ADEM, also known as postinfectious encephalomyelitis,
usually follows either a vaccination within the preceding
four weeks, or an infection which may be a childhood
exanthema such as measles, rubella or chickenpox, or else a
systemic infection characteristically affecting the respiratory
or gastrointestinal systems.7 There is very good evidence from
various sources to suggest an immune pathogenesis of this
disorder, with an abnormal immune reaction directed against
normal brain. Potential immune mechanisms include molecular
mimicry between epitopes expressed by myelin antigens
and viral or other pathogens, immune dysregulation, superantigen
induction, and a direct result of the infection.8 It is
important to distinguish ADEM from acute infectious
encephalitis, acute non-infectious encephalitis, and an acute
metabolic or toxic encephalopathy.8 While the diagnosis may
not always be immediately obvious, there are a number of
helpful clues in the history, examination, and investigative
profile that may greatly assist in the distinction between
ADEM and an infectious encephalitis. These differences have
been very clearly identified by Davis7 and are summarised in
table 4. The typical clinical picture is of a younger person with
a history of vaccination or infection presenting abruptly,
without fever at the onset of symptoms, and with multifocal
neurological signs affecting both the CNS and possibly the
peripheral nerve roots.9 It should also be appreciated that
ADEM can also present with a very restricted neurological
picture such as transverse myelitis, optic neuritis, and
cerebellar ataxia, and differentiation from acute multiple
sclerosis can sometimes be difficult. As is the case for table 3,
these various diagnostic pointers should be taken together
and not in isolation when trying to make the diagnosis of
ADEM.
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