CNS Infections

Bacterial Infections - Brain Under Siege

• Pyogenic Abscess:
  • Stages:
    • Cerebritis (Early/Late): Ill-defined edema, patchy enhancement → central necrosis, developing rim enhancement.
    • Capsule (Early/Late): Thin, uniform enhancing rim → thick, collagenous rim; may show daughter loculi.
  • Features: Ring enhancement, central DWI restriction, surrounding vasogenic edema.
• Empyema:
  • Subdural/Epidural collections, often crescentic or lentiform.
  • Shows DWI restriction.
• Ventriculitis:
  • Ependymal enhancement.
  • Intraventricular debris/pus with DWI restriction.
• Meningitis Complications:
  • Hydrocephalus.
  • Infarcts.
  • Cranial nerve enhancement.

⭐ Central diffusion restriction (bright on DWI, dark on ADC) is a hallmark of pyogenic brain abscess, differentiating it from many cystic neoplasms.

Viral Infections - Code Red Viruses

• Herpes Simplex Encephalitis (HSE)
  • Type 1: Adults - affects temporal lobes, insula, cingulate gyrus; Neonates - diffuse pattern.
  • Imaging: T2/FLAIR hyperintensity, DWI restriction, petechial hemorrhages, gyral enhancement.

  ⭐ Herpes Simplex Encephalitis (HSE) classically demonstrates bilateral but asymmetric involvement of the medial temporal lobes and limbic system.

• HIV Encephalopathy
  • Presents with progressive cognitive decline.
  • Imaging: Diffuse, symmetric periventricular/deep white matter T2/FLAIR hyperintensity, cerebral atrophy. Typically no mass effect or enhancement.
• Progressive Multifocal Leukoencephalopathy (PML)
  • Caused by JC virus in immunocompromised individuals (e.g., HIV).
  • Imaging: Asymmetric, multifocal subcortical and periventricular white matter lesions. Usually no enhancement or mass effect. Posterior fossa involvement is common.

Granulomas & Parasites - The Great Imitators

• Neurotuberculosis (TB):
  • Meningitis: Thick basal cisternal enhancement (exudates), communicating hydrocephalus, vasculitic infarcts.
  • Tuberculomas: Solitary/multiple ring-enhancing lesions; central caseation (often T2 hypointense, DWI restriction); 'target sign' (central calcification/T2 hypointensity with peripheral rim). Miliary spread possible.
• Neurocysticercosis (NCC) - Taenia solium:
  • Stages 📌 Mnemonic: "Very Cute Girl Now Calms Down" (Vesicular, Colloidal, Granular, Calcified).

-   Locations: Common: parenchyma, subarachnoid space (racemose), ventricles. Intraventricular cysts may cause hydrocephalus.
> ⭐ The presence of an eccentric 'scolex' within a cystic lesion is pathognomonic for the vesicular stage of Neurocysticercosis.

• Toxoplasmosis - Toxoplasma gondii:
  • Risk: Immunocompromised (esp. AIDS with CD4 < 100 cells/µL).
  • Imaging: Multiple (often <10) ring-enhancing lesions, typically in basal ganglia & corticomedullary junction; surrounding vasogenic edema.
  • 'Asymmetric target sign' (eccentric nodule).
  • Differentiation: Thallium SPECT: Cold. PET: Hypometabolic (vs. lymphoma: hot/hypermetabolic).

Fungal Infections - Opportunistic Invaders

Primarily affect immunocompromised patients. Key CNS fungal infections include:

• Cryptococcosis (Cryptococcus neoformans)

  • Commonly presents as meningitis.

  • Gelatinous pseudocysts: Basal ganglia/thalami; T2 hyperintense, often non-enhancing.

  • Cryptococcomas: Ring or nodular enhancement.

• Aspergillosis (Aspergillus fumigatus)
  • Angioinvasive, leading to infarcts and hemorrhage.
  • Forms granulomas or abscesses.
  • Often from sino-nasal disease extending intracranially.
• Mucormycosis (Rhizopus, Mucor spp.)
  • Rhino-orbito-cerebral disease: Affects diabetics (DKA) & immunocompromised.
  • Aggressive angioinvasion, thrombosis, infarction, perineural spread.
  • Imaging: Lesions often T2 hypointense (iron/manganese).

⭐ Rhino-orbito-cerebral mucormycosis is a life-threatening emergency often seen in patients with diabetic ketoacidosis, characterized by rapid progression and angioinvasion.

High‑Yield Points - ⚡ Biggest Takeaways

• Ring-enhancing lesions: Differentiate pyogenic abscess (DWI restriction), toxoplasmosis (multiple, basal ganglia, eccentric target), tuberculoma (target sign/calcification).
• Neurocysticercosis (NCC): Scolex in cyst is pathognomonic; stages progress from vesicular to calcified.
• HSV encephalitis: Affects temporal lobes and limbic system; often hemorrhagic.
• PML: Subcortical white matter lesions in immunocompromised; no mass effect/enhancement.
• Cryptococcosis: Gelatinous pseudocysts (dilated Virchow-Robin spaces) in basal ganglia; possible leptomeningeal enhancement.