Conditions · Infectious and tropical diseases

Brain abscess !

A brain abscess is a pocket of pus inside the brain, usually spread from a sinus, ear, tooth or blood infection; it is a medical emergency.

Updated 2026-11-01

Structures involved

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Why it happens

What it is

A focal collection of pus within the brain tissue, surrounded by a capsule, caused mostly by bacteria and less often by fungi or parasites. It acts as an expanding mass, so it causes headache, focal deficits and raised intracranial pressure.

Who gets it

Uncommon but serious; it affects all ages, with peaks in children with cyanotic congenital heart disease and in adults with dental, sinus or ear infections. In Latin America, chronic otitis media and dental infections remain frequent sources, and in people with HIV, toxoplasmosis is the main cause of ring-enhancing lesions. In Latin America, including Colombia, neurocysticercosis is the most frequent cause of focal brain lesions and the main mimic.

Causes

Contiguous spread from sinusitis (frontal lobe), otitis media or mastoiditis (temporal lobe and cerebellum) or dental infection
Haematogenous spread from lung infection, endocarditis or right-to-left cardiac shunts (often multiple, at the grey-white junction)
Penetrating head trauma or neurosurgery (Staphylococcus aureus)
Common organisms: viridans and anaerobic streptococci, anaerobes, S. aureus, Enterobacterales
Immunosuppressed: Toxoplasma, Nocardia, fungi (Aspergillus, Candida, Cryptococcus), Listeria, tuberculosis

Risk factors

Chronic sinusitis, otitis or mastoiditis and poor dental health
Cyanotic congenital heart disease (tetralogy of Fallot) and pulmonary arteriovenous malformations
Infective endocarditis and lung abscess or bronchiectasis
HIV, transplantation, chemotherapy or corticosteroid treatment
Head trauma, neurosurgery and CSF leak

Pathophysiology

Organisms seed the brain and cause a focal cerebritis with oedema. Over about two weeks the centre becomes necrotic and a collagen capsule forms, giving a ring-enhancing lesion with surrounding vasogenic oedema. The mass raises intracranial pressure and can herniate the brain or rupture into the ventricles, causing ventriculitis with high mortality.

Classification

By source: contiguous, haematogenous, post-traumatic or postoperative, cryptogenic
By stage: early and late cerebritis, early and late capsule
Single or multiple

Symptoms and signs

Headache, often progressive and localised, the most common symptom
Fever (absent in about half of patients)
Focal deficits: hemiparesis, aphasia or visual field loss depending on the lobe
Seizures
Confusion, drowsiness, nausea and vomiting from raised intracranial pressure
Papilloedema and neck stiffness in some cases

Diagnosis

Clinical: Classic triad of headache, fever and focal deficit, though all three occur in a minority. A known source (sinus, ear, teeth, heart, lung) supports the diagnosis.
Tests: Blood cultures, CBC and CRP (may be normal). Stereotactic aspiration for Gram stain, culture (aerobic and anaerobic), fungal and mycobacterial studies and PCR. HIV test and Toxoplasma serology. Lumbar puncture is avoided because of the risk of herniation and low yield.
Imaging: Contrast MRI is preferred: ring-enhancing lesion with surrounding oedema and restricted diffusion in the cavity (which helps tell it from necrotic tumour). Contrast CT if MRI is unavailable. Imaging of sinuses, mastoid, chest and echocardiography to find the source.

Differential diagnosis

Glioblastoma or brain metastases (ring-enhancing tumour)
Toxoplasmosis and primary CNS lymphoma in HIV
Neurocysticercosis and tuberculoma
Subdural empyema and bacterial meningitis
Ischaemic stroke in evolution or demyelinating lesion

Goal of treatment

Control the mass effect, drain the pus when possible, give prolonged antimicrobial therapy that penetrates the brain, treat the source and prevent seizures and herniation.

Treatment

Acute: Emergency stabilisation: airway, management of raised intracranial pressure and neurosurgical consultation; empirical intravenous antibiotics started promptly, after blood cultures and ideally after aspiration if it is not delayed.
Medical: Prolonged intravenous combination therapy chosen by source, typically a third-generation cephalosporin plus metronidazole, with an anti-staphylococcal agent after trauma or surgery; adjusted to culture and continued for weeks with imaging follow-up.
Surgical: Stereotactic aspiration or open excision for large abscesses (over about 2.5 cm), mass effect, proximity to the ventricles, gas, fungal infection or failure of medical treatment; source control of sinus, ear or dental focus.
Medical: Antiseizure medicines for seizures; corticosteroids only for severe oedema with mass effect, as they reduce antibiotic penetration and capsule formation.
Follow-up: Neurological checks and Glasgow scale, pupils and vital signs for raised intracranial pressure, seizure precautions and serial imaging until resolution.

Complications

Brain herniation
Intraventricular rupture with ventriculitis
Epilepsy
Persistent focal deficits or cognitive impairment
Hydrocephalus
Recurrence

Prognosis

Untreated it is almost always fatal. With modern imaging, drainage and antibiotics, mortality in modern series is under about 15%, higher with coma at presentation, ventricular rupture, fungal infection or immunosuppression. About one in five survivors has seizures, weakness, language or cognitive sequelae.

Prevention

Prompt treatment of chronic otitis, sinusitis and dental infections
Early repair of cyanotic heart disease
Prophylaxis against toxoplasmosis in advanced HIV

Test yourself

Think of the answer, then tap the question to see it.

What is the classic triad of brain abscess, and how often is it complete?

Headache, fever and a focal neurological deficit; all three are present in only a minority of patients.

An abscess in the temporal lobe or cerebellum most often comes from which source?

Contiguous spread from otitis media or mastoiditis.

Which congenital heart lesion predisposes children to brain abscess?

Cyanotic heart disease with a right-to-left shunt, such as tetralogy of Fallot.

Why is lumbar puncture usually avoided in a suspected brain abscess?

The mass can cause brain herniation, and CSF findings rarely identify the organism.

In a person with advanced HIV and multiple ring-enhancing lesions, what is the most likely cause?

Cerebral toxoplasmosis.

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Educational information, not medical advice. For any symptom, see a health professional.

Sources: Brain abscess: MedlinePlus Medical Encyclopedia (MedlinePlus (U.S. National Library of Medicine), 2024); Harrison's Principles of Internal Medicine, 21st ed., ch. 140 Brain Abscess and Empyema (McGraw Hill, 2022).

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Representative model, for educational purposes only.

Frontal lobe highlighted on the Durgical 3D model