Neuroinfectious and neuroimmunological disease, including neurotuberculosis and HIV neurology
This domain tests CSF interpretation, empirical treatment decisions made before confirmation, and the distinctive imaging of opportunistic infections in immunocompromised patients. Tuberculosis and HIV dominate in this region.
Meningitis patterns
Bacterial meningitis: neutrophilic pleocytosis (often 1000-5000 cells), low glucose (below 2.2 mmol/L or CSF/serum ratio under 0.4), high protein; start empirical ceftriaxone plus vancomycin (plus ampicillin in neonates and the elderly for Listeria) with dexamethasone before or with the first antibiotic dose in pneumococcal disease. Viral meningitis: lymphocytic pleocytosis with normal glucose. Tuberculous meningitis: lymphocytic pleocytosis, markedly raised protein, low glucose, basal meningeal enhancement, hydrocephalus and infarcts; start four-drug antituberculous therapy with adjunctive dexamethasone and monitor for paradoxical reactions. HSV encephalitis: fever, confusion, seizures, temporal lobe change on MRI; start IV acyclovir 10 mg/kg every 8 hours immediately - do not wait for PCR, which can be negative in the first 72 hours.
HIV neurology
Toxoplasmic encephalitis: multiple ring-enhancing lesions, often in the basal ganglia, with oedema; treat empirically with pyrimethamine, sulfadiazine and folinic acid. Primary CNS lymphoma: periventricular, homogeneous enhancement, EBV DNA in CSF, positive thallium SPECT or PET; steroids can make the lesion vanish ("ghost tumour") and compromise biopsy, so avoid them before tissue diagnosis when possible. Progressive multifocal leukoencephalopathy: JC virus, non-enhancing white matter lesions without mass effect, worse with immune reconstitution. Cryptococcal meningitis: CD4 below 100, headache, raised opening pressure, India ink or cryptococcal antigen. Also screen for neurosyphilis, CMV retinitis and HIV-associated neurocognitive disorder.
Autoimmune and other infections
Anti-NMDA receptor encephalitis: young women, psychiatric onset, dyskinesias, seizures, autonomic instability, ovarian teratoma; treat with steroids, IVIG or plasma exchange and tumour removal. Anti-LGI1 encephalitis: faciobrachial dystonic seizures with hyponatraemia. Neurocysticercosis: seizures with cystic lesions; albendazole plus steroids. Cerebral malaria: fever with impaired consciousness and parasitaemia.
Worked example
A patient with HIV and CD4 count 60 has headache and two ring-enhancing lesions with surrounding oedema. Start empirical therapy for toxoplasmosis and review imaging at 2 weeks; if lesions are solitary or progress, pursue lymphoma (EBV PCR, PET, biopsy). Prophylaxis and antiretroviral timing follow the CD4 count.
Traps
Giving steroids before biopsy in suspected CNS lymphoma can destroy the diagnosis. HSV PCR may be negative early - treat anyway. Do not delay antituberculous therapy for culture confirmation. Aseptic meningitis does not automatically mean viral disease when drugs or tuberculosis are possible.