Brain Abscess

A brain abscess is a focal, encapsulated collection of pus within the brain parenchyma. It is a neurosurgical emergency. Most cases need surgical drainage combined with prolonged antimicrobial therapy.

It arises in three ways:

• direct spread of infection from a contiguous source (sinusitis, otitis media, dental infection)
• haematogenous spread from a distant source (endocarditis, pulmonary infection, particularly with right to left cardiac shunting)
• penetrating trauma or neurosurgical intervention

Pathophysiology and source identification

The likely causative organisms and the anatomical location of the abscess are frequently predictable from the underlying source. This guides both empirical antibiotic selection and the search for the primary focus of infection.

• Frontal lobe abscesses commonly arise from frontal sinusitis.
• Temporal lobe abscesses commonly arise from otitis media or mastoiditis.
• Abscesses in unusual or atypical locations more often reflect haematogenous spread from a distant source such as endocarditis or a pulmonary source.

Headache is the commonest presenting symptom. It is often progressive and poorly responsive to simple analgesia.

Fever is present in only a proportion of cases, and its absence does not exclude the diagnosis. The diagnosis is easily deprioritised without overt systemic infective features.

Focal neurological deficit, seizures, and signs of raised intracranial pressure (declining consciousness, papilloedema, vomiting) reflect the mass effect and location of the abscess. The pattern of neurological signs correlates with the anatomical site.

The classical triad of fever, headache and focal neurological deficit is present together in only a minority of confirmed cases. Keep a high index of suspicion in any patient with new headache and neurological signs. This applies particularly with a recognised predisposing condition (sinusitis, otitis media, endocarditis, immunocompromise, cyanotic congenital heart disease).

Magnetic resonance imaging with contrast is the investigation of choice. It shows a characteristic ring enhancing lesion with surrounding oedema.

Diffusion weighted imaging helps distinguish an abscess from a necrotic tumour. An abscess shows restricted diffusion within the necrotic centre. A necrotic tumour does not typically show this pattern.

Computed tomography with contrast is a reasonable and more widely available alternative where magnetic resonance imaging is not immediately accessible. It shows a similar ring enhancing appearance. Sensitivity and specificity for distinguishing abscess from other ring enhancing lesions are somewhat lower.

Take blood cultures in every patient.

Stereotactic or open surgical aspiration of the abscess provides the definitive microbiological diagnosis and guides targeted antibiotic therapy. It is also the primary therapeutic intervention in most cases. Send material for Gram stain and culture (aerobic, anaerobic, fungal and, where relevant, mycobacterial) and, where indicated, histopathology.

Lumbar puncture is generally contraindicated in suspected brain abscess. A focal mass lesion combined with any raised intracranial pressure carries a significant risk of precipitating cerebral herniation. Cerebrospinal fluid analysis rarely contributes useful diagnostic information.

Identify the underlying source through imaging (sinus computed tomography, echocardiography where endocarditis is suspected) and clinical assessment. This is essential. It guides empirical treatment and allows the primary focus to be addressed alongside the abscess.

Empirical intravenous antibiotic therapy is guided by the likely source. It should cover streptococci and anaerobes. Where relevant to the source, it should also cover staphylococci and Gram negative organisms.

A typical regimen is ceftriaxone 2 g intravenously twice daily combined with metronidazole 500 mg intravenously three times daily. Add vancomycin where staphylococcal infection is a specific concern (recent neurosurgery, trauma, or haematogenous spread from a skin or line source).

Narrow antibiotic therapy once culture results are available from the aspirated abscess material. Blood cultures are a less ideal basis, used where surgical sampling is not feasible or has been deferred. Tailor treatment to the organism and susceptibility pattern identified.

Surgical intervention is either stereotactic needle aspiration or open craniotomy with excision. It is indicated for:

• abscesses larger than approximately 2.5 cm
• abscesses causing significant mass effect or neurological deficit
• abscesses not responding adequately to antibiotic therapy alone

Surgery relieves mass effect and obtains material for microbiological diagnosis. The approach is determined by abscess size, location, number and the patient's overall clinical condition.

Antibiotic therapy alone may manage:

• small abscesses (generally under 2.5 cm)
• multiple abscesses where surgical access to all lesions is impractical
• abscesses in a surgically inaccessible or high risk location

Monitor closely with clinical review and serial imaging. This confirms response and detects any need for delayed surgical intervention.

Total duration of antibiotic therapy is typically 6 to 8 weeks. It is guided by clinical response and serial imaging showing resolution or significant reduction in abscess size. Some patients transition to oral therapy for the latter part of the course. This needs clear clinical and radiological improvement and an appropriate oral agent with good central nervous system penetration.

Corticosteroids (dexamethasone) are reserved for significant surrounding oedema causing mass effect or midline shift. Use them cautiously and for the shortest effective duration. There is a theoretical concern about reduced antibiotic penetration into the abscess capsule and masking of clinical response.

Give anticonvulsant therapy for any seizure activity. Individualise the duration of ongoing prophylactic treatment.

Treat the identified underlying source alongside the abscess. This is an essential component of management (sinus surgery, mastoidectomy, dental treatment, valve surgery for endocarditis where indicated).