Acute Disseminated Encephalomyelitis in Children

Acute Disseminated Encephalomyelitis (ADEM) in children is a complex, acute demyelinating disease of the central nervous system. This condition typically presents as a monophasic disorder, meaning it usually occurs once, and is characterized by multiple neurological symptoms alongside encephalopathy. Understanding ADEM is crucial for timely diagnosis and management, especially given its distinctions from other inflammatory demyelinating conditions in childhood, such as multiple sclerosis. This article aims to provide a comprehensive overview for students and those seeking a clear understanding of ADEM in children.

What is Acute Disseminated Encephalomyelitis (ADEM) in Children?

ADEM is considered an autoimmune disorder, where the body's immune system mistakenly attacks its own central nervous system myelin. This immune response is often triggered by an environmental stimulus in individuals who are genetically predisposed. The proposed mechanism involves molecular mimicry, where myelin autoantigens (like myelin basic protein or myelin oligodendrocyte glycoprotein) share similarities with components of an infecting pathogen.

Understanding the Immune System's Role in ADEM

The pathogenesis of ADEM involves a series of immune events. First, in the priming and activation phase, myelin protein antigens are presented to T cells in systemic lymphoid organs. These activated T cells then migrate to the central nervous system.

The subsequent recruitment and effector phase sees further immune cell migration into the CNS, driven by cytokines and chemokines. Proteases released by mast cells, T cells, and monocytes cause a breakdown of the blood-brain barrier. This leads to demyelination and axonal injury through various inflammatory processes, including oxygen and nitrogen radicals, tumor necrosis factor-alpha, complement activation, and direct axonal injury.

Following the inflammatory period, a repair phase begins. Astrocytes activate, macrophages clear debris, and oligodendrocyte precursors work to remyelinate the damaged axons. While complete recovery is common, subtle differences in repaired myelin may explain recurrent forms of ADEM.

How Common is ADEM and Who Gets It?

ADEM is an uncommon illness, with an estimated annual incidence of 0.2 to 0.5 per 100,000 children. Regional medical centers in the United States, United Kingdom, and Australia typically see about three to six cases annually.

The average age of onset in children is between 3 to 7 years. There is no specific ethnic distribution, but several studies suggest a slight male predominance. Most cases are preceded by a febrile or viral infection, often a nonspecific upper respiratory infection, occurring in about 75% of children. Some studies also observe a seasonal distribution, with more cases presenting in winter and spring.

While early reports suggested a link between ADEM and vaccination, subsequent studies have found little to no association. A study using the Vaccine Safety Datalink found no increased risk for ADEM following most vaccines, with a possible, but weakly supported, exception for the Tdap vaccine based on very few cases.

Recognizing the Signs: Clinical Features of ADEM

Clinical features of ADEM typically appear 4 to 13 days after an infection or vaccination. About 75% of children experience preceding viral or febrile illness symptoms.

Classic ADEM Presentation

Fever, headache, vomiting, and meningismus are often present at the initial presentation and may persist. Encephalopathy is a required diagnostic feature in pediatric ADEM, developing rapidly alongside multifocal neurologic deficits. It can manifest as:

• Confusion or excessive irritability
• Altered level of consciousness, from sleepiness and lethargy to coma

Acute Hemorrhagic Leukoencephalitis: A More Severe Variant

Rare, hyperacute variants of ADEM, such as acute hemorrhagic leukoencephalitis, involve inflammatory hemorrhagic demyelination of CNS white matter. These variants are more rapidly progressive and severe than typical ADEM. Symptoms are similar to classic ADEM, including meningismus, headache, seizures, multifocal and asymmetric neurologic deficits, and coma, often following an upper respiratory infection. Prognosis for survival and neurologic recovery is generally worse in these hemorrhagic forms.

Key Neurological Symptoms to Watch For

Progression to maximum neurological deficits usually occurs over four to seven days. Common features include:

• Long tract (pyramidal) signs
• Acute hemiparesis
• Cerebellar ataxia
• Cranial neuropathies, including optic neuritis
• Myelopathy (transverse myelitis)

Optic neuritis can cause vision loss, pain with eye movement, and an afferent pupillary defect. Transverse myelitis leads to flaccid paralysis of the legs with a sensory level; arm involvement and even respiratory failure can occur with cervical cord lesions. Bowel and bladder dysfunction are also common.

Seizures occur in about one-third of patients, while aphasia, movement disorders, and sensory deficits are less common.

The Clinical Journey and Recovery

The severe phase of ADEM typically lasts two to four weeks. Many children require hospitalization, often for 13 to 27 days, and up to 25% may need intensive care for severe encephalopathy, seizures, or paralysis affecting the diaphragm. While ADEM can be fatal, the mortality rate is low, and most patients recover completely. Some may experience neurologic sequelae.

A minority of patients may develop multiphasic ADEM (MADEM) with long-term follow-up, characterized by relapsing episodes or new lesions on brain MRI.

Diagnosing ADEM in Children: The Evaluation Process

Diagnosing ADEM requires a comprehensive approach, as there are no specific biomarkers. The diagnosis relies on clinical and radiological features and is considered a diagnosis of exclusion.

When to Suspect ADEM

ADEM should be suspected in a child who develops multifocal neurologic abnormalities with encephalopathy (e.g., confusion, excessive irritability, or altered level of consciousness), especially if the onset occurs within two weeks after a viral infection.

Initial Evaluation Steps for Suspected ADEM

Children with suspected ADEM require urgent investigations. The general approach includes:

• Magnetic Resonance Imaging (MRI) of the brain and spine, with and without contrast.
• Lumbar puncture for cerebrospinal fluid (CSF) analysis, including assessment for oligoclonal bands.
• Additional laboratory studies to rule out infection.
• Testing for anti-myelin oligodendrocyte glycoprotein (MOG) and anti-aquaporin 4 (AQP4) antibodies.

Neuroimaging: Looking Inside the Brain and Spine

MRI is crucial for evaluating ADEM abnormalities, particularly T2-weighted images, FLAIR sequences, and contrast-enhanced MRI with gadolinium. CT scans are often normal or nondiagnostic.

Brain MRI typically reveals:

• Diffuse, poorly marginated, large (>1 to 2 cm) lesions, often bilateral and asymmetric, predominantly in the deep and subcortical white matter. The periventricular white matter is often spared.
• Multiple lesions, varying in number and size (from <5 mm to 5 cm).
• Frequent involvement of the thalami and basal ganglia, often symmetrically. Supratentorial lesions are usually asymmetrical.
• Gray matter lesions sometimes accompany white matter abnormalities, especially in children.
• Hemorrhagic demyelinating lesions may be seen in hyperacute variants like acute hemorrhagic leukoencephalitis, identifiable with susceptibility-weighted MRI sequences.
• Gadolinium contrast enhancement is variable but can be seen in acute lesions.
• Diffusion-weighted MRI (DWI) shows restricted diffusion in acute lesions (within seven days), which then increases within a few weeks.
• Lesions may progress over a short time and often improve with recovery, though some can persist.

Spinal cord MRI commonly shows abnormalities, often presenting as longitudinally extensive transverse myelitis, with lesions extending over multiple MRI segments. This contrasts with the more typical segmental myelitis of multiple sclerosis.

Orbital imaging with dedicated orbital MRI or retinal nerve fiber layer (RNFL) thinning on optical coherence tomography (OCT) may suggest ADEM due to MOGAD, particularly when clinical diagnosis of optic neuritis is difficult in young children.

Sequential MRI during follow-up is sometimes needed to confirm the diagnosis, as new lesions and relapses are not compatible with monophasic ADEM.

Cerebrospinal Fluid (CSF) Analysis

Lumbar puncture for CSF analysis is essential to rule out infection. CSF testing includes cell counts, protein, glucose, culture, and viral studies (e.g., influenza, Epstein-Barr virus, herpes, varicella, mycoplasma, cytomegalovirus, rubella).

While CSF can be normal, inflammation is common, with pleocytosis (usually <100 cells/microL, rarely up to 600 cells/microL) and/or increased protein concentration. Increased opening pressure may also be present. In acute hemorrhagic leukoencephalitis, CSF typically shows both white and red blood cells with increased protein.

CSF oligoclonal immunoglobulin G (IgG) bands are seen in ≤20% of children with ADEM. These bands, indicating intrathecal synthesis of immunoglobulins, are a nonspecific finding more often associated with multiple sclerosis or chronic CNS infections.

Additional Evaluation for Infection

Beyond CSF analysis, other investigations for infectious agents may include:

• Complete blood count (CBC)
• Blood cultures
• Viral cultures of the throat, nasopharynx, and stool
• Serologic testing for various agents like influenza, Epstein-Barr virus, herpes simplex, hepatitis A, HIV, measles, rubella, varicella zoster, West Nile, Borrelia burgdorferi, Chlamydia, Leptospira, Mycoplasma pneumoniae, Rickettsia, and Beta hemolytic Streptococcus.

Serum and CSF next-generation sequencing techniques can also help identify a broad array of potential causative pathogens.

Serum Autoantibodies

Testing for anti-myelin oligodendrocyte glycoprotein (MOG) IgG autoantibody and anti-aquaporin-4 (AQP4) IgG serum autoantibody is recommended. MOG-IgG seropositivity is found in 33% to 66% of children with ADEM. Patients with ADEM who are MOG antibody-positive meet criteria for MOG antibody-associated disease (MOGAD), a distinct antibody-mediated disorder that can present as ADEM, optic neuritis, or transverse myelitis.

Anti-AQP4 IgG antibody is a specific biomarker for neuromyelitis optica spectrum disorder (NMOSD).

Other Studies in Selected Patients

If atypical clinical features are present, further studies may be needed to rule out mimics such as central nervous system infection, malignancy, systemic autoimmune disease, genetic syndromes, or inborn errors of metabolism. This might involve lab studies for rheumatologic conditions, whole exome sequencing, or metabolic studies.

Limited Role of Brain Biopsy

Brain biopsy is infrequently needed, primarily to rule out ADEM mimics like CNS malignancy or primary CNS vasculitis when diagnostic uncertainty persists despite extensive work-up. Large lesions that mimic tumors can raise concerns for malignancy, and a biopsy may be necessary if other less invasive studies are inconclusive.

Making the Diagnosis: Key Criteria

The diagnosis of ADEM is based on clinical and radiological features, as outlined by the International Pediatric Multiple Sclerosis Study Group. The key diagnostic criteria are:

1. A first polyfocal, clinical central nervous system event with presumed inflammatory demyelinating cause.
2. Encephalopathy that cannot be explained by fever, systemic illness, or postictal symptoms.
3. No new clinical and MRI findings emerge three months or more after the onset.
4. Brain MRI is abnormal during the acute (three-month) phase, showing diffuse, poorly demarcated, large (>1 to 2 cm) lesions predominantly involving the cerebral white matter. Deep gray matter lesions (e.g., basal ganglia or thalamus) can be present, but T1 hypointense lesions in the white matter are rare.

Encephalopathy as a presenting symptom is a required feature that helps distinguish ADEM from other clinically isolated syndromes.

Differential Diagnosis: What Else Could It Be?

Given that ADEM is a diagnosis of exclusion, it's vital to consider and rule out other conditions with similar symptoms.

• Bacterial and Viral Meningitis or Encephalitis: These are critical to exclude, especially in children with fever and meningeal irritation. CSF analysis is key, looking for pleocytosis, elevated protein, and pathogen identification.
• Multiple Sclerosis (MS): A chronic demyelinating disease with recurrent episodes separated in space and time. ADEM usually has a monophasic course, and encephalopathy is a distinguishing feature.
• Myelin Oligodendrocyte Glycoprotein Antibody-Associated Disease (MOGAD): An antibody-mediated group of demyelinating conditions that can include ADEM, relapsing optic neuritis, or transverse myelitis. MOGAD is a common cause of ADEM in children.
• Neuromyelitis Optica Spectrum Disorder (NMOSD): An inflammatory disorder causing immune-mediated demyelination and axonal damage, predominantly leading to optic neuritis, acute transverse myelitis, or area postrema syndrome. AQP4 antibodies are a specific biomarker.
• Primary Angiitis of the Central Nervous System (PACNS): A vasculitis presenting with encephalopathy, focal deficits, or seizures. Suspicion is heightened with contrast-enhancing lesions with diffusion restriction in a single vascular territory, or relapsing disease. Brain biopsy may be needed for diagnosis.
• Central Nervous System Malignancy: Brain tumors can present with nonspecific symptoms (headache, vomiting) or CNS-specific signs (ataxia, cranial nerve palsies, seizures).

Frequently Asked Questions About ADEM in Children

What is the main cause of Acute Disseminated Encephalomyelitis in children?

ADEM in children is primarily an autoimmune disorder, meaning the immune system mistakenly attacks the body's own myelin. This is typically triggered by a recent infection, such as a viral illness, through a mechanism called molecular mimicry, where immune cells confuse parts of a pathogen with myelin proteins.

How is ADEM diagnosed in a child?

Diagnosing ADEM in children involves a combination of clinical evaluation, neuroimaging (especially MRI of the brain and spine), and cerebrospinal fluid (CSF) analysis via lumbar puncture. Doctors look for specific patterns of neurological symptoms, particularly encephalopathy, alongside characteristic lesions on MRI. Blood tests for antibodies like MOG-IgG are also important, and other conditions, especially infections, must be ruled out.

What are the typical symptoms of ADEM in children?

Typical symptoms of ADEM in children often follow a viral infection and can include fever, headache, vomiting, and meningismus. Crucially, children will develop encephalopathy, which means changes in mental state like confusion, excessive irritability, or altered consciousness. Other neurological signs can involve weakness or paralysis on one side of the body, problems with coordination (ataxia), vision loss (optic neuritis), or spinal cord issues leading to leg weakness and bladder/bowel problems. Seizures can also occur.

What is the prognosis for children with ADEM?

The prognosis for most children with ADEM is generally good, with many recovering completely from the acute illness. However, some children may experience lingering neurological deficits. While severe cases may require intensive care and can rarely be fatal, the overall mortality rate is low. A minority of children might experience a multiphasic form of ADEM (MADEM) with relapsing episodes.