Meningitis & encephalitis

Do not wait for the needle

About 95% of bacterial meningitis have ≥2 of fever, neck stiffness, altered mental status, and headache (). Elderly may show only lethargy or hypothermia. Petechiae or palpable purpura is meningococcemia until it is not. Give antibiotics within 1 hour — mortality rises ~12% per hour of delay (; NICE NG240).

Order of operations

1
Blood cultures STAT
Before antibiotics. Positive in ~70%. Do not delay drugs for imaging or LP (; ).
2
Antibiotics ± steroids
Dexamethasone 0.15 mg/kg q6h × 4 days, started before or with the first dose if pneumococcus is likely (greatest benefit GCS 8–11). Stop if it is not S. pneumoniae.
3
LP, with or without CT
Opening pressure (normal ~200 mm H2O; bacterial often ~350). Tube 1/4 cell count, tube 2 glucose/protein, tube 3 Gram stain/culture and the meningitis/encephalitis panel.

CT before LP?	Indication
No — go to LP	Immunocompetent adult, no focal findings, awake, no papilledema, no known CNS mass or stroke, no new seizure
Yes — but still give antibiotics first if CT will wait	Immunocompromised; known CNS disease (mass, CVA, focal infection); new seizure; papilledema; decreased consciousness; focal neurologic deficit (Clin Neuroradiol 2022;32:857)

Kernig and Brudzinski are ~5% sensitive. Nuchal rigidity ~30% sensitive. Jolt accentuation is not a rule-out. Exam can vanish if age >65, immunocompromised, or on analgesics ().
Repeat LP if no improvement at 48 hours, documented resistance, or fever past day 8 on appropriate therapy.
Vancomycin is for penicillin-resistant pneumococcus, not “for MRSA.”

Empiric by age and risk

Community vs nosocomial split: NEJM 2011;364:2016. Listeria is a Gram-positive rod and intrinsically CTX-resistant.

Host	Community microbiology	Start
Adults 18–34	S. pneumoniae ~50%, N. meningitidis ~35%, H. influenzae, GBS, rare Listeria	Vancomycin + CTX 2 g q12h
Adults 35–49	Pneumococcus dominates (~75%); meningococcus falls	Vancomycin + CTX 2 g q12h
Adults ≥50	Pneumococcus ~76%; GBS, Listeria (~7%), H. influenzae, meningococcus	Vancomycin + CTX 2 g q12h + ampicillin 2 g q4h
Immunocompromised	Add Listeria, GNR, fungi, viruses	Vanc + (cefepime 2 g q8h or meropenem 2 g q8h) + ampicillin 2 g q4h (drop amp if already on meropenem)
Nosocomial / hardware / trauma	Aerobic GNR ~40%, S. aureus ~10%, CoNS with foreign material, C. acnes (slow grower)	Vanc + cefepime 2 g q8h, ceftazidime 2 g q8h, or meropenem 2 g q8h. Neurosurgery for shunt removal ().
Severe β-lactam allergy	Same bugs, different drugs	Vanc + meropenem 2 g q8h, or moxifloxacin 400 mg daily. Listeria if ≥50 or immunocompromised and not on meropenem: TMP-SMX 5 mg/kg IV q6–12h.

Close contacts of meningococcus (and selected Hib): post-exposure prophylaxis. Vaccines: MenACWY/MenB, Hib, pneumococcal.
VP-shunt peritonitis and abdominal pain can be the meningitis equivalent — treat as hardware CNS infection.

Aseptic meningitis and encephalitis

Aseptic meningitis is meningeal inflammation with negative bacterial cultures — usually less toxic, lymphocytic CSF, lower CSF lactate. Encephalitis is abnormal brain function: AMS and focal findings, not just a stiff neck ( ; ).

Aseptic meningitis

Infectious: enteroviruses first, HSV-2 > HSV-1, VZV, HHV-6, EBV, CMV, partially treated bacteria, syphilis, Lyme, rickettsia, leptospira, measles/mumps, Nocardia, TB, primary HIV, LCMV, fungi, parameningeal abscess.
Noninfectious: Behçet, sarcoid, SLE, Sjögren, GPA, leukemia/lymphoma, NSAIDs, antimicrobials, IVIG, checkpoint inhibitors.
Add CSF/serum arbovirus Abs, VDRL/RPR, Lyme, mycobacterial studies, HIV, HSV swabs of lesions.

Encephalitis

HSV-1 >> HSV-2, VZV, arboviruses (WNV, EEE/WEE, St. Louis, JEV), enteroviruses, HIV, JC, influenza, Powassan, Zika.
Noninfectious: ADEM, autoimmune and paraneoplastic (anti-NMDAR / ovarian teratoma, anti-Hu / SCLC, anti-Ma2 / testicular).
MRI, EEG, ME panel, arbovirus panel, autoimmune antibody panel. Temporal T2/FLAIR and CSF RBCs support HSV — they do not delay acyclovir.