Acute disseminated encephalomyelitis overview

# Overview
Acute disseminated encephalomyelitis (ADEM) is an acute neurologic disease of the central nervous system characterized by scattered foci of demyelination and perivascular inflammation.  The disease may occur withoutprecipitant, or may develop after infection or vaccination.

# Historical Perspective
Encephalomyelitis was primarily an experimental finding discovered after injection of rabbits with rabbit spinal cord infused with solutions of sodium chloride and lecithinase, adjuvants, like ,] and killed tubercle bacilli, and human spinal cord treated with formaldehyde solution. Clinical signs were spasticity, ataxia, weakness and even death. The perivascular inflammatory lesions had a predilection for the white matter and demyelination was seen in all ages. Older lesions had glial scars.

# Classification
Acute disseminated encephalomyelitis can occur after an episode of infection (post-infectious), vaccination or even, spontaneously without an obvious trigger (idiopathic).

# Pathophysiology
The exact mechanism of acute disseminated encephalomyelitis is not determined. However, it is usually preceded by an environmental trigger, e.g. an infection or vaccination and affects individuals with a genetic predisposition.

# Causes
In the past, ADEM was a sequelae of common childhood infections like measles, smallpox and chickenpox. With the improvements in infectious disease management practices, ADEM in developed countries frequently follow non-specific upper respiratory tract infections. Failure to identify a definite cause could suggest that the inciting agents are unusual and not recovered by standard laboratory tests. ADEM is much more common in the developing countries where measles and other viral infections still account for a major proportion of cases.
Another common variant is the postimmunisation encephalomyelitis, which has a preference for the peripheral nervous system. Currently, the most common implicated vaccines are measles, mumps and rubella. The risk of the condition developing after measles vaccination is considerably lower than the natural infection.

# Differentiating Acute disseminated encephalomyelitis from Other Diseases
The differential diagnoses of Acute disseminated encephalomyelitis include:
- Viral encephalitis
- HIV encephalopathy
- Multiple sclerosis
- Antiphospholipid antibody syndrome

# Epidemiology and Demographics
The average annual incidence of ADEM is between 0.07 and 0.6 per 1,00,000 individuals per year. It is more common during childhood, with a median age on onset between 5-8 years and a male predominance (1.8:1).

# Risk Factors
The risk factors for ADEM include:
- Infections (most common)
- Vaccinations
- Genetic predisposition

# Screening
There is no screening tool currently used for Acute disseminated encephalomyelitis.

# Natural History, Complications, and Prognosis
The classic form, accounting for 70-90% of cases, typically follows a monophasic pattern. Residual severe disability is quite rare in pediatric ADEM cases (7%). Adult patients frequently suffer from residual ataxia, clumsiness, hemiparesis or epilepsy. The poor prognosis and long-term outcomes of ADEM have changed dramatically owing to efficient vaccination coverage and widespread, early use of high-dose steroids.

# Diagnosis

## Diagnostic Study of Choice
MRI is the best method for further evaluation after an initial suspicion of ADEM. MRI brain with gadolinium enhancement is indicated in stable patients whereas, MRI of the dorsal and cervical spinal cord can determine the extent of inflammation in symptoms and signs suggestive of myelopathy.

## History and Symptoms
Classic ADEM is monophasic, with a history of usually a preceding illness or less commonly, a vaccination. It is characterised by an acute onset of focal neurologic symptoms, often with rapid deterioration of consciouness, after a variable latent period of several days to few months.

## Physical Examination
Physical examination of ADEM may reveal positive findings in different parts of the nervous system namely, the cerebral cortex, brainstem, cranial nerves and the sensory and motor tracts. Respiratory failure may prove fatal in some cases.

## Laboratory Findings
Antibodies like anti-MOG antibodies are found in the serum of patients suffering from ADEM. A lumbar puncture is also useful for evaluation of the changes in the Cerebrospinal fluid (CSF).

## Imaging Findings
The appearance of ADEM on cranial CT has rarely been reported. There is a delay between the onset of the clinical signs and the appearance of lesions on CT scan. The correlation was limited between the clinical course and the anatomical distribution and type of abnormality seen on CT scan.
MRI is the best method for further evaluation after an initial suspicion of ADEM. MRI brain with gadolinium enhancement is indicated in stable patients whereas, MRI of the dorsal and cervical spinal cord can determine the extent of inflammation in symptoms and signs suggestive of myelopathy.

## Other Diagnostic Studies
Many experts discourage diagnostic biopsies in the absence of structured histopathological guidelines. Nevertheless, typical lesions of ADEM may be identified by histopathology anywhere in the white matter of brain and spinal cord.

# Treatment

## Medical Therapy
The analogy between the pathogenesis of ADEM and MS forms the basis of the use of high-dose steroids, plasma exchange and intravenous immunoglobulin for the treatment of ADEM.

## Surgery
Surgical interventions can be helpful for managing complications like cerebral edema, optic neuritis and elevated intracranial pressure. The options include:
- Decompressive hemi-craniectomy
- Bilteral optic nerve sheath decompression]]
- Lumboperitoneal shunting

## Prevention
The management protocol for ADEM does not routinely include strategies for primary prevention.Case reports and literature reviews have highlighted the importance of early MRI scans of the brain as a guide for immediate intervention for ADEM.Plasmapheresis should be considered early in the disease course for severe or life-threatening cases of ADEM.