What is ADEM

Acute disseminated encephalomyelitis (ADEM) accounts for around 10% of all known cases of encephalitis.  ADEM usually affects children and begins after a childhood rash (exanthema), other viral infections or immunisations. There is usually a latent period of days to two to three weeks before symptoms emerge. The illness has been poorly understood and a variety of terminologies are used to describe it, these including post-viral, post-infectious or para-infectious encephalitis.

The white matter of the brain (which contains nerve fibres and myelin) is predominantly affected in ADEM. Under the microscope it can be seen that there is invasion with immune cells from the blood. Where these cells accumulate, myelin (the protective fatty substance around nerves) is destroyed.

Symptoms

The symptoms can be similar despite different causes. The illness usually begins with less-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 and occasionally comatose. Neurological features which may be detected include visual deterioration, clumsiness in arms and legs, paralysis down one side and seizures. The duration of these symptoms is variable. Some cases last a few weeks to a month, while other fatal cases have a rapid progressive course over several days. The symptom that correlates most closely with prognosis is the level of consciousness.

The illness usually has a 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 develop multiple sclerosis, although this area remains controversial.

Investigations

The cerebrospinal fluid (CSF) is frequently abnormal showing an increase in white cells and protein. The electroencephalogram (EEG) is abnormal in most cases showing diffuse slowing. Magnetic resonance imaging (MRI) typically shows multiple areas of abnormality in the white matter of the brain that can be rather characteristic.

Differential diagnosis

The differential diagnosis of ADEM includes acute meningitis, acute viral encephalitis, autoimmune 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 a specific viral antibody may occur. To distinguish ADEM from multiple sclerosis in the initial phase may be more difficult. MRI and CSF examination may help.

Evidence for an immunological cause

There is a general agreement that a causative organism cannot be isolated from the brain of patients with ADEM.  The association of the disease with a previous infection or immunisation suggests an immunological process. Detailed laboratory studies involving measurement of anti-brain antibodies and of cellular immune responses to specific brain antigens suggest that these patients have developed an allergic immune response attack against their own brain constituents, and this is an ‘autoimmune’ response.

Treatment

The ideal form of treatment is immunomodulation (to dampen down the immune system) which should be started once the diagnosis is made as it has more benefit when given early.  The diagnosis, however, may be difficult to make swiftly.  High doses of steroids (drugs used to reduce inflammation) can often lead to a rapid resolution of symptoms with an excellent prognosis.

By Professor Clive Hawkins, Professor of Clinical Neurology, Consultant Neurologist, University Hospital of North Staffordshire and reviewed by Dr Sarosh Irani, John Radcliffe Hospital, Oxford and Dr. Arun Venkatesan, Johns Hopkins Hospital, Baltimore, MD, USA

FS009V4  ADEM

Date created November 2001; Date updated: October 2021; Review date: October 2024

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