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Acute Disseminated Encephalomyelitis
This paper was prepared by Dr Clive Hawkins Consultant Neurologist / Senior Lecturer Royal Infirmary, Stoke on Trent
Acute disseminated encephalomyelitis (ADEM) accounts for up to one third of all known cases of encephalitis. This illness usually follows in the wake of exanthema or after other viral infections or immunisations. There is usually a latent period of days to two to three weeks. This illness was first described 250 years ago by the distinguished English physician, Clifton who noted that it occurred occasionally in patients who had smallpox. The white matter of the brain is predominantly affected and under the microscope it can be seen that there is invasion around small veins by white blood cells from the blood. Where these cells accumulate myelin is destroyed. The illness has been poorly understood and a variety of terminologies used to describe it, these including post infectious, parainfectious or post vaccinial.
Clinical Presentation
The clinical presentation of ADEM despite different causes is similar. The illness usually begins with non specific symptoms such as fever, headache, stiff neck, vomiting and anorexia. These are rapidly followed by depression of consciousness in which the patient may become confused, stuporous, delirious and occasionally entering into coma. During this early period neurological examination usually shows focal neurological signs such as bilateral optic neuritis, ataxia of the limbs, clumsiness in walking, paralysis down one side and seizures may occur. The duration of these symptoms is variable, some cases lasting a few weeks to a month, and other fatal cases having a rapid progressive course over a number of days. The clinical sign that correlates most closely with the prognosis is the level of consciousness. The illness usually has monophasic course i.e. once it is over, further attacks rarely develop. Recently long term studies of patients with ADEM have shown that a small number later on develop multiple sclerosis.
Investigations
The cerebrospinal fluid is frequently abnormal showing an increase in white cells and protein. The electroencephalogram is abnormal in most cases showing diffuse slowing. Magnetic resonance imaging typically shows multiple areas of abnormality in the white matter of the brain.
Differential Diagnosis
The differential diagnoses of ADEM include acute meningitis, acute viral encephalitis and acute multiple sclerosis. Differentiation of these diseases is not easy, certainly in the early stages. In viral encephalitis the CSF is often abnormal and a rise in specific viral antibody may occur. To distinguish ADEM from multiple sclerosis in the initial phases may be more difficult. Magnetic resonance imaging and CSF examination may help.
Pathology
The brain at post mortem may appear entirely normal or may show the signs of congestion. Histologically the basic lesions consist of infiltrations of mononuclear cells from the blood which occur around small veins in the white matter. Demyelination occurs and is limited to the area of the perivenous cellular cuff. These are different from the lesions found in multiple sclerosis.
Evidence for an Immunological aetiology
There is general agreement that a causative organism cannot be isolated from the central nervous system of patients with ADEM. The association of the disease with an antecedent infection or immunisation suggests an immunological process and detailed laboratory studies involving measurement of anti-brain antibodies and of cellular immune responses to specific myelin antigens have shown that these patients indeed have mounted an allergic response against their own brain constituents.
Treatment
The ideal form of treatment is immunomodulation to be instituted without delay once the diagnosis is made. High doses of steroids can often lead to a very rapid resolution of symptoms with an excellent prognosis. Overall the prognosis is good where the diagnosis is made early and the appropriate therapy instituted without delay.
Last modified: 15/11/2006
