STEPTRACKERNERVOUS SYSTEM | PATHOLOGY + MICROBIOLOGY + PHARMACOLOGY | HIGH-YIELD REVIEW

CNS Infections

Meningitis by age and by CSF, the encephalitides, abscess, and the infectious protein that causes rapidly progressive dementia.

FOCUS ON
Bacterial meningitis organisms by age: newborn GBS, E coli, Listeria; children and adults S pneumoniae, N meningitidis; elderly add Listeria. CSF: bacterial = neutrophils, ↓ glucose; viral = lymphocytes, normal glucose; TB/fungal = lymphocytes, ↓ glucose, ↑↑ protein. HSV → temporal lobes. Prion → myoclonus + rapid dementia.
INFECTION
ORGANISM · MECHANISM
FEATURES
CLUE · TREATMENT
Bacterial meningitis — by age
0–6 mo: GBS, E coli, Listeria · 6 mo–60 y: S pneumoniae, N meningitidis (teens/young adults), Hib (unvaccinated) · > 60: S pneumoniae, N meningitidis, Listeria
Fever, headache, neck stiffness, photophobia, confusion; Kernig/Brudzinski signs
Purulent exudate in the subarachnoid space
Bacterial meningitis — treatment
Cover likely organisms; reduce inflammation
Ceftriaxone + vancomycin; add ampicillin for Listeria (newborns, elderly, immunocompromised); dexamethasone before antibiotics
Meningococcal contacts: rifampin, ciprofloxacin, ceftriaxone
Complications
Inflammation, vasculitis, ↑ ICP
Sensorineural hearing loss (most common sequela), seizures, hydrocephalus, SIADH, infarcts; Waterhouse-Friderichsen (meningococcus)
Petechial rash → N meningitidis
Viral (aseptic) meningitis
Enteroviruses (summer–fall, most common), HSV-2, HIV, mumps, arboviruses
Milder; lymphocytic CSF, normal glucose
Supportive care
Tuberculous meningitis
Hematogenous seeding; thick basal exudate
Subacute; cranial nerve palsies, hydrocephalus, infarcts; ↑↑ protein, ↓ glucose, lymphocytes
Endemic area, HIV
Cryptococcal meningitis
Cryptococcus neoformans, encapsulated yeast (pigeon droppings); AIDS with CD4 < 100
Headache, fever, very ↑ opening pressure; India ink, capsular antigen; "soap bubble" lesions
Amphotericin B + flucytosine → fluconazole
HSV encephalitis
HSV-1 reactivation; hemorrhagic necrosis of temporal lobes
Fever, confusion, personality change, seizures, aphasia; CSF lymphocytes + RBCs
CSF PCR · start acyclovir immediately
Other viral encephalitis
Rabies (bats; Negri bodies in Purkinje cells and hippocampus), West Nile (birds/mosquitoes; flaccid paralysis), HIV (microglial nodules, multinucleated giant cells)
Rabies: hydrophobia, agitation, paralysis
Rabies post-exposure: vaccine + immunoglobulin
Toxoplasmosis (CNS)
T gondii reactivation in AIDS (CD4 < 100)
Multiple ring-enhancing lesions, often basal ganglia; headache, focal deficits
Pyrimethamine + sulfadiazine · vs CNS lymphoma (single, EBV)
Brain abscess
Contiguous (sinus, ear, dental) or hematogenous (endocarditis, right-to-left shunt); viridans strep, S aureus, anaerobes
Fever, headache, focal deficit; ring-enhancing lesion with edema
No LP (herniation) · drainage + antibiotics
Neurocysticercosis
Eggs of Taenia solium (fecal-oral) → larval cysts in brain
Seizures; cystic lesions with a scolex, later calcifications
Leading cause of acquired epilepsy worldwide · albendazole
Prion disease (CJD)
Normal PrPc (α-helix) → misfolded PrPsc (β-sheet, protease-resistant) that converts more
Rapidly progressive dementia, startle myoclonus, ataxia; periodic sharp waves on EEG; 14-3-3 protein; spongiform vacuoles without inflammation
Sporadic, familial, iatrogenic (dura grafts, corneas), variant (beef)

Mechanism — bacterial meningitis

Nasopharyngeal colonization
→
BacteremiaCapsule resists phagocytosis
→
Crosses BBB / choroid plexus
→
Cytokines (IL-1, TNF)Neutrophils
→
Edema, ↑ ICP, vasculitis

Dexamethasone given with or before the first antibiotic dose limits the inflammatory burst from dying bacteria.

CSF patterns

CAUSE
CELLS
PROTEIN
GLUCOSE
Bacterial
↑↑ neutrophils
↑
↓
Viral
↑ lymphocytes
Normal / mild ↑
Normal
TB
↑ lymphocytes
↑↑
↓
Fungal
↑ lymphocytes
↑
↓
Guillain-Barré
Normal
↑↑
Normal

Opening pressure is high in bacterial and especially cryptococcal meningitis.

How it's tested

COMMON CONFUSIONToxoplasmosis (multiple ring lesions, Toxo IgG ⁺) vs primary CNS lymphoma (usually single, periventricular, EBV DNA in CSF) in an AIDS patient.
THE TRAPDon't delay antibiotics for imaging or LP in suspected bacterial meningitis — draw blood cultures and treat immediately.
YOUR APPROACHAge + immune status + exposure → CSF pattern → organism → antibiotics (add ampicillin for Listeria) → prophylaxis for contacts.

If you see… think…

3-day-old with fever and bulging fontanelle → GBS meningitis
College student with fever, neck stiffness and petechial rash → N meningitidis
Elderly or pregnant patient with meningitis → add ampicillin for Listeria
Lymphocytic CSF, normal glucose, summer → enteroviral meningitis
Basal meningitis with cranial nerve palsies, very ↑ protein → TB meningitis
AIDS, headache, positive capsular antigen → Cryptococcus
Fever, bizarre behavior, temporal lobe lesions, RBCs in CSF → HSV encephalitis — acyclovir
Bat exposure, agitation, hydrophobia → rabies (Negri bodies)
Immigrant with seizures and calcified brain cysts → neurocysticercosis
Rapid dementia with startle myoclonus and spongiform change → Creutzfeldt-Jakob disease