= term generally reserved for diffuse inflammatory process of viral etiology, most commonly arthropod-borne arboviruses (Eastern + Western equine encephalitis, California virus encephalitis, St. Louis encephalitis)
- diffuse mild cerebral edema
- small infarctions / hemorrhage (less frequent)
- hyperintensity on T2WI in areas of cortical involvement
Granulomatous Amebic Encephalitis
= usually in immunocompromised patients
Organism: Acanthamoeba histolytica, Balamuthia species
Endemic: southern California, Texas, Georgia, Florida
Transmission:
- hematogenous: skin / lower respiratory tract for Acanthamoeba
- inhalation: airborne cysts in soil
- direct contamination: from skin lesion, organ transplantation, stagnant water
- headache, altered mental status, focal neurologic deficits
- hypoattenuated / T1 hypointense lesions
- heterogeneous hyperintensity on T2WI
- variably restricted diffusion + enhancement
Dx: identification of trophozoites with “spiky” pseudopodia on wet mount / Giemsa stains of CSF; positive polymerase chain reaction test
Prognosis: death within 7–10 days after onset of illness
DDx: neoplasm; acute disseminated encephalomyelitis; neurocysticercosis; toxoplasmosis
Primary Amebic Meningoencephalitis
= extremely aggressive amebic infection
Organism: free-living thermophilic ameba Naegleria fowleri = “brain-eating ameba” with predilection for warm fresh water bodies and soil
Transmission: through nasal cavity during swimming / diving activity in nature or poorly chlorinated swimming pool water
- NOT by drinking contaminated water!
Path: amebic collections in olfactory grooves + extensive destruction of olfactory tracts
- leptomeningeal enhancement, brain edema
- cerebral infarction
Herpes Simplex Encephalitis (HSE)
= most common cause of nonepidemic necrotizing meningo-encephalitis in immunocompetent individuals in USA
- Neurologic emergency due to high morbidity + mortality
Organism: HSV type I (in adults); HSV type II (in neonates from transplacental infection)
- preceding viral syndrome, low-grade fever, headache, seizures
- mental status changes: confusion, disorientation, hallucination, personality change, aphasia
Location: inferomedial temporal >frontal >parietal lobes; propensity for limbic system (olfactory tract, temporal lobes, cingulate gyrus, insular cortex); initially predominantly unilateral
- mild patchy peripheral / gyral / cisternal enhancement (50%), may persist for several months
CT:
- may be negative in first 3 days
- poorly defined bilateral areas of mildly decreased attenuation in one / both temporal lobes + insulae
- spared putamen forms sharply defined concave / straight border (DDx: infarction, glioma)
- mild mass effect with compression of lateral ventricles + loss of sylvian fissure (brain edema)
- tendency for hemorrhage + rapid dissemination in brain
MR (study of choice, positive within 2 days):
- increased signal intensity on T2WI + mild to moderate hypointensity on T1WI
- increased signal on DWI ← cytotoxic edema
- small foci of hemorrhage (common)
NUC:
Agents: standard brain imaging (eg, 99mTc-DTPA); newer brain agents (eg, 123I-iodoamphetamine / 99mTc-HMPAO)
- SPECT imaging improves sensitivity
- characteristic focal increase in activity in temporal lobes on brain scintigraphy ← breakdown of blood-brain barrier
Dx:
- identification of virus within CSF (using polymerase chain reaction technique)
- fluorescein antibody staining / viral culture from brain biopsy
Mortality: 30–70%
Rx: adenine arabinoside
DDx:
- Infarction (involves either medial or lateral temporal lobe, almost exclusively unilateral)
- Low-grade glioma
- Abscess
Human Immunodeficiency Virus Encephalitis (HIV)
often in combination with CMV encephalitis
Histo: microglial nodules + perivascular multinucleated giant cells accompanying gliosis of deep white + gray matter
- predominantly central CNS atrophy
- symmetric periventricular / diffuse white matter disease without mass effect:
- hypodense on CT, hyperintense on T2WI
following exanthematous viral illness (measles, mumps, rubella, smallpox, chickenpox, Epstein-Barr virus, varicella, pertussis) / vaccination
Acute Disseminated Encephalomyelitis (ADEM)
= POSTVIRAL LEUKOENCEPHALOPATHY
= autoimmune reaction against patient's white matter
- 7–14 days / several weeks following an exanthematous viral infection / vaccination
- confusion, headaches, fever
- seizures, focal neurologic deficits
Histo: diffuse perivenous inflammatory process resulting in areas of demyelination
Location: subcortical white matter of both hemispheres asymmetrically; may involve brainstem / posterior fossa
- lesions may demonstrate contrast enhancement
CT:
- multifocal hypodense white matter abnormalities
- sparing of cortical gray matter, occasionally deep gray matter involvement
- no additional lesions on follow-up exam
MR:
- multifocal punctate / large confluent areas of hyperintensity on FLAIR / T2WI
Rx: corticosteroids result in dramatic improvement
Prognosis: complete resolution of neurologic deficits within 1 month (80–90%) / some permanent neurologic damage (10–20%)
DDx: multiple sclerosis (rarely recurrent episodes as in multiple sclerosis); autoimmune vasculitis; aging brain
Acute Hemorrhagic Leukoencephalitis
= fulminant myelinoclastic disease of CNS
= hyperacute form of acute disseminated encephalomyelitis
Cause: immunoreactive disease following prodromal illness (minor upper respiratory viral infection, ulcerative colitis)
Path: marked edema, brain softening
Histo: necrotizing angiitis of venules + capillaries within white matter with extravasation of PMNs and lymphocytes; fibrinoid necrosis of affected capillaries + surrounding tissues; confluent hemorrhages with ball-and-ring configuration due to diapedesis of RBCs
- progressive coma, motor disturbance, speech difficulty, seizures, pyrexia, leukocytosis
- pleocytosis (= ↑ WBC in spinal / other bodily fluid)
- elevated protein in spinal fluid
Location: unilateral disease; parietal + posterior frontal white matter at level of centrum semiovale (sparing subcortical U-fibers + cortex) >basal ganglia, cerebellum, brainstem, spinal cord
- rapid development of profound mass effect resembling infarction
- multiple punctate white matter hemorrhages
- extensive hypoattenuation virtually confined to hemispheric white matter
Prognosis: usually results in death
DDx:
- Herpes simplex encephalitis (cortical lesions in temporal + inferior frontal lobes + insular region, no imaging findings until 3–5 days after onset of significant symptoms)
- Tumefactive multiple sclerosis
- Osmotic demyelination
- Toxic encephalopathy: lipophilic solvent, methanol
- Hypertensive encephalopathy: eclampsia, thrombotic thrombocytopenic purpura