Infectious Disease · Central Nervous System Infections
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Cryptococcal meningitis is the most common opportunistic fungal infection of the central nervous system in patients with advanced HIV/AIDS (CD4 count < 100 cells/mm³).
The causative organism, Cryptococcus neoformans, is an encapsulated yeast typically acquired via inhalation of aerosolized spores from pigeon droppings.
The gold standard for diagnosis is the detection of cryptococcal antigen (CrAg) in the cerebrospinal fluid (CSF) via latex agglutination or lateral flow assay.
CSF analysis typically reveals elevated opening pressure, lymphocytic pleocytosis, low glucose, and elevated protein.
India ink staining of the CSF demonstrates the classic encapsulated yeast with a wide halo appearance.
Induction therapy for symptomatic patients consists of amphotericin B plus flucytosine for at least two weeks.
Delayed initiation of antiretroviral therapy (ART)—typically 2 to 6 weeks after starting antifungal treatment—is required to prevent immune reconstitution inflammatory syndrome (IRIS).
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A 38-year-old male with a history of untreated HIV presents to the emergency department with a two-week history of progressive headache, nausea, and confusion. Physical examination reveals nuchal rigidity and papilledema. His CD4 count is 45 cells/mm³. A lumbar puncture is performed, revealing an opening pressure of 28 cm H2O, a white blood cell count of 40/µL with lymphocytic predominance, a glucose of 35 mg/dL, and an elevated protein level.
What is the most appropriate initial diagnostic test to confirm the suspected diagnosis?
Cryptococcal antigen (CrAg) testing of the CSF
The patient's presentation of subacute meningitis in the setting of severe immunosuppression is classic for cryptococcal meningitis, and the CrAg test is the most sensitive and specific diagnostic modality.
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Etiology / Epidemiology
Caused by Cryptococcus neoformans; primarily affects HIV/AIDS patients with CD4 < 100 cells/µL.
Clinical Manifestations
Presents as subacute meningitis with headache, fever, and elevated intracranial pressure.
Diagnosis
Cryptococcal antigen (CrAg) in CSF is the gold standard; India ink stain shows encapsulated yeast.
Treatment
Induction therapy: Amphotericin B plus Flucytosine; followed by Fluconazole consolidation.
Prognosis
High mortality if untreated; increased intracranial pressure is the primary cause of early death.
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Epidemiology & Etiology
Caused by the encapsulated yeast Cryptococcus neoformans, typically acquired via inhalation of aerosolized spores from pigeon droppings. It is an AIDS-defining illness occurring almost exclusively in patients with CD4 counts < 100 cells/µL. Rarely seen in immunocompetent hosts or those on chronic corticosteroids.
Pertinent Anatomy
The yeast has a predilection for the central nervous system, crossing the blood-brain barrier to cause meningoencephalitis. The thick polysaccharide capsule inhibits phagocytosis, leading to massive fungal burden in the subarachnoid space.
Pathophysiology
Inhalation leads to pulmonary colonization, followed by hematogenous dissemination to the meninges. The polysaccharide capsule prevents immune clearance, causing obstructive hydrocephalus due to impaired CSF resorption at the arachnoid villi. This results in elevated opening pressure on lumbar puncture.
Clinical Manifestations
Presentation is often indolent, evolving over weeks. Look for headache, nuchal rigidity, and altered mental status. Red flag: elevated intracranial pressure manifesting as papilledema, cranial nerve palsies, or vision loss. Unlike bacterial meningitis, nuchal rigidity may be absent in up to 50% of patients.
Diagnosis
Perform lumbar puncture; CSF Cryptococcal antigen (CrAg) is the most sensitive and specific test. India ink stain of CSF reveals encapsulated yeast with a clear halo. Opening pressure > 20 cm H2O is common and requires therapeutic drainage.
Treatment
Induction: Amphotericin B plus Flucytosine for at least 2 weeks. Caution: Flucytosine requires dose adjustment in renal failure and monitoring for bone marrow suppression. Consolidation: Fluconazole for 8 weeks, followed by lifelong maintenance therapy until CD4 > 100 for 3 months.
Prognosis
Early mortality is driven by increased intracranial pressure; serial lumbar punctures are often required. Immune Reconstitution Inflammatory Syndrome (IRIS) is a major risk if ART is started too early; delay ART for 2–6 weeks after antifungal initiation.
Differential Diagnosis
Bacterial meningitis: rapid onset with high CSF neutrophils
Tuberculous meningitis: basilar meningeal enhancement on MRI
Viral meningitis: lymphocytic pleocytosis with normal glucose
Neurosyphilis: positive VDRL/RPR in CSF
Primary CNS lymphoma: ring-enhancing lesions on MRI