Merck Manual

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Introduction to Brain Infections

By

John E. Greenlee

, MD, University of Utah Health

Reviewed/Revised Mar 2022
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Brain infections can be caused by viruses, bacteria, fungi, or, occasionally, protozoa or parasites.

SARS-CoV-2 infection COVID-19 COVID-19 is a respiratory illness caused by the novel coronavirus SARS-CoV-2. Infection may be asymptomatic or have symptoms ranging from mild upper respiratory symptoms to acute respiratory... read more can result in encephalopathy, although it is not clear to what extent the cause is direct viral invasion of the brain, virus-induced stroke or hemorrhage, or secondary brain insults, such as hypoxia, electrolyte abnormalities, and liver and/or kidney dysfunction.

Certain noninfectious disorders can mimic encephalitis. An example is the autoimmune condition anti-NMDA (N-methyl-d-aspartate) receptor encephalitis, which involves an autoimmune attack on neuronal membrane proteins.

Brain infections often also involve other parts of the central nervous system (CNS), including the spinal cord. The brain and spinal cord are usually protected from infection, but when they become infected, the consequences are often very serious.

Infections may cause the meninges to become inflamed (meningitis) Overview of Meningitis Meningitis is inflammation of the meninges and subarachnoid space. It may result from infections, other disorders, or reactions to drugs. Severity and acuity vary. Findings typically include... read more . Often, bacterial meningitis spreads to the brain, causing encephalitis, infecting mainly the brain parenchyma. Similarly, viral infections that cause encephalitis often also cause meningitis. Technically, when both the brain and the meninges are infected, the disorder is called meningoencephalitis. However, the term meningitis is usually used to refer to infection that affects mainly the meninges, and encephalitis is usually used to refer to infection that affects mainly the brain.

CNS infections may manifest as follows:

Brain involvement may also be a manifestation of postinfectious mechanisms, such as acute disseminated encephalomyelitis.

Bacteria and other infectious organisms can reach the brain and meninges in several ways:

Many symptoms and signs of brain infections (eg, acute or subacute obtundation, seizures, focal neurologic deficits, signs of increased intracranial pressure) can develop in various brain infections (eg, brain abscess, intracranial epidural abscess, subdural empyema); thus neuroimaging is frequently necessary to differentiate among the infections.

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NOTE: This is the Professional Version. CONSUMERS: View Consumer Version
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