Meningitis

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Meningitis

Overview

Meningitis is inflammation of the meninges (pia mater, arachnoid, and dura) and subarachnoid space, most commonly caused by infection. It is a neurological emergency with significant morbidity and mortality if not recognized and treated promptly.

Etiology

Type	Common Pathogens / Causes
Bacterial	S. pneumoniae (most common in adults), N. meningitidis, L. monocytogenes (neonates/elderly/immunocompromised), H. influenzae, E. coli, Group B Strep (neonates)
Viral (Aseptic)	Enteroviruses (most common), HSV-2, HIV, mumps, arboviruses, CMV
Fungal	Cryptococcus neoformans (esp. in HIV/immunocompromised), Histoplasma
Tuberculous	Mycobacterium tuberculosis
Non-infectious	Malignancy (carcinomatous meningitis), drugs (NSAIDs, TMP-SMX, IVIG), sarcoidosis, SLE

Pathophysiology

(Harrison's, p. 1101–1102)

Bacteria colonize the nasopharynx → bacteremia → blood-brain barrier disruption
Bacterial components (LPS, teichoic acid) trigger cytokine release (TNF-α, IL-1)
Cytokine-mediated inflammation → increased BBB permeability → cerebral edema
Raised intracranial pressure (ICP) → impaired cerebral blood flow → neuronal injury
Exudate accumulates in subarachnoid space → impairs CSF flow → hydrocephalus

Clinical Features

(Harrison's, p. 978, 1102)

Classic Triad (present in <50% of patients):

Fever
Nuchal rigidity (neck stiffness)
Altered mental status

Other features:

Headache (severe, progressive — most common symptom)
Photophobia / phonophobia
Nausea and vomiting
Seizures (~30% of bacterial cases)
Focal neurological deficits

Meningeal Signs:

Kernig's sign: inability to fully extend the knee with the hip flexed to 90°
Brudzinski's sign: passive neck flexion causes involuntary hip/knee flexion
Jolt accentuation: worsening headache with horizontal head rotation at 2–3 Hz

Meningococcal-specific:

Petechial/purpuric, non-blanching rash (septicemia) — a medical emergency
May progress to Waterhouse-Friderichsen syndrome (adrenal hemorrhage, DIC)

Diagnosis

CSF Analysis (Lumbar Puncture) — Key Differentiator

Parameter	Bacterial	Viral	Tuberculous	Fungal
Appearance	Turbid/purulent	Clear	Fibrin web/clear	Clear/turbid
Opening pressure	↑↑ (>250 mmH₂O)	Normal/↑	↑	↑↑
WBC	>1000 cells/μL (PMN)	10–1000 (lymph)	100–500 (lymph)	10–500 (lymph)
Protein	↑↑ (>100 mg/dL)	Normal/↑	↑↑	↑
Glucose (CSF:serum)	↓ (<0.4)	Normal	↓↓ (<0.3)	↓
Gram stain	Positive ~60–90%	Negative	Negative (AFB stain)	India ink/Crypto Ag
Culture	Positive ~80%	Negative	Positive (slow)	Positive

(Harrison's, p. 1102–1103)

Before LP: CT head is required first if any of:

Papilledema
New focal neurological deficit
Severely depressed consciousness (GCS ≤10)
Seizure history
Immunocompromised

Do not delay antibiotics while waiting for CT/LP — if bacterial meningitis is suspected, treat immediately, then investigate.

Other Investigations

Blood cultures (×2, before antibiotics if possible)
CBC, CRP, procalcitonin, serum glucose, coagulation profile
CT/MRI brain — evaluate complications, rule out other diagnoses
PCR (bacterial, viral, TB) on CSF
Cryptococcal antigen (serum + CSF in immunocompromised)

MRI Findings in Bacterial Meningitis

MRI findings in acute bacterial meningitis showing leptomeningeal enhancement, FLAIR signal in sulci, and DWI restriction from intracranial pus

Axial MRI sequences in acute bacterial meningitis: (a) DWI showing hyperintense signals in cortical sulci and ventricular horns (diffusion restriction from intracranial/intraventricular pus); (b) FLAIR showing increased signal in frontal/parietal sulci and sub-ependymal lining (protein-enriched CSF); (c) Contrast-enhanced T1 showing leptomeningeal and ependymal enhancement. (Source: PMC Clinical VQA)

Management

Empirical Antibiotic Therapy

(Harrison's, p. 4159, Table 138-1)

Patient Group	Empirical Regimen
Adults (18–50 yrs)	Ceftriaxone 2g IV q12h + Vancomycin 15–20 mg/kg IV q8–12h
>50 yrs / immunocompromised	Above + Ampicillin 2g IV q4h (covers Listeria)
Neonates	Ampicillin + Cefotaxime (or Gentamicin)
Penicillin allergy	Chloramphenicol ± Vancomycin; Meropenem

Adjust antibiotics once cultures and sensitivities return.

Adjunctive Dexamethasone

0.15 mg/kg IV q6h × 4 days, given 15–20 min before or with the first dose of antibiotics
Reduces TNF-α-mediated inflammation
Most benefit in pneumococcal meningitis in adults (reduces hearing loss, mortality)
Should NOT delay antibiotics

Supportive Care

Elevate head of bed to 30°
Strict fluid balance (avoid hypotension and hyponatremia — risk of SIADH)
Seizure management (benzodiazepines acutely; consider prophylaxis)
ICP management if raised (mannitol, hyperventilation, neurosurgery if needed)
ICU monitoring for septic shock / DIC

Viral Meningitis

Mostly supportive care
Acyclovir 10 mg/kg IV q8h for HSV or VZV meningitis/encephalitis
Antiretrovirals if HIV-associated
Ganciclovir/Foscarnet for CMV (immunocompromised)

Cryptococcal Meningitis (HIV/Immunocompromised)

Induction: Liposomal Amphotericin B + Flucytosine × 2 weeks
Consolidation: Fluconazole 400mg/day × 8 weeks
Maintenance: Fluconazole 200mg/day
Serial LPs or lumbar drain for elevated ICP

Complications

(Harrison's, p. 1105–1106, 1200)

Hearing loss (most common long-term sequela — especially pneumococcal)
Hydrocephalus (communicating or obstructive)
Cerebral venous sinus thrombosis
Cerebral infarction / stroke
Subdural effusion / empyema
Cranial nerve palsies (II, III, VI, VII, VIII)
Cognitive impairment / learning disability
Septic shock, DIC, multi-organ failure (bacterial)
Death — bacterial meningitis case fatality rate: ~20–30% for pneumococcal, ~5–10% for meningococcal

Prophylaxis / Prevention

Indication	Intervention
Close contacts of meningococcal disease	Rifampicin 600mg BD × 2 days OR Ciprofloxacin 500mg single dose OR Ceftriaxone 250mg IM single dose
Vaccination	MenACWY, MenB vaccines (adolescents, asplenic patients, pilgrims)
Pneumococcal	PCV13/PPSV23 (elderly, immunocompromised, asplenic)
Hib	Routine childhood vaccination (virtually eliminated Hib meningitis in immunized populations)
Recurrent bacterial meningitis	Investigate for CSF leak or complement deficiency

Differential Diagnosis

Encephalitis (prominent altered consciousness, seizures, focal signs without meningismus)
Subarachnoid hemorrhage (thunderclap headache, xanthochromia on LP)
Brain abscess (focal signs, fever, CT ring-enhancing lesion)
Migraine, tension headache
Subdural empyema
Malignant meningitis (cytology positive, low glucose)

Key teaching point: In any patient with fever + headache + neck stiffness, bacterial meningitis must be ruled out immediately. Blood cultures and empirical antibiotics should never be delayed for imaging or LP.

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