Infections of the Nervous System

Bacterial Meningitis & Abscess - Brain Under Siege

• Bacterial Meningitis: Acute inflammation of leptomeninges.
  • Pathogens: S. pneumoniae (adults), N. meningitidis (teens, petechiae), GBS/E.coli (neonates), Listeria (elderly/immunocompromised).
  • CSF: ↑Pressure, ↑PMNs (>1000/µL), ↑Protein (>45mg/dL), ↓Glucose (<40mg/dL). Gram stain often positive.
  • Complications: Hydrocephalus, seizures, deafness.
• Brain Abscess: Focal collection of pus within brain parenchyma.
  • Etiology: Direct spread (sinusitis, otitis), hematogenous.
  • Imaging: CT/MRI shows characteristic ring-enhancing lesion.
  • Stages: Early cerebritis → Late cerebritis → Capsule formation.

⭐ CSF in bacterial meningitis: Glucose <40 mg/dL, Protein >45 mg/dL, WBC (Neutrophils) >1000/µL.

Viral Encephalitis & Myelitis - Viral Voltage

• Viral inflammation of brain parenchyma (encephalitis) or spinal cord (myelitis).
• Common Etiologies & Key Features:
  • Herpes Simplex Virus (HSV-1): Most common sporadic encephalitis; affects temporal lobes. Rx: Acyclovir.
  • Arboviruses (e.g., Japanese Encephalitis, West Nile Virus): Epidemics, mosquito-borne; JE affects thalamus, basal ganglia.
  • Enteroviruses (Poliovirus, Coxsackie): Aseptic meningitis; Polio targets anterior horn cells (myelitis).
  • Rabies Virus: Ascending myelitis/encephalitis; Negri bodies (intracytoplasmic inclusions).
  • JC Virus: Progressive Multifocal Leukoencephalopathy (PML) in immunocompromised (CD4 < 200).
• Clinical: Fever, headache, altered sensorium, seizures, focal neurological deficits. Myelitis: weakness, paralysis.
• Diagnosis: CSF (lymphocytic pleocytosis, ↑protein), PCR for viral nucleic acid, MRI.

⭐ HSV encephalitis is the most common cause of fatal sporadic encephalitis in adults and classically involves the inferomedial temporal lobes and limbic system; early Acyclovir is crucial.

Fungal & Parasitic CNS - Unwelcome Guests

• Fungal Infections:
  • Cryptococcosis: C. neoformans. HIV/AIDS. India ink (CSF), soap bubble lesions (MRI). Rx: Ampho B + Flucytosine.
  • Mucormycosis: Rhino-orbital-cerebral. Diabetics (DKA). Angioinvasion, black eschar. Rx: Surgery + Ampho B.
  • Aspergillosis: Angioinvasive, hemorrhagic infarcts. Neutropenia. Galactomannan antigen.
• Parasitic Infections:
  • Neurocysticercosis (NCC): T. solium larvae. Seizures. CT/MRI: cysts, scolex, calcifications. Rx: Albendazole + steroids.

    ⭐ NCC is the most common cause of adult-onset seizures in endemic areas like India.

  • Cerebral Toxoplasmosis: T. gondii. HIV (CD4 < 100). Multiple ring-enhancing lesions. Rx: Pyrimethamine + Sulfadiazine.
  • Cerebral Malaria: P. falciparum. Coma, seizures. Retinal hemorrhages. Rx: IV Artesunate. oka

TB Meningitis & Prions - Chronic Creepers

Tuberculous (TB) Meningitis:

• Thick, gelatinous basal exudates; often leads to cranial nerve palsies (VI, VII, III, IV).
• Complications: Hydrocephalus, vasculitis, infarcts.
• CSF: Lymphocytic pleocytosis, ↑ protein (often >100 mg/dL), ↓ glucose (<40 mg/dL or <40% of blood glucose), ↑ ADA.
• Diagnosis: AFB smear (low sensitivity), GeneXpert/MTB PCR (high specificity).
• Treatment: Anti-Tubercular Therapy (ATT) + Corticosteroids (e.g., Dexamethasone for 6-8 weeks).

⭐ CSF in TB meningitis classically shows a "cobweb coagulum" upon standing due to high fibrinogen content.

Prion Diseases (Transmissible Spongiform Encephalopathies - TSEs):

• Agent: Abnormal prion protein (PrPSc) - protease-resistant, induces conformational change in normal PrPC.
• Examples: Creutzfeldt-Jakob Disease (CJD), Kuru, Gerstmann-Sträussler-Scheinker (GSS).
• CJD: Rapidly progressive dementia, myoclonus (startle myoclonus), ataxia.
• EEG (sCJD): Periodic sharp wave complexes.
• Histology: Spongiform degeneration, neuronal loss, astrogliosis.

High‑Yield Points - ⚡ Biggest Takeaways

• Bacterial meningitis: S. pneumoniae (adults). CSF: ↑Protein, ↓Glucose, ↑Neutrophils.
• Viral meningitis: CSF: Normal/↑Protein, Normal Glucose, ↑Lymphocytes.
• Tuberculous meningitis: CSF: Markedly ↑Protein, ↓↓Glucose, ↑Lymphocytes, cobweb coagulum.
• Brain abscess: Ring-enhancing lesion on imaging; central necrosis, surrounding fibrous capsule.
• Herpes Simplex Encephalitis (HSE): Affects temporal lobes; hemorrhagic necrosis, Cowdry A inclusions.
• Progressive Multifocal Leukoencephalopathy (PML): JC virus in immunocompromised; widespread demyelination affecting oligodendrocytes.
• Rabies: Negri bodies (intracytoplasmic inclusions) in hippocampal pyramidal cells and Purkinje cells of cerebellum.