Key Facts

Brain abscess requires a high index of suspicion. Combined surgical and antibiotic treatment is standard. The source of infection must be identified and treated. DWI-MRI is key to distinguishing abscess from tumour.

Epidemiology

Incidence ~1-2 per 100,000/year. M:F 2-3:1. All ages but peak in 3rd-4th decade. Immunocompromised patients at higher risk (HIV, transplant, steroids).

Aetiology

• Contiguous spread (~50%): otitis media/mastoiditis (temporal lobe, cerebellum), frontal sinusitis (frontal lobe), dental infection (frontal/temporal lobe)
• Haematogenous (~25%): infective endocarditis, lung abscess, cyanotic congenital heart disease (right-to-left shunt), IV drug use
• Direct inoculation (~10%): penetrating trauma, neurosurgery
• Cryptogenic (15-20%): no source identified

Pathophysiology

Cerebritis stage (days 1-3): focal inflammation → early capsule (days 4-9) → late capsule (days 10-14+): well-formed capsule surrounded by oedema and gliosis. Ring enhancement on contrast imaging reflects capsule vascularity. Medial capsule is thinner than lateral (closer to white matter, less blood supply) → tendency to rupture into ventricle (ventriculitis — high mortality).

Symptoms

• Headache (70%): progressive, often localising
• Fever (50%): may be absent, especially if on antibiotics
• Nausea/vomiting: raised ICP
• Seizures (25-30%): focal or generalised

Signs

• Focal neurological deficit (50%): hemiparesis, dysphasia, visual field defect
• Papilloedema: raised ICP
• Meningism: if abscess close to meninges or ruptures

Red Flags

• Rapid deterioration → abscess rupture into ventricle (ventriculitis — meningism, rapid GCS decline, very high mortality)
• Multiple abscesses → haematogenous source (endocarditis, lung)
• Immunosuppressed patient with ring-enhancing lesion → consider toxoplasma, fungal, TB

Imaging

• CT head with contrast: ring-enhancing lesion with surrounding oedema; urgent first-line
• MRI brain with DWI: gold standard; restricted diffusion (high signal on DWI, low ADC) — distinguishes abscess from tumour (tumour shows no restriction)
• MR spectroscopy: amino acid peaks (from bacterial breakdown products) — supports abscess diagnosis

Bloods

• FBC: leucocytosis (may be normal)
• CRP, ESR: raised
• Blood cultures: positive in ~15-30% (especially haematogenous)
• HIV test: if immunosuppression suspected

Source Identification

• Echocardiography (TTE/TOE): endocarditis screening
• CT sinuses/petrous bones: contiguous spread from sinusitis/otitis
• Dental assessment: if dental source suspected
• CT chest: lung abscess, pneumonia

Microbiology

• Aspirate culture: from stereotactic drainage — essential for targeted antibiotic therapy
• CSF: LP is generally CONTRAINDICATED (risk of coning); if performed — raised protein, lymphocytic pleocytosis, culture usually negative

Antibiotics (Empirical — Then Targeted)

• IV ceftriaxone 2g BD + IV metronidazole 500mg TDS: first-line empirical (covers streptococci, anaerobes, Gram-negatives)
• Add IV vancomycin if MRSA risk (neurosurgery, trauma)
• Duration: 6-8 weeks IV (minimum 4 weeks IV; may convert to oral after clinical and radiological improvement)
• Adjust based on culture results from aspirate

Surgical

• Stereotactic aspiration: CT/MRI-guided; diagnostic + therapeutic; first-line for most abscesses >2.5 cm
• Craniotomy and excision: for superficial, well-encapsulated abscesses; fungal abscesses; failed aspiration
• Conservative (antibiotics alone): small abscesses (<2.5 cm), cerebritis stage, surgically inaccessible location, multiple small abscesses
• Repeat imaging: serial CT/MRI to monitor resolution (every 1-2 weeks initially)

Supportive

• Dexamethasone: if significant oedema/mass effect (vasogenic oedema); controversial — may reduce antibiotic penetration and slow capsule formation
• Anticonvulsants: if seizures (levetiracetam, phenytoin); prophylactic use is debated
• ICP management: if raised (head elevation, osmotic agents, EVD if hydrocephalus)

Treat Source

• Mastoidectomy for otogenic abscess; sinus surgery for sinogenic
• Dental extraction for odontogenic
• Treat endocarditis

Referral Criteria

• All brain abscesses: neurosurgery + infectious diseases
• ENT/dental if contiguous source
• Cardiology if endocarditis suspected

Mortality ~10% with treatment (higher if ventricular rupture — ~80%). Seizures develop in ~30-50% (long-term antiepileptic therapy may be needed). Neurological sequelae in ~30-50%: focal deficits, cognitive impairment, epilepsy. Prognosis worse with: ventricular rupture, multiple abscesses, immunosuppression, deep/eloquent location, delayed treatment. Imaging resolution lags behind clinical improvement by weeks-months.

Brain Abscess — Location by Source

DWI-MRI: Abscess vs Tumour