Brain abscess

Overview

•Classic triad of fever, headache, and focal neurological deficit - present together in only ~50% of cases

•Headache - progressive, often earliest and most consistent symptom

•Focal neurological deficits - hemiparesis, dysphasia, visual field defects depending on abscess location

•Seizures - focal or generalised tonic-clonic; may be the presenting event

•Raised ICP signs - papilloedema, Cushing's triad, meningism if meningeal irritation present

•Source clues: frontal lobe abscess - sinusitis; temporal/cerebellar - otitis media/mastoiditis; multiple abscesses - haematogenous spread (endocarditis, cyanotic CHD, dental caries)

🚨
Sudden catastrophic worsening of headache with new meningism = abscess rupture into the ventricular system - neurosurgical emergency with very high mortality.

Investigations

🥇 First-line

•CT head with contrast - rim-enhancing hypodense central cavity with surrounding oedema; identifies location, number, size

•Gold standard (diagnosis): MRI with DWI - abscess restricts diffusion (bright/hyperintense on DWI); distinguishes abscess from necrotic tumour (e.g. glioblastoma), which does not restrict diffusion

•Gold standard (microbiology): stereotactic aspiration and pus culture - confirms organism and sensitivities

•Bloods: FBC (leukocytosis), CRP/ESR (elevated); blood cultures x2 before antibiotics - positive in only ~15%

⚠️
Lumbar puncture is contraindicated - raised ICP is almost always present and LP risks transtentorial (uncal) herniation. CSF findings would be nonspecific anyway.

Brain abscess vs glioblastoma - key differentiator
FeatureBrain abscessGlioblastoma
FeverPresent (often)Absent
DWI (diffusion restriction)Restricted (bright)Not restricted
CT appearanceRim-enhancing, central cavityRim-enhancing, necrotic core
Meningism possibleYes (if rupture)No

Feature	Brain abscess	Glioblastoma
Fever	Present (often)	Absent
DWI (diffusion restriction)	Restricted (bright)	Not restricted
CT appearance	Rim-enhancing, central cavity	Rim-enhancing, necrotic core
Meningism possible	Yes (if rupture)	No

Management

•First-line (empirical antibiotics): ceftriaxone + metronidazole - start immediately after blood cultures drawn

•Ceftriaxone - third-generation cephalosporin; excellent BBB penetration; covers streptococci and gram-negatives but no anaerobic cover

•Metronidazole - excellent CNS penetration; bactericidal against anaerobes; essential because sinusitis/otitis abscesses are often polymicrobial

•Surgical: urgent neurosurgical review; stereotactic aspiration for drainage and culture

•Seizure control: anti-epileptic medication (e.g. levetiracetam)

•Adjunct: dexamethasone may reduce cerebral oedema but antimicrobials remain first-line

•Immunocompromised - alternative organisms: Toxoplasma (sulfadiazine + pyrimethamine); Cryptococcus (amphotericin B + fluconazole); fungal (Aspergillus)

🎯
DWI restriction is the key imaging differentiator: brain abscess restricts diffusion (bright on DWI); glioblastoma does not. Fever + ring-enhancing lesion + diffusion restriction = brain abscess until proven otherwise.