Brain abscess

01OverviewDefinition, clinical context and the essential points that orientate the chapter.

A brain abscess evolves from cerebritis to a necrotic pus-filled cavity surrounded by capsule and oedema. Contiguous dental, sinus or ear spread commonly produces polymicrobial streptococcal and anaerobic infection; haematogenous spread follows endocarditis, lung infection or right-to-left shunt and may be multifocal. Neurosurgery and trauma add staphylococci and resistant hospital organisms.

The capsule limits antimicrobial and immune penetration, while oedema and mass effect cause neurological injury. Imaging identifies location and safe drainage route but cannot reliably distinguish pyogenic abscess from tumour, toxoplasma, tuberculoma or fungal disease. Aspirated material therefore requires broad microbiological processing, and control of the extracranial source is as important as drainage of the brain lesion.

Key points

Brain abscess presents with headache, focal neurology, seizure, cognitive change or raised pressure; fever and the full triad are often absent.
Do not perform lumbar puncture because a focal mass and pressure gradient can precipitate herniation and CSF rarely identifies the organism.
Use contrast MRI as the most sensitive imaging test; CT is appropriate immediately when MRI is unavailable or the patient is unstable.
Take blood cultures and identify dental, sinus, ear, lung, cardiac, trauma, surgical and haematogenous sources.
A stable patient should undergo stereotactic aspiration before antibiotics when this can occur promptly, preserving bacterial, fungal, mycobacterial and molecular specimens.
An unstable patient receives ceftriaxone 2 g intravenously every 12 hours plus metronidazole 500 mg every 8 hours immediately, modified by source and local protocol.
Add MRSA-active treatment after neurosurgery, penetrating trauma or defined resistant staphylococcal risk under microbiology guidance.
Treatment generally continues for weeks and fails when a collection, contiguous source, infected device or endocarditis remains uncontrolled.

02AetiologyUnderlying causes, associations and risk factors, with why each one matters.

Contiguous oral and ENT infection

Dental, sinus, middle-ear and mastoid infection crosses bone or veins, producing adjacent polymicrobial streptococcal and anaerobic abscess.

Bloodstream or procedural seeding

Endocarditis, lung infection, right-to-left shunt, neurosurgery and penetrating trauma introduce pathogenic organisms haematogenously or by direct inoculation.

03PathophysiologyThe causal sequence from the underlying abnormality to symptoms and harm.

1
Cerebritis and necrosis
Local bacterial replication recruits neutrophils, destroys cerebral tissue and progressively creates a liquefied necrotic centre containing pus.
2
Capsule and oedema
A fibrovascular capsule contains pus but restricts drug entry, while surrounding vasogenic oedema produces clinically important mass effect.
3
Vascular and ventricular spread
Septic emboli create multifocal lesions and rupture into ventricles disseminates infection through CSF with abrupt deterioration.

04Clinical features and red flagsSymptoms, examination findings, patterns of presentation and time-critical warnings.

Focal mass syndromeRed flag

Progressive headache, focal weakness, dysphasia, field loss, personality change or seizure suggests a focal cerebral lesion requiring urgent imaging.

Raised pressureRed flag

Vomiting, papilloedema, reduced consciousness, sixth-nerve palsy or abnormal pupils indicates mass effect and makes lumbar puncture unsafe.

Contiguous infectionRed flag

Recent sinusitis, otitis, mastoiditis or dental sepsis with adjacent frontal or temporal lesion supports direct spread.

Haematogenous patternRed flag

Multiple lesions at grey-white junction with bacteraemia, murmur, lung abscess or cyanotic heart disease suggests bloodstream seeding.

Red flags requiring action

Falling consciousness, abnormal pupils or Cushing physiology indicates impending herniation and requires emergency neurosurgical management.
A new focal deficit or seizure with fever or a contiguous ear, sinus or dental source requires urgent contrast brain imaging.
Ventricular rupture, multiloculation or posterior-fossa location carries high mortality and changes operative urgency.
Immune suppression broadens the differential to toxoplasma, fungal and mycobacterial disease and changes specimen handling.

05InvestigationsWhat to request, why it matters and how to interpret it.

Investigation order

Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.

01
Contrast MRI brainFirst step
Why
Define abscess number, location, diffusion restriction, oedema and ventricular involvement.
Interpretation and limitations
Ring enhancement with central diffusion restriction supports pus but is not pathognomonic; spectroscopy and serial imaging assist when aspiration is unsafe.
02
Blood cultures
Why
Identify haematogenous bacteria before antimicrobials.
Interpretation and limitations
Take multiple sets when safe. S aureus or streptococcal bacteraemia prompts endocarditis and metastatic-focus evaluation.
03
Stereotactic aspirate
Why
Provide definitive bacterial, fungal, mycobacterial and molecular diagnosis and decompress the collection.
Interpretation and limitations
Send adequate separate specimens without formalin for Gram stain, aerobic and anaerobic culture, histology and host-directed additional testing.
04
Source imaging and echocardiography
Why
Find sinus, dental, ear, pulmonary, cardiac or postoperative origin.
Interpretation and limitations
Treat positive source findings actively; a drained cranial lesion can recur while endocarditis or a sinus focus persists.

06Differential diagnosisRealistic alternatives and the features that help distinguish them.

Necrotic tumour

Glioblastoma and metastasis ring enhance and cause oedema; diffusion, perfusion and tissue distinguish them from infection.

Toxoplasma, fungal or TB lesion

Advanced immune suppression changes organism probability and demands broad tissue microbiology, histopathology and formal HIV status assessment.

Resolving haematoma or infarct

Blood products and subacute infarction can enhance peripherally, but clinical timing and advanced imaging clarify mechanism.

07ManagementImmediate care, first-line treatment, alternatives and escalation.

01ASSESSImage and protect the brainFirst stepA focal neurological infection or intracranial mass is suspected.

1Use ABCDE, neurological observations and seizure treatment and obtain urgent contrast imaging without lumbar puncture.
2Take blood cultures and involve neurosurgery and infection specialists immediately; start empirical therapy before aspiration when unstable.
3Use corticosteroid only for life-threatening mass effect under neurosurgical advice because it can reduce capsule and diagnostic yield.
4Assess immune state and contiguous, cardiac, pulmonary, traumatic and postoperative sources to guide initial coverage and specimens.

02TREATDrain and treat the collectionImaging supports abscess and the patient needs microbiological and pressure control.

1Aspirate or excise accessible large, uncertain, pressure-producing or non-responding lesions and send broad deep microbiology.
2Give ceftriaxone plus metronidazole for community oral or sinus patterns and add specialist MRSA or antipseudomonal cover when exposure warrants.
3Drain sinus, mastoid, dental, cardiac-device or other source and investigate persistent bacteraemia for endocarditis.
4Narrow to organism and susceptibility and define intravenous and oral duration with serial imaging, drainage and host response.

03REVIEWProve resolution and source controlTreatment is underway or response is incomplete.

1Repeat neurological examination and interval MRI or CT at a frequency determined by size, location, operation and trajectory.
2Re-image urgently for new deficit, seizure, fever or stalled improvement and check drainable residual, ventricular rupture or wrong diagnosis.
3Monitor FBC, renal, liver and agent-specific levels or interactions through prolonged therapy and maintain seizure management.
4Arrange neurorehabilitation, dental or ENT follow-up and confirm ownership of endocarditis, congenital-heart or immune investigation.

Key medicines and prescribing safety3 treatments · regimens, roles and cautions

Covers common streptococci, Enterobacterales and anaerobes while aspirate cultures are obtained.

Ceftriaxone and metronidazole empirical regimen

Give ceftriaxone 2 g intravenously every 12 hours plus metronidazole 500 mg intravenously or orally every 8 hours for a community-acquired oral, sinus or ear pattern.

Check allergy, renal and hepatic function, warfarin and alcohol advice and narrow after deep microbiology; prolonged duration is specialist and imaging guided.

Provides resistant Gram-positive activity while postoperative or traumatic microbiology is unresolved.

Vancomycin resistant-staphylococcal component

Add intravenous vancomycin using local weight, renal and concentration-guided dosing after neurosurgery, trauma or MRSA risk.

Monitor concentrations and kidney function and remove when cultures and exposure no longer support benefit.

Reduces vasogenic oedema temporarily while definitive drainage proceeds.

Dexamethasone for critical mass effect

Use the minimum neurosurgical dose and duration only when oedema causes life-threatening pressure or impending herniation.

Routine use can reduce antibiotic penetration and diagnostic inflammation; taper promptly and never substitute it for source control.

08ComplicationsImportant consequences, why they occur and why they matter clinically.

Herniation and death

Expanding abscess and oedema distort brain compartments and can cause abrupt neurological, respiratory and circulatory collapse from herniation.

Ventriculitis

Rupture into the ventricular system spreads pus widely, causes acute hydrocephalus and carries a particularly poor prognosis.

Epilepsy and focal disability

Cortical injury and surgical scarring can produce chronic seizure disorders, weakness, language dysfunction and persistent cognitive impairment.

09Monitoring and follow-upTreatment response, safety checks and longer-term review.

Perform frequent consciousness, pupil, focal-deficit and seizure assessment and escalate any deterioration immediately.
Trend blood and aspirate cultures, inflammatory markers and source investigations while documenting antimicrobial narrowing.
Repeat MRI or CT to demonstrate reduction and investigate non-response rather than relying on fever or CRP alone.
Monitor prolonged antimicrobial marrow, renal, hepatic and interaction toxicity and coordinate rehabilitation and seizure follow-up.

10Special situationsVariants, exceptions and circumstances that change the usual approach.

Fever may be missing

A capsule can contain systemic inflammation, so a progressive focal syndrome after dental, ear or bloodstream infection still warrants urgent imaging.

Diffusion helps but tissue decides

Restricted diffusion strongly supports pus, yet tumour, fungal and parasitic lesions can overlap in immune-suppressed hosts.

Source can sit outside brain

Endocarditis, sinus disease or an infected tooth can reseed a well-drained abscess unless addressed definitively.

Lumbar puncture is low yield and high risk

A focal pressure gradient can cause herniation, while aspirate rather than CSF best represents the lesion.

11Common pitfallsFrequent interpretation and management errors.

01
Performing lumbar puncture in a patient with focal deficits or suspected intracranial mass.
02
Starting antibiotics before planned prompt aspiration in a stable patient and unnecessarily reducing culture yield.
03
Treating serial scans while ignoring infected tooth, sinus, ear, heart valve or device source.
04
Assuming every ring-enhancing lesion is pyogenic abscess in advanced HIV or cancer.