This disorder may follow upper respiratory and gastrointestinal infections (viral), viral exanthems (measles, chickenpox, rubella, etc.) or immunisation with live or killed virus vaccines (influenza, rabies).
Measles is the commonest cause occurring in 1 per 1000 primary infections; next Varicella zoster (chickenpox), 1 per 2000 primary infections.
Clinical features: Within days or weeks of resolution of the viral infection, fever, headache, nausea and vomiting develop. Meningeal symptoms (neck stiffness, photophobia) are then followed by drowsiness and multifocal neurological signs and symptoms - hemisphere brain stem/cerebellar/spinal cord and optic nerve involvement. Myoclonic movements are common.
Predominantly spinal, cerebral or cerebellar forms occur, though usually the picture is mixed. Optic nerve involvement takes the form of optic neuritis. Rarely the peripheral nervous system is involved.
Generalised asynchronous delta activity
Diagnosis: No diagnosis test. CSF - 20-200 mononuclear cells. Total protein and y globulin raised. Peripheral blood may be normal or show neutrophilia, lymphocytosis or lymphopenia.
The electroencephalogram (EEG) shows diffuse slow wave activity.
CT scan is normal. MRI shows small focal white matter changes, simultaneously enhancing with contrast indicating that all are of the same degree of acuteness (unlike MS).
Diagnosis is straightforward when there is an obvious preceding viral infection or immunisation. When viral infection immediately precedes, distinction from acute encephalitis is often impossible.
Separation from acute MS may be difficult. Fever, meningeal signs with elevated CSF protein above 100 mg/ml with cell count greater than 50 per mm3 suggest ADEM.
Pathology: demyelination is limited to perivascular areas and lesions do not approach the same size as in MS.
Outcome: The illness is typically monophasic. The mortality rate is 20%. Full recovery occurs in 50%.
Poor prognosis in associated with an abrupt onset and the degree of deficit.
Treatment: Steroids are used, although no controlled trials have been conducted. Large dosage is recommended during the acute phase. Cyclophosphamide may be used in refractory cases.
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