Neuroinfectious and Neuroimmunologic Disorders
Bacterial meningitis
Do not delay antibiotics for imaging or lumbar puncture. Empiric therapy is vancomycin plus ceftriaxone 2 g every 12 hours, adding ampicillin when Listeria is a concern (age over 50, immunocompromise, alcohol use). Give dexamethasone 0.15 mg/kg every 6 hours before or with the first antibiotic dose and continue for four days when pneumococcal disease is likely; giving it after antibiotics reduces vancomycin penetration into CSF. Obtain CT before lumbar puncture only with focal deficits, seizures, papilledema, or immunocompromise. Expect neutrophil-predominant CSF with low glucose and high protein.
HSV encephalitis
Start acyclovir 10 mg/kg every 8 hours (renally adjusted) on clinical suspicion, not on PCR results: PCR can be negative in the first 72 hours. Treat 14-21 days and repeat CSF PCR if relapse is suspected. Temporal lobe involvement with fever and confusion is the classic picture.
Autoimmune encephalitis
Anti-NMDA receptor encephalitis often begins with psychiatric symptoms in young women, followed by dyskinesias, seizures, autonomic instability, and hypoventilation requiring ICU care. Treat with IVIG or plasma exchange plus tumor resection when a teratoma is found; rituximab or cyclophosphamide is second line.
Trap: labeling fever and altered mentation in an ICU patient as sepsis without considering CSF analysis, or stopping acyclovir too early when the PCR is negative but the syndrome fits.
Rule of thumb: in suspected meningitis, antibiotics first, then imaging, then lumbar puncture, and never delay treatment to obtain a scan.