Second edition

14.qxd 3/10/08 9:50 AM Page 497
degree of inflammation. At least initially, these foci are
generally few in number and confined to one hemisphere;
most commonly they are seen in the subcortical white matter
and the centrum semiovale; however, rarely they may
be prominent in the cerebellar white matter or the brainstem.
Over time, the foci increase in size and number, and
bilateral involvement occurs.
Differential diagnosis
In patients with AIDS, consideration must be given to
AIDS dementia and to other opportunistic infections, such
as CMV or toxoplasmosis, and to primary central nervous
system lymphoma.
Treatment
No specific treatment is available. In patients with AIDS it
is clear that treatment with HAART slows the progression
of the disease. The general treatment of dementia is discussed
in Section 5.1.
14.4 ARBOVIRUS MENINGOENCEPHALITIS
Viruses transmitted by arthropods are known as arboviruses,
a term derived from the fact that they are all arthropod borne.
In North America, there are seven arboviruses known to
cause meningoencephalitis, including six mosquito-borne
viruses (eastern equine, western equine, Venezualan,
St. Louis, La Crosse, and the newest member, West Nile)
and one tick-borne virus (Powassan virus). Most cases
occur in the late summer and early fall, when mosquitoes
are most active. Mention should also be made of Japanese
encephalitis, which, although not endemic in North
America, is a very common cause of meningoencephalitis
in the Far East (Lewis et al. 1947; Solomon et al. 2000,
2002) and may be contracted by travellers there.
Clinical features
The onset is typically acute, over a matter of days or, exceptionally,
merely hours. Patients present with delirium,
fever, and, typically, meningeal signs such as headache, stiff
neck, and photophobia. Seizures, focal signs, and abnormal
movements may or may not occur, and some patients
may develop a syndrome of inappropriate antidiuretic
hormone (ADH) secretion. Patients may progress to stupor
or coma. Although distinguishing among the various
pathogens on clinical grounds is difficult, some features
may be helpful: St. Louis encephalitis may be marked by a
coarse tremor (Wasay et al. 2000); West Nile encephalitis
by a rash, flaccid paralysis, and cranial nerve palsies (Davis
et al. 2006); and Japanese virus by parkinsonism (Pradhan
et al. 1999) or dystonia (Kalita and Misra 2000).
14.4 Arbovirus meningoencephalitis 497
The peripheral white blood cell count is typically
elevated.
Magnetic resonance scanning may be normal early on;
however, eventually both T2-weighted and FLAIR imaging
will reveal areas of increased signal intensity, which may be
seen in the thalami, basal ganglia, and the cortex.
The CSF may be under increased pressure. The white
blood cell count is typically increased. Early on, polymorphonuclear
cells may predominate; however, over time the
pleocytosis becomes lymphocytic. The total protein is
increased but the glucose is normal. Both PCR assay and
assays for specific IgM antibodies are available for all of the
arboviruses.
The EEG typically shows generalized slowing, at times
with focal predominances; in some cases interictal epileptiform
discharges may be present.
Course
The mortality rate varies from as little as 1 percent for La
Crosse virus to close to 50 percent for eastern equine
encephalitis. For those who survive, the encephalitis tends
to run its course within a matter of a few weeks, sometimes
longer. A minority of patients will be left with sequelae,
such as dementia, personality change, or a persistence of
any focal signs or abnormal movements seen during the
acute illness (Herzon et al. 1957; Przelomski et al. 1988;
Smardel et al. 1958); in this regard, post-encephalitic parkinsonism
has been noted after western equine encephalitis
(Mulder et al. 1951; Schultz et al. 1977) and dementia after
Japanese encephalitis (Solomon et al. 2002), which, in some
cases, may have prominent psychotic symptoms (Richter
and Shimojyo 1961).
Etiology
After the mosquito or tick bite, hematogenous spread carries
the virus to the brain. Although the severity of the
pathologic changes varies widely depending on the responsible
virus, in general one finds widespread perivascular
inflammation and areas of focal cerebritis in the leptomeninges,
cortical gray matter, cerebral white matter,
subcortical gray structures, and, in some cases, brainstem.
At times, thrombus formation may occur in the vessels
involved, with infarction (Leech and Harris 1977; Reyes
et al. 1981).
Differential diagnosis
As discussed in Section 7.6, arboviral meningoencephalitis
represents just one cause of acute encephalitis, and the
reader is directed to that section for a discussion of the differential
possibilities.