Pathophysiology of Meningococcal Meningitis - Step by Step

Step 1: Nasopharyngeal Colonization

• Pili (fimbriae) and Opa proteins mediate adhesion to nasopharyngeal epithelial cells
• The organism produces IgA1 protease, which cleaves and inactivates secretory IgA - the first line of mucosal defense
• Prior viral infections or smoking damages ciliated epithelial cells, promoting adhesion
• The polysaccharide capsule (serogroups A, B, C, W, X, Y) protects the organism from desiccation and from host immunity

Step 2: Mucosal Invasion and Transcytosis

• Meningococci invade nasopharyngeal mucosal cells by endocytosis and are transported to the basolateral (abluminal) side in membrane-bound vacuoles
• They can also transit directly through venules in the nasopharynx into the circulation
• Alternatively, hematogenous seeding can occur from bacteremia originating elsewhere

Step 3: Bacteremia and Immune Evasion

• The polysaccharide capsule resists phagocytosis and inhibits complement-mediated bactericidal activity (terminal complement components C5-9 are the principal defense; complement deficiency increases disease risk up to 600-fold)
• During growth, bacteria release outer membrane blebs containing lipopolysaccharide (LPS/endotoxin) and outer-membrane proteins
• Endotoxin (LPS) binds CD14 and TLR4 on host cells, triggering an intense inflammatory cascade

Step 4: Endothelial Injury and Systemic Effects (Meningococcemia)

• Loss of endothelial glycosaminoglycans and proteins increases vascular permeability - leading to capillary leak, hypovolemia, tissue edema, and pulmonary edema
• Intravascular thrombosis occurs from upregulation of tissue factor on the endothelium, while natural anticoagulant pathways (protein C, protein S, antithrombin III, thrombomodulin) are downregulated
• High levels of PAI-1 profoundly impair fibrinolysis
• Net result: Disseminated intravascular coagulation (DIC), microvascular occlusion, and purpura fulminans
• Myocardial depression (from cytokines, hypoxia, metabolic derangements) plus hypovolemia drives distributive and cardiogenic shock

Step 5: Crossing the Blood-Brain Barrier (BBB)

• Disruption of the BBB (endothelial tight junctions) - facilitated by LPS-mediated endothelial activation and cytokines
• Choroid plexus (blood-CSF barrier) - bacteria can cross here into CSF
• Once through, bacteria spread rapidly throughout the subarachnoid space

Step 6: Subarachnoid Inflammation

• Leukocytes (predominantly neutrophils) are recruited into the CSF
• Purulent exudate accumulates in the subarachnoid cisterns at the base of the brain and over the cortical sulci
• Cytokine release (TNF-alpha, IL-1, IL-6) within the CSF - similar in type to the systemic septicemia response - drives meningeal inflammation and is thought to determine the severity of neuronal injury

Step 7: Blood-Brain Barrier Breakdown and Brain Edema

• Increased BBB permeability - cytokines (especially TNF-alpha, IL-1) degrade tight junctions between endothelial cells
• Three types of cerebral edema develop:
  • Vasogenic edema - from BBB breakdown, protein-rich fluid leaks into the parenchyma
  • Cytotoxic edema - direct neuronal and glial cell swelling from ischemia and toxic injury
  • Interstitial edema - from obstruction of CSF flow (hydrocephalus)
• In fulminant meningococcal meningitis, cerebral edema can be severe even when CSF pleocytosis is only moderate - a key distinguishing feature

Step 8: Raised Intracranial Pressure (ICP)

• Inflammatory exudate can obstruct CSF pathways: obstruction of the aqueduct of Sylvius or foramina of Magendie and Luschka causes obstructive hydrocephalus; obstruction of arachnoid granulations causes communicating hydrocephalus
• Cerebral blood flow initially increases early in meningitis, then decreases - sometimes substantially
• Loss of cerebrovascular autoregulation means cerebral blood flow becomes passively dependent on mean arterial pressure, causing hyperperfusion or hypoperfusion
• Focal areas of severe hypoperfusion occur from focal vasculitis or thrombosis
• Raised ICP can lead to temporal lobe herniation through the tentorium (compressing cranial nerve III and contralateral corticospinal tract) or tonsillar herniation through the foramen magnum (apnea, hemodynamic instability, coma)

Step 9: Vascular Complications

• Cortical thrombophlebitis from venous stasis and inflammation can cause venous infarction
• Arteritis, aneurysm formation, and vascular spasm of cortical/pial arteries - particularly the anterior and middle cerebral arteries - occurs in ~15% of patients
• Internal carotid artery involvement at the skull base can cause supraclinoid stenosis
• These cause focal cerebral infarction and contribute to neurologic deficits

Step 10: Cranial Nerve Damage and Other Sequelae

• Dense purulent exudate accumulates around cranial nerves, especially III, VI, VII, and VIII - causing palsies
• Sensorineural hearing loss (CN VIII involvement) is a major long-term sequela; this risk is reduced by dexamethasone
• Subdural effusions form as sterile transudates and can be seen on CT/MRI
• Ventriculitis accompanies most cases and may rarely progress to ventricular empyema

Step 11: Late Immune Complex Complications

• Immune complex deposition - complexes of meningococcal polysaccharide or other antigens, specific IgG, and C3 deposit in tissues
• Manifests as arthritis, cutaneous vasculitis, iritis, episcleritis, pleuritis, or pericarditis
• Complicates 6-15% of cases; treated with NSAIDs; resolves within ~14 days without sequelae

Summary Flow

Nasopharyngeal colonization (pili, IgA protease, capsule)
        ↓
Mucosal invasion via endocytosis / transcytosis
        ↓
Bacteremia → endotoxin (LPS) release → TLR4/CD14 activation
        ↓
Massive cytokine storm (TNF-alpha, IL-1, IL-6, PAI-1)
        ↓
Endothelial injury → capillary leak + DIC + shock
        ↓
Crossing BBB via choroid plexus or disrupted endothelium
        ↓
Subarachnoid replication (minimal CSF defenses)
        ↓
Local inflammation → purulent exudate, more cytokines
        ↓
BBB breakdown → cerebral edema (vasogenic + cytotoxic)
        ↓
Raised ICP → herniation risk, loss of autoregulation
        ↓
Vasculitis, thrombosis → infarction; CN damage
        ↓
Late: immune complex disease