Meningitis — Robbins Pathology

Definition

• Chemical meningitis — response to irritants (e.g., debris from a ruptured epidermoid cyst)
• Carcinomatous meningitis — spread of metastatic cancer cells to the subarachnoid space
• Autoimmune meningitis — seen with systemic autoimmune disease

Classification

1. Acute Pyogenic (Bacterial) Meningitis

Causative Organisms by Age

• Haemophilus influenzae was previously a major cause in infants but immunization has dramatically reduced its incidence.
• In immunosuppressed patients, organisms such as Klebsiella or anaerobes may be causative with atypical clinical presentations.

Clinical Features

• Systemic signs of infection (fever, malaise)
• Meningeal irritation: headache, photophobia, neck stiffness
• Neurologic impairment: irritability, clouding of consciousness

CSF Findings

• Increased pressure; cloudy or frankly purulent
• Abundant neutrophils
• Elevated protein
• Markedly reduced glucose

Morphology (Gross and Microscopic)

Fig. 21.16 / Fig. 28.23 — Pyogenic meningitis. A thick layer of suppurative exudate covers the brain surface and thickens the leptomeninges. — Robbins Pathology

• Exudate evident within the leptomeninges on the brain surface
• Meningeal vessels are engorged and prominent
• Tracts of pus extend along blood vessels; distribution varies by organism (H. influenzae → basal; pneumococcal → over cerebral convexities near the sagittal sinus)
• In fulminant cases, inflammation extends to the ventricles (ventriculitis); the choroid plexus lacks a blood-brain barrier and may serve as a portal of entry

• Neutrophils fill the subarachnoid space in severe cases; in milder cases they surround leptomeningeal blood vessels
• Gram stain reveals bacteria, particularly in untreated cases
• In fulminant meningitis: inflammatory cells infiltrate leptomeningeal vein walls and extend focally into the brain parenchyma (cerebritis); secondary vasculitis and venous thrombosis → hemorrhagic cerebral infarction

• Leptomeningeal fibrosis → hydrocephalus (more common with TB meningitis, but occurs in bacterial meningitis too)
• In pneumococcal meningitis specifically: large quantities of capsular polysaccharide produce a gelatinous exudate → chronic adhesive arachnoiditis (arachnoid fibrosis)
• Waterhouse-Friderichsen syndrome: meningitis-associated septicemia with hemorrhagic infarction of the adrenal glands — most often with meningococcal and pneumococcal infection

2. Aseptic (Viral) Meningitis

Key Features

• A clinical term for meningitis manifestations (meningeal irritation, fever, altered consciousness) without organisms identifiable by bacterial culture
• Generally viral in etiology; may be rickettsial or autoimmune
• Enteroviruses account for ~80% of identified cases
• Usually self-limited; treated symptomatically

CSF Findings

• Lymphocytic pleocytosis
• Moderate protein elevation
• Normal glucose (key distinction from bacterial meningitis)

Morphology

• Less dramatic than bacterial meningitis
• Leptomeningeal infiltration by lymphocytes and monocytes rather than neutrophils
• No suppurative exudate

3. Chronic Meningitis

A. Tuberculous Meningitis

• Symptoms: headache, malaise, mental confusion, vomiting
• CSF: moderate ↑ cellularity (mononuclear or mixed), ↑↑ protein, moderately ↓ or normal glucose
• Tuberculoma: well-circumscribed intraparenchymal mass of M. tuberculosis; may coexist with meningitis
• Complication: chronic tuberculous meningitis → arachnoid fibrosis (particularly at the base of the brain) → hydrocephalus from impaired CSF resorption

B. Spirochetal Infections (Neurosyphilis)

• Occurs in ~10% of untreated Treponema pallidum infection (tertiary syphilis); more aggressive with HIV co-infection
• Patterns of CNS involvement:
  • Meningovascular neurosyphilis: chronic meningitis (usually basal), associated with obliterative endarteritis of meningeal/cortical vessels → strokes
  • Paretic neurosyphilis (general paresis): progressive dementia from direct parenchymal invasion
  • Tabes dorsalis: degeneration of posterior spinal columns and dorsal nerve roots
• Lyme Neuroborreliosis (Borrelia burgdorferi): can cause meningitis, cranial nerve palsies, and radiculopathy

C. Fungal Meningitis

• Cryptococcus neoformans: most important fungal cause of meningitis, especially in HIV/immunocompromised patients
• Produces a gelatinous meningitis as the organisms fill the subarachnoid space
• Characteristic "soap-bubble" lesions in basal ganglia/cortex (pseudocysts of cryptococcal organisms)

Summary of CSF Interpretation

• Definition & classification table
• Acute Pyogenic Meningitis — organisms by age, clinical features, CSF findings, gross & microscopic morphology, sequelae
• Gross pathology image from Robbins (Fig. 21.16)
• Aseptic (Viral) Meningitis section
• Chronic Meningitis — TB, neurosyphilis (with pattern table), and cryptococcal
• Full CSF comparison table across all three types
• Sourced from both Robbins editions throughout