Approach to Fever with Altered Sensorium
Immediately initiate empiric acyclovir for presumed herpes simplex encephalitis while simultaneously pursuing urgent diagnostic evaluation, as this is the most treatable and time-sensitive cause of infectious encephalitis 1.
Immediate Management Algorithm
Step 1: Stabilize and Start Empiric Treatment (Within 30 Minutes)
- Start acyclovir immediately (before any diagnostic testing) if encephalitis is suspected—delay in treatment significantly increases mortality and morbidity 1
- Ensure airway, breathing, and circulation are stable
- Obtain rapid glucose and correct if hypoglycemic
- Administer thiamine if any concern for alcohol use disorder or malnutrition
Step 2: Determine if This is Encephalitis vs. Encephalopathy
The critical distinction determines your entire management pathway. Encephalitis requires altered mental status lasting ≥24 hours PLUS at least 2 of the following 2:
- Documented fever ≥38°C within 72 hours of presentation
- Generalized or partial seizures (not from prior seizure disorder)
- New focal neurologic findings
- CSF white blood cell count ≥5/mm³
- Brain imaging abnormality suggesting encephalitis
- EEG abnormality consistent with encephalitis
If these criteria are NOT met, you are likely dealing with encephalopathy from systemic causes (metabolic, toxic, infectious outside CNS, organ failure) 2, 1.
Step 3: Urgent Diagnostic Workup (Within 1-2 Hours)
For suspected encephalitis, obtain simultaneously:
- Neuroimaging (MRI preferred, CT if MRI unavailable) before lumbar puncture to rule out mass effect 1
- Lumbar puncture with CSF studies: cell count with differential, glucose, protein, Gram stain, bacterial culture, HSV PCR, enterovirus PCR, and consider additional testing based on epidemiology (West Nile virus, arboviral panel, TB, fungal studies) 1
- Blood cultures, complete metabolic panel, liver function tests, ammonia
- EEG if seizures suspected or to evaluate for subclinical seizures (common pitfall: subclinical seizures can cause persistent altered mental status) 2
- Serology for specific pathogens based on exposure history and season
For suspected encephalopathy:
- Comprehensive metabolic panel, liver and renal function, ammonia, thyroid function
- Toxicology screen (including specific levels for acetaminophen, salicylates, alcohol)
- Arterial blood gas
- Chest X-ray and urinalysis to identify systemic infection
- Blood cultures if sepsis suspected
Step 4: Refine Antimicrobial Therapy Based on Clinical Context
Continue acyclovir until HSV encephalitis is definitively ruled out (negative CSF HSV PCR with alternative diagnosis confirmed) 1.
Add empiric bacterial meningitis coverage (vancomycin + third-generation cephalosporin ± ampicillin if >50 years or immunocompromised) if you cannot immediately distinguish between meningitis and encephalitis—the presentations overlap significantly 1.
Consider specific antimicrobials based on exposure history:
- Doxycycline for rickettsial diseases (tick exposure, summer months)
- Antimalarials if travel to endemic areas
- Antituberculous therapy if subacute presentation with risk factors
Critical Pitfalls to Avoid
Delaying acyclovir: HSV encephalitis has 70% mortality untreated; early acyclovir dramatically improves outcomes. Start before LP if imaging shows no contraindication 1.
Missing nonconvulsive status epilepticus: Up to 20% of encephalitis patients have subclinical seizures causing altered mental status. Obtain EEG if mental status doesn't improve as expected 2.
Assuming fever always means infection: Fever occurs in autoimmune encephalitis, neuroleptic malignant syndrome, and malignant catatonia. If extensive infectious workup is negative and patient has psychiatric history, consider these diagnoses 3.
Stopping workup after correcting metabolic abnormalities: If encephalopathy persists after correcting hyponatremia, hypoglycemia, or other metabolic derangements, you must pursue CNS-specific causes 4.
Overlooking immunosuppression: Immunocompromised patients require broader antimicrobial coverage including fungi (cryptococcus, aspergillus), toxoplasmosis, and atypical mycobacteria 1.
When Infectious Workup is Negative
If >50% of encephalitis cases remain unexplained despite extensive testing 2, consider:
- Autoimmune encephalitis (anti-NMDA receptor, voltage-gated potassium channel antibodies)
- Paraneoplastic syndromes
- Acute disseminated encephalomyelitis (ADEM): treat with high-dose corticosteroids 1
Do not use corticosteroids empirically in infectious encephalitis as they may worsen outcomes, particularly in HSV encephalitis 1. Only use steroids once infection is ruled out or for specific indications (ADEM, cerebral malaria adjunctively, Baylisascaris with albendazole).
Special Populations
Immunocompromised patients: Expand coverage to include toxoplasmosis (pyrimethamine + sulfadiazine), cryptococcus, CMV, and consider empiric antifungals 1.
Recent travel: Consider malaria (quinine/artemether), trypanosomiasis, or parasitic causes based on geographic exposure 1.
Elderly or nursing home residents: Higher likelihood of systemic infection causing encephalopathy; thoroughly evaluate for pneumonia, urinary tract infection, and intra-abdominal sources 5.