Infectious Brain Pathology 

Acute bacterial meningitis
- Affects meninges
- Hematogenous to CSF or brain vs direct inculation
- Usually immunocompetent host
- Edema of brain parenchyma
- S pneumo invades subarachnoid space but spares brain parenchyma - inflammation of vessels in subarachoid space can cause thrombosis/vasculopathic changes - mottled edematous cortex (ischemia from artery supply, venous congestion from venous compromise, bacterial toxins and chemokines - edema. Edema can cause seizures - arachnoid granulation damage - hydrocephalus.

Chronic bacterial meningitis
- Most common - TB
- Hematogenous spread
- chronic course
- gross anatomy - lesions more fibrotic, rather than pururlent
- path: necrotizing granulomas, vasculopathic changes (dmg to vessels - ischemia)
- can present at skull base - "basal meningitis"
- May present with individual cranial nerve deficits (CN run in subarachnoid space)

Bacterial Abscess
- hematogenous - presumed septic microemboli
- granulation tissue like response - leaky vessels - edema
- may be ring enhancing
- surrounding edema - from angiogenesis nearby to bring increased inflammation - new vessels have compromised BBB
- may have fibrotic capsule
- intraparenchymal, usually hemtogenous spread

HSV encephalitis
- hemorrhagic necrosis of temporal lobes and inferior frontal lobes. - can cause focal neuro exam (sz, complex temporal seizures, memory loss, hallucinations, frontal or brainstem sx )
- path: immune cells around blood vessels
- nuclear inclusions - cowdry A
- chronically, can cause atrophy/encephalomalacia of temporal lobes
- affects parenchyma
- typically accute course
- typically immunocompetent hosts
- no seasonal variation, no vector
- tx: high dose acyclovir for 14 - 21 days
- dx with hsv1 pcr

Seasonal viral encephalitis
- west nile can mimic polio
- parenchymal distribution
- immunocompetent host
- typically has a vector (bird, mosquito)
- eastern equine encephalitis, western equaline encephalitis, etc
- "microglial nodules" on path - hypercellular areas, with spindle nuclei (?) representing microglia

PML
- JC viruls - affects oligodendroglial cells (ie spares grey matter)
- hematogenous spread
- typically activation of latent infetion - no effect in immunocompetnet hosts
- infects and transforms astrocytes to make them look darker and atypical - can mimic astrocytoma
- immunocompromised host
- path: foamy macrophages (ie mphage repsonding to tissue injury) - plum colored inclusions in oligodedroglial cells

HIV/AIDS
- can lead to CNS lymphoma
- Toxo, cryptococus, PML, CMV
- HIV encephalitis/dementia - AIDS defining illness, atrophy with microgila nodules, patchy demyelination, low grade inflammation, multinucleated cells

Fungal infections
- immunocompromised
- hematongeous - usu pulm, systemic disease
- intraparenchymal disease
- favor vessels
- aspergillus : vasculocentric growth, hemorrhagic necrosis - hyphae forms on path with 45 degree branching - grow next/into to blood vessels - can cause hemorrhage or ischemia from occlusion of vascular supply
- mucor - also causes necrosis and hemorrhage. grow as ribbony hyphae with 90 degree branches, can be result of local extension of fungal sinusitis. immunocomp or diabetic
- cryptococcus - subacute/crhonic (vs mucor/aspergillus which are more acute). "slime capsule" - sclear space around organisms on some stains - small narrow budding eyast - test c crypto Ag (99%sensitivie in csf, 95% in blood) immunocomproimsmised (not nly aids but lupus, maglinancy, steroids). can affect mengines or parenchymal "soap bubble" lesions - grow as yeast; tx IV amphotericin b and PO fluctyosine for 2 weeks, then high dose oral fluconaozole
- Histoplasma - yeast form - grow inside cytoplasm of histiocyte - look like foamy macrophages. small budding yesat - thickened leptiomingines - granulomatous - may mimic TB

Toxo
- sporozoan- unicellular parasite
- cat feces/undercooked meat
- can get congenital infection
- immunocompromised (reactivation of latent infection)
- necrotizing hemorrhagic parenchymal infection (tachyzoites - extracellular), bradyzoites - encysted - IHC can be helpful.
- path: cysts that look almost like nuclei (bradyzoites) - cells proliferating inside shielded from immune system - rupture and release organisms - attract inflammatory cells

Amoebas
- primary amebic encephalitis - acute, rapidly fatal, immunocompetent, direct invasion, usually nagleria
- granulomatous amebic encephalitis - chronic/subachute, immunocompromised, debliited, malnourished, probably hematogenous from skin/respiratory tracts - acanthameoba Balmuthia
- rarely entamameboa

Cysticercosis
- Taneia solium
- adult worm infect humans - eggs shed in feces - pig (intermediate host) consumes eggs - larval cysts in pig
- human becomes intermediate host from eating eggs
- larvae form single or multiple cysts