CNS Infections Study Guide for Medical Students
Classification and Etiology
CNS infections are categorized by anatomic location and causative organism, with bacterial causes being most prevalent (73% combined), followed by fungal (10%), viral (4%), and parasitic (4%) infections. 1
Major Categories:
- Meningitis: Inflammation of meninges (bacterial, viral, tuberculous, fungal)
- Encephalitis: Brain parenchymal inflammation (predominantly viral, especially HSV)
- Cerebritis/Brain Abscess: Focal brain tissue infection
- Meningoencephalitis: Combined meningeal and parenchymal involvement 2, 3
Common Pathogens by Type:
- Bacterial: Tuberculous meningitis (36%), pyogenic bacteria (23%), tuberculoma (14%) 1
- Viral: HSV, VZV, enteroviruses 4
- Fungal: Mucormycosis (9%), Cryptococcus (1%) - particularly in immunocompromised 1
- Parasitic: Neurocysticercosis (3%), toxoplasmosis 1
Clinical Presentation
Cardinal Features (in order of frequency):
- Fever (80% of cases) 1
- Headache (67%) 1
- Altered mental status/encephalopathy 1, 5
- Neck stiffness (62%) 1
- Seizures (less common but significant) 1
- Focal neurologic deficits 6
Presentation Timeline:
- Acute presentations dominate (83% of cases) 1
- Most commonly affects middle-aged adults (41-60 years, 43%), followed by young adults (21-40 years, 31%) 1
- Male predominance (58%) 1
High-Risk Populations:
- HIV/AIDS patients (31% of CNS infection cases) 1
- Recent neurosurgery patients 7
- Patients with CSF shunts, drains, or implantable devices 7
- Head/spinal trauma 7
Critical Diagnostic Algorithm
Step 1: Immediate Clinical Assessment
If altered consciousness with inability to protect airway → immediate intubation before further workup. 8
Key clinical predictors for CNS infection (use for rapid screening): 6
- Age <56 years
- Fever ≥38°C (100.4°F)
- Glasgow Coma Scale <14
- Focal neurologic deficit
- Seizure
- AIDS/HIV status
Step 2: Distinguish Encephalitis from Encephalopathy
This distinction fundamentally changes management: 8
Encephalitis indicators:
- Fever + altered mental status + focal findings
- Requires immediate empiric acyclovir
- Requires urgent LP (if no contraindications)
Encephalopathy indicators:
- Metabolic derangements present
- No fever or focal findings typically
- Requires identification of precipitating factors
Step 3: Neuroimaging BEFORE Lumbar Puncture
CT brain scan must be performed before LP in most patients to exclude contraindications. 8
Brain MRI with diffusion-weighted imaging is preferred and should be performed within 24 hours to confirm diagnosis and determine extent of inflammation. 9
Step 4: Obtain Blood Cultures
Draw blood cultures before initiating antimicrobials (positive in 28% of brain infection cases). 9
Consider HIV testing in ALL patients regardless of risk factors. 9
Step 5: Lumbar Puncture and CSF Analysis
Perform LP if no contraindications for culture, PCR, and analysis. 9
CSF Interpretation: The Critical Differentiator
Viral Meningitis Pattern:
- WBC: 5-1000 cells/μL (lymphocyte predominance, though neutrophils may predominate early) 4
- Appearance: Clear 4
- Protein: Mildly elevated (<0.6 g/L) 4
- Glucose: Normal or slightly low 4
- CSF/plasma glucose ratio: Normal or slightly low but >0.36 4
- Opening pressure: Normal or mildly raised 4
- Lactate: <2 mmol/L (effectively rules out bacterial disease) 4
Bacterial Meningitis Pattern:
- WBC: >100 cells/μL (typically much higher, neutrophil predominance) 4
- Appearance: Turbid, cloudy 4
- Protein: Raised 4
- Glucose: Very low 4
- CSF/plasma glucose ratio: Very low 4
- Opening pressure: Raised 4
Tuberculous Meningitis Pattern:
- WBC: 5-500 cells/μL (lymphocyte predominance) 4
- Appearance: Clear or cloudy 4
- Protein: Markedly raised 4
- Glucose: Very low 4
- Opening pressure: Raised 4
Critical CSF Pearls:
- In 5-10% of viral encephalitis (especially HSV), initial CSF may be completely normal 4
- If first CSF is normal with high clinical suspicion, repeat LP in 24-48 hours 4
- CSF red cells elevated in ~50% of HSV encephalitis due to hemorrhagic nature 4
- PCR for HSV-1/2, VZV, and enteroviruses identifies 90% of viral cases 4
Empiric Treatment: Time-Critical Decisions
For Suspected Encephalitis (Viral):
START IV ACYCLOVIR IMMEDIATELY before diagnostic results return - HSV encephalitis has high mortality without treatment. 8
- Dosing: Standard IV acyclovir (reduce dose in renal impairment) 9
- Duration: 14 days if HSV confirmed 8, or 14-21 days per alternative guidelines 9
- Critical pitfall: NEVER delay acyclovir while awaiting LP or imaging 8, 9
For Community-Acquired Cerebritis (Bacterial):
Empirical treatment must include: 9
- Third-generation cephalosporin (covers streptococci and gram-negatives)
- PLUS Metronidazole (covers anaerobes)
- PLUS Acyclovir (covers possible HSV)
If bacterial meningitis also suspected, follow meningitis-specific guidelines. 9
Duration: IV antimicrobials for 6-8 weeks for bacterial cerebritis 9
For Encephalopathy (Non-Inflammatory):
Identify and correct precipitating factors (resolves 90% of cases). 8
For hepatic encephalopathy specifically:
- Lactulose: 30-45 mL (20-30 g) PO three to four times daily, titrated to 2-3 soft stools/day 8
- Rifaximin: 550 mg twice daily (reduces recurrence by 58% when added to lactulose) 8
Supportive Care and ICU Management
Indications for ICU Admission:
- Falling level of consciousness → urgent ICU assessment for: 9
- Airway protection
- Ventilatory support
- Raised intracranial pressure management
- Electrolyte correction
Management of Cerebral Edema:
- Elevate head of bed to 30° 9
- Osmotic therapy: Mannitol or hypertonic saline for impending herniation 9
- Corticosteroids: May be indicated for severe perifocal edema 9
Monitoring Requirements:
- Frequent neurological status checks to detect deterioration 9
- Renal function monitoring (acyclovir causes crystalluria/obstructive nephropathy after 4+ days) 9
- Adequate hydration essential with acyclovir 9
Follow-Up and Complications
End-of-Treatment Confirmation:
Repeat LP at end of treatment to confirm CSF negative for causative pathogen by PCR. 9
Repeat brain imaging if clinical deterioration or to monitor treatment response. 9
Neurosurgical Considerations:
If cerebritis progresses to abscess formation, neurosurgical intervention may be necessary. 9
Long-Term Outcomes:
- Long-term neurologic sequelae occur in ~45% of brain infection patients 9
- Referral to specialized neurorehabilitation required for many survivors 9
- Overall mortality: 21% with appropriate treatment 1, but can reach 60% in healthcare-associated infections 7
Common Pitfalls to Avoid
Diagnostic Pitfalls:
- Never delay antimicrobials while awaiting diagnostic results - treatment should start immediately when CNS infection suspected 9
- Don't rely on ammonia levels alone for hepatic encephalopathy diagnosis or monitoring 8
- Don't assume normal initial CSF rules out viral encephalitis - repeat in 24-48 hours if suspicion high 4
- Empirical antimicrobials without diagnostic workup can halt the diagnostic pathway 9
Treatment Pitfalls:
- Never delay acyclovir for imaging or LP in suspected encephalitis 8, 9
- Don't forget to adjust acyclovir dose in renal impairment 9
- Monitor for acyclovir nephrotoxicity with adequate hydration 9