]