PA34.1-3 | CNS Infections & Tumors — Summary & Reflection

REFLECT

Think about the 22-year-old student in the hook scenario. You now have all the tools to manage him. His CSF — turbid, 12,000 neutrophils/µL, protein 280 mg/dL, glucose 15 mg/dL — screams bacterial meningitis before the culture comes back. His age (22, college student, possibly in a hostel) makes Neisseria meningitidis the most likely organism. He needs empirical ceftriaxone immediately — not after culture confirmation.

Now push further:
• What if the same patient's CSF had come back crystal clear, 120 lymphocytes/µL, protein 70 mg/dL, glucose 58 mg/dL? (Viral — reassure, supportive care)
• What if he were 45 years old, an Indian patient, with 3 weeks of fever and weight loss, and his CSF showed lymphocytes, protein 380 mg/dL, glucose 28 mg/dL, and a pellicle on standing? (TBM — start empirical ATT)
• What if he were HIV-positive with CD4 = 50 and the India ink was positive? (Cryptococcal — amphotericin B)

The same headache-and-meningeal-signs presentation leads to four entirely different treatments depending on the CSF profile. This is the power of pathological reasoning over pattern matching. How has your understanding of the inflammatory response in each type of infection changed how you'll approach a CSF report in the future?

KEY TAKEAWAYS

CNS Infections — Key Points:

• Meningitis is classified by CSF cell type: neutrophilic (bacterial), lymphocytic (viral, TB, fungal).
• Bacterial meningitis organisms vary by age: neonates (E. coli, GBS, Listeria), infants/toddlers (H. influenzae), young adults (N. meningitidis), adults (S. pneumoniae). All share polysaccharide capsule as virulence factor.
• Bacterial meningitis pathology: purulent subarachnoid exudate, neutrophils, vasculitis; complications include hydrocephalus, CN palsies, deafness, infarcts.
• Viral meningitis: lymphocytic, self-limiting; enteroviruses commonest; normal glucose.
• Tubercular meningitis: basal gelatinous exudate, granulomas with Langhans giant cells + caseous necrosis; low glucose; high protein; AFB/PCR for diagnosis; India-relevant.
• Cryptococcal meningitis: opportunistic (HIV/immunosuppressed); encapsulated yeast; India ink pathognomonic; soap-bubble lesions.

CSF Table Summary: Bacterial = turbid + neutrophils + very low glucose + very high protein. Viral = clear + lymphocytes + normal glucose + mildly raised protein. TB = clear/turbid + lymphocytes + low glucose + high protein + cobweb clot. Fungal (Crypto) = clear + lymphocytes + low glucose + India ink positive.

CNS Tumors — Key Points:

• Metastases most common in adults (lung, breast, melanoma, kidney, colon).
• Glioblastoma (Grade 4): pseudopalisading necrosis + microvascular proliferation; IDH-wild type (primary) vs IDH-mutant (secondary); MGMT methylation → better chemotherapy response.
• Oligodendroglioma: 1p/19q codeletion + IDH mutation; fried-egg cells; better prognosis.
• Ependymoma: perivascular pseudorosettes; 4th ventricle (children), filum terminale (adults).
• Meningioma: most common benign; arachnoid cap cells; whorls + psammoma bodies; extra-axial; NF2 mutation.
• Medulloblastoma: most common malignant brain tumor in children; cerebellum; Homer-Wright rosettes; CSF drop metastases.
• Schwannoma: Schwann cells; CN VIII (acoustic neuroma); Antoni A/B + Verocay bodies; NF2 (bilateral).
• Raised ICP: morning headache, vomiting, papilloedema; uncal herniation → blown pupil + hemiplegia; tonsillar herniation → death.