Acute disseminated
encephalomyelitis
What
is acute disseminated encephalomyelitis?
Acute disseminated encephalomyelitis (ADEM) is a rare
neurological disorder. It affects children more than adults, but can affect
anyone.
What
are the symptoms of acute disseminated encephalomyelitis?
More
than half of patients have an illness, usually an infection, two to four weeks
before developing ADEM. Most of these illnesses are viral or bacterial, often
no more than an upper respiratory tract infection. In children with ADEM,
prolonged and severe headaches occur. In addition, the patient develops fevers
during the ADEM course.
Along
with this pattern, the patients usually get neurological symptoms which may
include:
- confusion,
drowsiness, and even coma
- unsteadiness
and falling
- visual
blurring or double vision (occasionally)
- trouble
swallowing
- weakness
of the arms or legs
In
adults with ADEM, motor (movement) and sensory (tingling, numbness) symptoms
tend to be more common. Overall, what triggers a diagnosis of ADEM is a rapidly
developing illness with neurological symptoms, often with fever and headache,
usually following an upper respiratory tract infection, and which has
significant MRI and spinal fluid findings consistent with ADEM.
What causes ADEM?
The cause of ADEM is not clear but in
more than half of the cases, symptoms appear following a viral or bacterial
infection, usually a sore throat or cough and very rarely following
vaccination. ADEM is thought to be an autoimmune condition where the body’s
immune system mistakenly identifies its own healthy cells and tissues as
foreign and mounts an attack against them. This attack results in
inflammation. Most cases of ADEM begin about 7 to 14 days after an
infection or up to three months following a vaccination. In some cases of ADEM,
no preceding event is identified.
Pathology
Thought to occur from
a cross reactivity in immunity to viral antigens triggering a subsequent
autoimmune attack on the CNS.
The pathological
hallmark is perivenular inflammation with limited ‘‘sleeves of
demyelination", which is also a feature of multiple sclerosis.
However, multiple sclerosis typically present confluent sheets of
macrophage infiltration mixed with reactive astrocytes in completely
demyelinated regions
How is ADEM different
from MS?
- In most but not
all cases, ADEM occurs only once, while patients with MS have further,
repeated attacks of inflammation in their brains and spinal cords.
- In most cases, ADEM
patients do not develop new scars on a repeat MRI scan whereas MS patients
typically experience new scars on their follow-up MRI scans.
- Typical symptoms
of ADEM such as fever, headache and confusion, vomiting, and seizures are
not usually seen in people with MS, although they can be seen in pediatric
MS onset especially in patients younger than 11 years.
- Sometimes the
pattern of MRI abnormalities helps differentiate these two disorders.
- Most patients with
MS are treated with ongoing medication to prevent attacks. Patients with
ADEM do not require such medication.
- ADEM occurs more
frequently in males; MS more frequently in females.
- ADEM is more
common in children; MS is more common in adults.
- ADEM occurs more
frequently in winter and spring; MS has no seasonal variation.
Radiographic
features
Appearances vary from small punctate
lesions to tumefactive regions, which have less mass effect than one would
expect for their size, distributed in the supratentorial or
infratentorial white matter. Compared to multiple sclerosis,
involvement of thecallososeptal
interface is unusual. Lesions are usually bilateral but
asymmetrical. Involvement of cerebral cortex, subcortical grey matter,
especially the thalami, and thebrainstem is not very common, but if present are
helpful in distinguishing frommultiple sclerosis .
In addition to lesions involving grey
matter, especially in the setting of post-streptococcal pharyngitis, antibodies
to basal ganglia can also develop resulting in more diffuse involvement
Spinal cord may show confluent
intramedullary lesions with variable enhancement.
CT
The lesions
are usually indistinct regions of low density within the white matter and may
demonstrate ring enhancement.
MRI
MRI is far more sensitive
than CT:
·
T2: demonstrates regions of high
signal, with surrounding oedema typically situated in subcortical locations;
the thalami and brainstem can also be involved
·
T1 C+ (Gd): punctate, ring
or arc enhancement (open ring
sign) is often demonstrated along the leading edge of
inflammation; absence of enhancement does not exclude the diagnosis
·
DWI: there can be peripheral
restricted diffusion; the center of the lesion, although high on T2 and low on
T1 does not have increased restriction on DWI (c.f. cerebral abscess); nor
does it demonstrate absent signal on DWI as one would expect from a cyst, this
is due to increase in extracellular water in the region of demyelination
Magnetization
transfer may help
distinguish ADEM from MS, in that normal appearing brain (on T2 weighted
images) has normal magnetization
transfer ratio (MTR) and normal diffusivity,
whereas in MS both measurements are significantly decreased
What are the causes and/or risk factors associated
with acute disseminated encephalomyelitis?
We
know that ADEM usually follows an infection of some kind. In 50 percent to 75
percent of cases, the beginning of the disease is preceded by a viral or
bacterial infection, usually a sore throat or cough (upper respiratory tract
infection). Many different bacteria, viruses and other infections have been
related to ADEM, but the disease does not appear to be caused by any one
infectious agent. Most cases of ADEM begin about 7 to 14 days after the
infection.
On occasion, ADEM occurs after a
vaccination. This is rare overall, but when it happens, it usually occurs after
the measles, mumps, and
rubella vaccination. In these cases, ADEM may occur up to three months after
the vaccination.
ADEM
appears to be an immune reaction to the infection. In this reaction, the immune
system, instead of fighting off the infection, causes inflammation in the
central nervous system. Inflammation is defined as the body's complex
biological response to harmful stimuli, such as infectious agents, damaged
cells, or irritants. Inflammation is a protective attempt to remove the
injurious stimuli and initiate the healing process. In the case of ADEM, the
immune response is also responsible for demyelination, a process in which the
myelin that covers many nerve fibers is stripped off.
How is acute disseminated encephalomyelitis
treated?
ADEM
is a rare disease, and so there are no well-designed clinical trials comparing
one treatment with placebo, or one treatment with another. Everything we know
about treatment in ADEM comes from small published series of cases, and there
are no guidelines for treatment of ADEM yet.
At this time, intravenous
methyl-prednisolone (for instance, Solu-Medrol®) or other steroid medications are the front-line treatment for ADEM.
Usually these medications are given over a five- to seven-day course, followed
by a tapering dose of oral steroids. The aim is to reduce inflammation and
speed recovery from the disease.
Patients
on steroids need to be monitored for increased blood glucose, low potassium,
and sleep disturbance. There may be mood changes (irritability, crying,
anxiety) when people are on steroid therapy. Other short-term complications of
steroid therapy include weight gain, flushed cheeks, facial swelling, and a
metallic taste (when using IV Solu-Medrol).
If
a patient does not respond to IV methylprednisolone, the next line of treatment
may be intravenous immune globulin (IVIG). This is an intravenous treatment
using a blood product which has been shown to reduce the activity in certain
immune diseases, including ADEM. Treatment is usually given for a few hours
daily over five days for ADEM. IVIG has the same risks as any blood product
(allergic reaction, infection); it also sometimes causes shortness of breath
due to fluid overload. Rarely, patients lack an antibody important to the
system and may react more strongly to IVIG.
Another approach to treatment is a
process called plasmapheresis. This
is a treatment in which the blood is circulated through a machine that
withdraws components of the immune system from the circulation, reducing immune
activity. It is usually a process which takes a few hours and is done every
other day for 10 to 14 days, often as part of a hospital stay. It may require
the placement of a central venous catheter to allow for blood to be removed
from the system rapidly. Risks of plasmapheresis include discomfort from taking
blood, sometimes a tendency to bleed due to a reduction in platelets, and
infections.
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