Acute disseminated encephalomyelitis and demyelinating disease — formative SAQs

Question 1 (10 marks)

A previously well six-year-old boy presents four days after a viral upper-respiratory illness with fever, drowsiness, and irritability, then over the next twenty-four hours develops a right hemiparesis, slurred speech, and an unsteady gait. His conscious level fluctuates and is not explained by the fever alone. Brain MRI with gadolinium shows diffuse, bilateral, poorly demarcated white-matter lesions with thalamic involvement and patchy enhancement. [1]

(a) Define acute disseminated encephalomyelitis and identify the single clinical feature that distinguishes it from a clinically isolated syndrome such as optic neuritis or transverse myelitis. (3 marks) [1]

(b) Outline the first-line acute treatment, including the drug, the dose, and the duration, and state when and how you would escalate. (4 marks) [4]

(c) Describe the antibody-led workup that resolves the diagnosis and governs the long-term management, naming the three antibody tests and the imaging. (3 marks) [1] [2]

Model answer

Acute disseminated encephalomyelitis is a first, polysymomatic, inflammatory demyelinating event of the central nervous system, defined under the International Pediatric Multiple Sclerosis Study Group criteria by the presence of encephalopathy - an altered conscious level or behaviour not explained by fever alone - alongside multifocal neurological deficits and a diffuse, bilateral white and deep grey matter lesion pattern on MRI. The single distinguishing feature is encephalopathy: its presence defines ADEM, and its absence defines a clinically isolated syndrome, which by definition is non-encephalopathic beyond the irritability expected in a sick child. This child's fluctuating conscious level, dissociated from the fever, establishes the ADEM phenotype. [1]

The first-line acute treatment is high-dose intravenous methylprednisolone at 20 to 30 mg/kg per day to a maximum of 1 g per day, given over three to five days, followed by a short oral prednisolone taper. If the encephalopathy or deficit has not improved after forty-eight to seventy-two hours of corticosteroids - defining steroid-refractory disease - the response is to escalate to intravenous immunoglobulin at a total dose of 2 g/kg (over two to five days) or to plasma exchange (typically five to seven exchanges on alternate days), rather than repeating the steroid course. Plasma exchange is favoured for the most severe attacks because it removes the pathogenic antibody and complement directly. [4]

The workup is run in parallel with the acute treatment. An urgent brain and spinal MRI with gadadolium assesses the lesion pattern and dissemination in space; the serum is sent for myelin-oligodendrocyte-glycoprotein IgG and aquaporin-4 IgG by a live cell-based assay (the assay matters, because fixed-cell ELISA assays produce false positives); and a lumbar puncture provides cerebrospinal-fluid oligoclonal bands on a paired serum sample, which, if unique to the CSF, support paediatric multiple sclerosis and can substitute for dissemination in time under the 2017 McDonald criteria. The antibody triad - MOG-IgG, AQP4-IgG, and oligoclonal bands - is what resolves the provisional ADEM phenotype into a confirmed diagnosis and governs the long-term treatment. [1] [2]