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be near a ventricle, in the basal ganglia, or near subarachnoid space. Mass effect and edema are
frequently present.[561] By magnetic resonance imaging (MRI) the lesions are hyperintense
with T1 weighting and isointense to hyperintense masses with moderate edema and mass effect
with T2 weighting, and there is homogenous or ring enhancement with contrast.[778] Features
that suggest malignant lymphoma, rather than toxoplasmosis, include: periventricular location
(particularly in deep white matter), solitary lesion, homogenous enhancement of a lesion greater
than 2 cm in size, and limited edema or mass effect.[759]
Grossly, the most common pattern for CNS lymphomas is that of widespread infiltration
without a discrete mass lesion, whether unifocal or multifocal. Most occur above the tentorium.
Microscopically, they are often difficult to classify, particularly in small biopsies with extensive
necrosis, though all are high grade and most are of an immunoblastic or large cell type. Almost
all demonstrate bcl-2 gene expression.[782] Whether a prominent mass is seen or not, there is
generally extensive perivascular spread in the brain or spinal cord. Necrosis may also be
extensive. In about one fourth of AIDS patients with lymphoma, only the CNS is involved.
Prognosis is poor, with survival of only a few months despite treatment.[746,755,756]

KAPOSI'S SARCOMA.-- Kaposi's sarcoma involvement of the CNS is extremely rare.
It may represent widespread involvement.[783]

HERPES VIRUSES.-- Herpes simplex virus type 1 (HSV-1) is occasionally reported in
the central nervous system in AIDS. Varicella-zoster virus (VZV) and even HSV-2 have been
identified in the brain lesions of AIDS patients who have had a clinical and radiologic picture
corresponding to that of progressive multifocal leukoencephalopathy (PML). Although these
cases may mimic PML very closely, computed tomographic or magnetic resonance imaging
scans can show evidence of hemorrhage, a mass effect, or gray matter involvement.[755,756]
Grossly, areas of necrosis may appear most commonly in temporal lobe, inferior frontal lobe,
insula, or cingulate gyrus. Microscopically, the lesions can have petechiae with fibrinoid
necrosis, perivascular mononuclear inflammatory cell infiltrates, and Cowdry type A inclusions
in either neurons or glial cells. Immunohistochemical staining for HSV is helpful.
Herpes simplex virus infection of the CNS can have a varied clinical presentation,
including confusion, fever, headache, anxiety, depression, and memory loss. The diagnosis can
be made in most, but not all, cases by PCR performed on CSF. Most patients respond to therapy
with acyclovir or valacyclovir.[784]
Varicella-zoster virus (VSV) involvement of the central nervous system with AIDS can
have several patterns. There can be multifocal leukoencephalitis, mainly involving the deep
white matter and grey-white junction. Ventriculitis and/or periventriculitis may be accompanied
by vasculitis and necrosis of the ventricular wall. The large amount of virus present leads to the
appearance of many intranuclear Cowdry type A inclusions. Also seen are acute hemorrhagic
meningo-myeloradiculitis with necrotizing vasculitis, focal necrotizing myelitis, and
leptomeningeal arterial vasculopathy with cerebral infarction. A characteristic VSV skin
eruption may not be seen in cases of brain involvement. However, infections can involve skin,
viscera, spinal cord, and brain. Patients may have headache, confusion, and focal weakness.
The clinical course can be protracted. The syndrome of post herpetic neuralgia, which is the
persistence of pain lasting for more than 4 to 6 weeks following resolution of the skin lesions of
VSV, may be seen in 8 to 15% of persons with HIV infection, particularly those that are
elderly.[462,785]