CNS infections are medical emergencies — delayed recognition or treatment can lead to death or permanent neurological damage. Empirical therapy depends on likely pathogen, patient age, immune status, and clinical context. Always involve microbiology early.

💉 Meningitis – Community Acquired

⚠️ Medical Emergency: Do not delay antibiotics for imaging or LP if contraindicated. Outcome correlates directly with time to antibiotic administration.

Likely pathogens: Neisseria meningitidis and Streptococcus pneumoniae. Less common: Listeria monocytogenes in elderly, pregnant, or immunocompromised patients.
Initial investigations:
- CSF microscopy, culture, and PCR (if safe to perform LP).
- Blood cultures ± EDTA sample for meningococcal PCR.
- Throat swab for meningococcal carriage.
Empirical therapy:
- Ceftriaxone 2 g IV every 12 h.
- If Listeria risk → add Amoxicillin 2 g IV every 4 h.
- If sensitivities allow, step down to Benzylpenicillin 2.4 g IV every 4 h.
Duration:
- Meningococcal — 7 days
- Pneumococcal — 10–14 days
- Listeria — 14–21 days
Public Health: Notify UKHSA Health Protection Team immediately — they arrange chemoprophylaxis for close contacts.
Penicillin allergy: Seek microbiology advice; third-generation cephalosporins may still be safe if non-anaphylactic.

💡 Teaching Pearl: If meningococcal disease is suspected pre-hospital, give Benzylpenicillin 1.2 g IM/IV immediately — early treatment improves survival.

🧩 Meningitis after Head Injury / Neurosurgery & Brain Abscess

Likely organisms: Gram-negative bacilli (e.g. Pseudomonas), Staphylococcus aureus, and anaerobes.
Empirical therapy:
- Meropenem 2 g IV every 8 h OR
- Ceftriaxone 2 g IV every 12 h + Metronidazole 500 mg IV every 8 h
Neurosurgical review: Essential — abscesses frequently require drainage.
Obtain imaging prior to LP if raised ICP or focal neurological signs present.

🧬 Viral Encephalitis

⚠️ Key fact: HSV-1 is the commonest cause of sporadic viral encephalitis. Mortality exceeds 70% without treatment.

First-line therapy: Aciclovir 10 mg/kg IV every 8 h (adjust for renal function).
Duration: Typically 10–14 days; extend to 21 days in immunocompromised or severe cases.
Investigations: CSF PCR for HSV, EEG, MRI (temporal lobe changes typical).
Rationale: Early aciclovir markedly reduces death and long-term neurological sequelae.

📋 Adjunctive Measures & Key Notes

Do not delay antibiotics/antivirals while awaiting imaging or LP in unstable patients.
Consider Dexamethasone 10 mg IV every 6 h for 4 days in suspected pneumococcal meningitis (start before or with first antibiotic dose).
Close liaison with Microbiology, Neurology, and Infectious Diseases for complex or resistant cases.
Monitor for complications: seizures, hydrocephalus, cranial nerve palsies, SIADH.

🧠 Teaching Point

The key skill in CNS infection management is timing: a lumbar puncture should never delay antibiotics when the patient is unwell. Empiric therapy is guided by age and risk factors — Listeria coverage in the elderly, Pseudomonas coverage post-neurosurgery, and aciclovir for encephalitis are must-know distinctions for finals and MRCP.

📚 References

The content on this website is provided for educational and informational purposes only to support exam preparation (e.g., MLA, MRCP, USMLE) and learning. This is NOT medical advice, diagnosis, treatment, or professional guidance. It does not replace consultation with a qualified healthcare professional, official guidelines (e.g., NICE, GMC, BNF), or supervised clinical practice. Always verify information with current, authoritative sources. Makindo and its contributors accept no liability for any reliance on this content, including errors, omissions, or any resulting harm, loss, or consequences. By using this site, you agree to these terms.

CNS infections

🧠 Central Nervous System (CNS) Infections