23.4 Acute CNS Infections (03.M)
Key Takeaways
- In suspected community bacterial meningitis, give dexamethasone immediately before or with the first antibiotic dose when pneumococcus is possible; do not wait for CSF Gram stain if that wait delays drugs.
- Start intravenous acyclovir as soon as HSV encephalitis is in the differential; temporal/insular imaging and CSF PCR confirm, they do not license a delay.
- Brain abscess, subdural empyema, and spinal epidural abscess are not interchangeable: empyema and spinal epidural abscess with deficit are surgical emergencies plus antibiotics.
- EVD or shunt ventriculitis needs CSF sampling and hardware removal; empiric therapy is vancomycin plus an anti-pseudomonal beta-lactam (IDSA 2017).
- Empiric meningitis drugs follow age and neurosurgical context; immunocompromised pathogens (Toxoplasma, Cryptococcus, and related) are covered in chapter 10.4, not by ceftriaxone alone.
Minutes matter more in CNS infection than in almost any other 03.M-adjacent topic. Independent OpenExamPrep teaching for ABIM Neurocritical Care is the community-meningitis bundle, HSV treated on suspicion, the surgical infections that antibiotics alone will not drain, and ventriculitis on an EVD. Hosts without a working immune system need extra organisms; that list lives with 10.4 Antimicrobial Use and Infection in the Immunosuppressed — point there rather than duplicating toxoplasmosis protocols here.
Community bacterial meningitis
The classic triad (fever, nuchal rigidity, altered mental status) is incomplete in many adults. Still treat empirically when the pretest probability is real. Sequence that does not delay drugs:
- Two sets of blood cultures.
- Dexamethasone and antibiotics immediately if LP will wait for CT.
- Lumbar puncture when safe; CT first only for the IDSA-style red flags (immunocompromise, known CNS disease, new seizure, papilledema, focal deficit, moderate-to-severe impairment of consciousness).
Adjunctive dexamethasone 10 mg IV every 6 hours for 4 days (adult dosing from de Gans and van de Beek, NEJM 2002) is given 15–20 minutes before or with the first antibiotic dose. That trial showed fewer unfavorable outcomes (15% vs 25%) and lower mortality (7% vs 15%), with the clearest benefit in Streptococcus pneumoniae. IDSA 2004 bacterial meningitis guidance endorses dexamethasone in suspected or proven pneumococcal disease in adults. If the pathogen is later not pneumococcus (and not H. influenzae in children), stop the steroid. Do not start dexamethasone hours after antibiotics “because someone remembered.” Do not use it as routine therapy for healthcare-associated gram-negative meningitis or for septic shock from meningococcemia without meningitis — those are different inflammatory problems.
Empiric intravenous regimens are conceptual, then narrowed to MIC and CNS penetration:
| Setting | Typical organisms | Empiric backbone (adults; local antibiogram applies) |
|---|---|---|
| Community, age 16–50 years | Pneumococcus, meningococcus | Vancomycin plus ceftriaxone 2 g every 12 hours (or cefotaxime) |
| Community, age >50 or cell-mediated immune defect | Above plus Listeria | Add ampicillin 2 g every 4 hours |
| Basilar skull fracture / CSF leak | Pneumococcus, streptococci, H. influenzae | Vancomycin plus a third-generation cephalosporin |
| Post-neurosurgery, penetrating trauma, or CSF shunt/EVD | Staphylococci including MRSA, Cutibacterium, hospital gram-negatives including Pseudomonas | Vancomycin plus cefepime, ceftazidime, or meropenem (IDSA 2017 healthcare-associated ventriculitis/meningitis) |
Ceftriaxone doses used for meningitis are CNS doses, not 1 g daily community-pneumonia doses. Vancomycin is included because penicillin-nonsusceptible pneumococcus is not rare. Ampicillin is the Listeria drug; cephalosporins do not cover Listeria. After neurosurgery, ceftriaxone alone is the wrong gram-negative choice.
Droplet precautions for meningococcus until 24 hours of effective therapy. Chemoprophylaxis for close contacts is a public-health task, not an ICU afterthought.
HSV encephalitis: treat, then confirm
Herpes simplex virus (HSV-1) encephalitis is a necrotizing temporal and insular process. Personality change, fever, new seizures, and a CSF lymphocytic pleocytosis with a high protein and variable red cells should start acyclovir 10 mg/kg of ideal body weight IV every 8 hours immediately. MRI: T2/FLAIR hyperintensity in the medial temporal lobe, insula, and cingulate, often asymmetric. EEG may show periodic discharges over the temporal region. CSF HSV PCR is the confirmatory test; it can be falsely negative in the first 72 hours. A negative day-1 PCR does not stop acyclovir if the MRI and story still fit — repeat LP. Dose-adjust for GFR; early treatment changes mortality. Add bacterial coverage if you cannot exclude bacterial meningitis at the door. VZV, autoimmune encephalitis, and abscess enter the differential; they do not justify withholding the first acyclovir dose.
Abscess, subdural empyema, spinal epidural abscess
These three are all “pus near the neuraxis.” They are not the same operation.
| Entity | Where the pus lives | Typical source | First definitive move besides antibiotics |
|---|---|---|---|
| Brain abscess | Parenchymal, often ring-enhancing with diffusion restriction | Hematogenous (endocarditis, pulmonary shunt) or contiguous sinus/ear | Stereotactic aspiration or excision if large, mass-effect, or diagnosis unclear; small, multiple abscesses may be medical after a causative organism is known |
| Subdural empyema | Between dura and arachnoid, spreads over the convexity | Sinusitis, mastoiditis, post-craniotomy | Emergency surgical drainage; antibiotics alone fail as pus layers over cortex |
| Spinal epidural abscess | Spinal epidural space | Skin/soft-tissue S. aureus, injection, endocarditis, contiguous osteomyelitis | Emergency surgical decompression when there is deficit, sepsis with collection, or failure of medical therapy; MRI of the whole spine because skip lesions exist |
Empiric drugs while you go to the scanner and the OR often include vancomycin plus a third-generation cephalosporin plus metronidazole for community brain abscess (staph, strep, anaerobes from sinus). Post-neurosurgical collections follow the hospital-meningitis table (vancomycin plus anti-pseudomonal beta-lactam). Spinal epidural abscess is S. aureus until proven otherwise: vancomycin plus a gram-negative agent that covers the local epidemiology, then narrow.
The classic spinal triad (back pain, fever, neurologic deficit) is often missing a piece. Do not wait for paralysis to call MRI. Once plegia is established, recovery odds collapse. That is why this is a surgical emergency plus antibiotics, not a 48-hour vancomycin trial in a ward bed.
Ventriculitis and EVD infection
An EVD is a foreign body in the ventricle. Infection risk rises with duration, breaks in the closed system, and CSF leaks at the tunnel. Fever, unexplained GCS drop, or a change in CSF appearance should trigger CSF sampling from the drain (Gram stain, culture, cell count, glucose, protein). Interpretation is messy because blood and the catheter itself inflame the fluid. Culture and a falling glucose with a rising neutrophil count still matter. Repeat sampling when the first draw is ambiguous.
IDSA 2017: remove the infected CSF drain; if diversion is still required, replace at a new site. Infected VP shunts are fully removed and replaced by a temporary EVD, not “suppressed” in situ. Empiric intravenous therapy is vancomycin plus cefepime, ceftazidime, or meropenem. Intraventricular antibiotics are reserved for selected persistently positive cultures after removal, not as a way to leave hardware in. Chapter 26 returns to EVD technical complications (malposition, hemorrhage, overdrainage); the infection rule is already this: sample, remove, cover widely, then narrow.
Immunocompromised extras — pointer, not a second chapter
HIV with a CD4 count in the cellar, transplant recipients, and patients on high-dose steroids do not get “community meningitis plus a shrug.” Add Listeria (ampicillin) when risk is present even under age 50. Cryptococcus needs antigen testing and induction amphotericin plus flucytosine, not ceftriaxone. Toxoplasma ring lesions are pyrimethamine-based therapy after imaging, not empiric metronidazole for pyogenic abscess without a host clue. PML, CMV, endemic fungi, and nocardia live in section 10.4. If the stem is a transplant patient with fever and a ring lesion, do not stop at the table above.
Exam traps
Holding antibiotics for a CT that is two hours away. Giving dexamethasone the next morning. Waiting for HSV PCR before acyclovir. Treating subdural empyema with 6 weeks of antibiotics and no surgeon. Sampling an EVD, leaving it in, and calling that a cure. Using 1 g of ceftriaxone daily for meningitis. Forgetting ampicillin in a 72-year-old. Forgetting vancomycin plus cefepime after a craniotomy.
In practice
A 68-year-old with fever and coma: blood cultures, dexamethasone 10 mg IV, vancomycin plus ceftriaxone plus ampicillin, then CT if needed and LP — that is community meningitis with Listeria coverage. A 34-year-old with new temporal seizures: acyclovir now, MRI and PCR in parallel. A teenager with frontal sinusitis and a rapidly spreading hemiparesis: subdural empyema, call the OR. A thoracic back-pain patient who cannot lift the legs: whole-spine MRI and a spine surgeon the same hour. An EVD day-8 fever: CSF from the drain, new catheter, vancomycin plus cefepime.
A 61-year-old with fever, neck stiffness, and confusion is going to CT before lumbar puncture. Which timing of adjunctive steroid and the reason for it is correct?
A patient develops fever and a falling GCS on EVD day 9. CSF from the drain shows neutrophilic pleocytosis and Gram-positive cocci. Which management matches IDSA 2017 healthcare-associated ventriculitis guidance?
Which pair correctly matches a CNS infectious syndrome with the action that must not wait?