CHAPTER FIVE
INFECTIONS OF THE NERVOUS SYSTEM

FUNGAL INFECTIONS OF THE NERVOUS SYSTEM

The most common CNS mycoses are, in order of frequency, Candidiasis, Aspergillosis, and Cryptococcosis. They are seen mainly in patients with AIDS and other immunosuppressed states.

Candidiasis. Extensive brain necrosis	Diseminated neonatal candidiasis. Multiple microabscesses

Candida is also frequent in the neonatal period. Candida is a commensal but rarely causes disease in normal people. Infection is caused by organisms that are already present in the intestine and other locations, and in neonates it is trasmitted from external sources. Most disseminated infections are nosocomial, and the key risk factors are catheters, and antibiotics. Candida consists of budding yeasts and hyphae. It causes meningitis, multiple microabsesses, or extensive brain necrosis. At first, the inflammation consists of neutrophils; later of epithelioid cells and giant cells.

Aspergillus arteritis	Mucor arteritis of the basilar artery with pontine infarct

Aspergillus and the related soil fungus mucor (rhizopus) are branching hyphae. They are ubiquitous in nature but only cause disease in immunosuppressed persons. The most important risk factors for disseminated infection are neutropenia, cytotoxic chemotherapy, and corticosteroids. Aspergillus enters the body through the lungs. Mucor, which is also common in patients with diabetic ketoacidosis, infects the nasal mucosa from where is spreads to the brain. Both fungi have the tendency to invade blood vessels and cause thrombosis with cerebral infarction or vascular rupture with cerebral hemorrhage.

Cryptococci	Cryptococcosis. Microcysts	Cryptococci in perivascular space

Cryptococcus is an oval yeast about the size of a red cell, surrounded by a gelatinous capsule. It is a worldwide fungus, present in bird droppings, vegetables, and soil. It speads to the brain from the lungs and often causes meningitis without involving other organs. It is most often community-acquired. It may affect healthy individuals, but is particularly common in immunodeficient patients. It grows extensively in the subarachnoid space and perivascular spaces, which become cystically distended to the point that brain sections look like Swiss cheese. In immunosuppressed individuals, inflammation is absent or mild. In immunocompetent hosts, cryptococcus elicits a cell-mediated immune reaction with lymphocytes and epithelioid cell granulomas. Cryptococcus meningitis has an insidious onset and may go on from weeks to years. It can cause hydrocephalus, dementia, and focal neurological deficits. The CSF in cryptococcosis shows mononuclear pleocytosis, elevated protein, and low glucose, similar to tuberculous meningitis. Yeasts can be identified by microscopy of the CSF and their antigens can be detected by latex agglutination.