A cerebral abscess is a localized, focal, suppurative infection within the brain parenchyma that begins as an area of cerebritis and evolves into a collection of pus surrounded by a capsule. It represents the host’s attempt to contain infection.

 

Q. What are the stages of evolution of a brain abscess?

 

1. Early cerebritis (1–3 days): Perivascular inflammation and necrosis, no capsule.
2. Late cerebritis (4–9 days): Expanding necrosis, peripheral fibroblast proliferation.
3. Early capsule (10–13 days): Capsule formation begins with fibroblast collagen deposition.
4. Late capsule (≥14 days): Thick collagen capsule surrounded by gliosis and reduced edema.
Steroids slow capsule formation and should be avoided unless there is significant mass effect.

 

Q. What are the routes of spread of infection? Which is most common?

 

1. Hematogenous spread (most common): From lung abscess, bronchiectasis, empyema, endocarditis, skin sepsis, pelvic infection, or cyanotic congenital heart disease. In congenital heart disease, right-to-left shunts bypass pulmonary filtration, allowing septic emboli to reach the brain. Usually results in multiple abscesses at the gray–white junction.

2. Contiguous spread (30–40 %): From sinusitis (especially frontal and sphenoid), otitis media, mastoiditis, or dental sepsis. Typically causes solitary abscess in adjacent lobe.

3. Direct inoculation: Post-neurosurgical procedures, compound fractures, or penetrating trauma.

4. Cryptogenic: No identifiable source (10–15 %).

 

Q. What are the common causative organisms?

 

Most common overall: Streptococcus species (esp. S. milleri group).
Polymicrobial: Found in up to 60 % of cases.
Anaerobes: Bacteroides, Peptostreptococcus, Fusobacterium.
Gram-negative bacilli: Proteus, E. coli, Klebsiella, Pseudomonas.
Neurosurgical procedures: Staphylococcus aureus, Staphylococcus epidermidis, Pseudomonas aeruginosa.
Penetrating trauma: Clostridium, Staphylococcus aureus, Pseudomonas.
In AIDS / immunocompromised: Nocardia, Toxoplasma gondii, Aspergillus, Candida, Mucor.

 

Q. What are the common locations and their sources?

 

Temporal lobe and cerebellum: Otitis media, mastoiditis
Frontal lobe: Frontal or sphenoid sinusitis, dental sepsis
Parietal lobe: Postoperative or post-traumatic
Occipital lobe: Chronic pulmonary sepsis, cyanotic heart disease
Multiple abscesses: Bacteremia or endocarditis

 

Q. What investigations will you perform?

 

CBC: Leukocytosis in ~60%.
ESR / CRP: CRP elevation in nearly all; sensitivity ~97%, specificity ~80%.
Blood cultures: Positive in 10–20%.
 

CT brain with contrast:
- Early cerebritis: Poorly defined hypodensity.
- Mature abscess: Ring-enhancing lesion with smooth inner margin, hypodense center, vasogenic edema.
- Double ring sign characteristic (outer capsule + inner granulation).
 

MRI:
- T1: Central hypointensity with enhancing capsule.
- T2: Central hyperintensity with hypointense rim.
- DWI: Restricted diffusion in abscess cavity (distinguishes from tumor necrosis).
 

Leukocyte (HMPAO) scan: Localizes active suppuration and differentiates abscess from tumor or sterile cystic lesions. (Most accurate)
 

MR spectroscopy: Shows lactate, acetate, and succinate peaks (unique to bacterial metabolism).
Aspiration for culture and sensitivity is the diagnostic gold standard.

 

Q. What are the imaging features distinguishing late cerebritis from early capsule stage?

 

In late cerebritis, rim enhancement appears irregular and incomplete.
In early capsule stage, the rim becomes smooth and complete with reduced central diffusion restriction.
With steroid therapy, enhancement and perilesional edema may prematurely decline, making differentiation challenging.

 

Q. What are the differential diagnoses of a ring-enhancing lesion?

 

High-grade glioma, metastasis, tuberculoma, toxoplasmosis, subacute infarct, demyelination, or resolving hematoma.

 

Q. How will you manage a case of cerebral abscess?

 

1. Medical management (selected cases only):
Indications:
- Abscess < 3 cm in diameter
- No significant mass effect or midline shift
- Glasgow Coma Scale > 12
- Etiology and organism known
- Abscess in deep or eloquent area (thalamus, brainstem) not surgically accessible
- Early cerebritis on imaging
- Multiple small abscesses without mass effect
- Post-surgical patients with sterile collections
- Concomitant meningitis or ependymitis (surgery contraindicated until sterilization)

Regimen:
- Ceftriaxone 2 g IV q12h or Cefotaxime 2 g IV q6h
- Metronidazole 500 mg IV q6h
- Vancomycin 15 mg/kg IV q8–12h
Duration: Minimum 6–8 weeks IV, continued until contrast enhancement resolves.
 

Endpoint: Disappearance of enhancement and edema on serial MRI.
Monitoring: ESR, CRP, weekly CT/MRI.
Steroids: Avoid unless mass effect is severe; they delay capsule formation and antibiotic penetration.

 

Q. What are the indications for surgical treatment?

 

1. Size > 3 cm
2. Neurological deterioration or low GCS
3. Mass effect or midline shift
4. Diagnostic uncertainty (to exclude tumor)
5. Failure of medical therapy after 2 weeks
6. Multiple abscesses with one dominant lesion
7. Superficial or traumatic abscess near dura
8. Posterior fossa abscess
9. Gas-containing abscess (suggests anaerobes)
10. Impending rupture into ventricles
 

Contraindications: Concomitant meningitis or ependymitis until infection controlled

 

Q. What are the surgical options?

 

Needle aspiration (stereotactic or open) is the mainstay for most cases; allows decompression and culture with minimal morbidity.
 

Excision (en bloc removal): Reserved for encapsulated abscesses near cortical surface, recurrent abscess, fungal or foreign-body-related lesions.
 

Multiple abscesses: Target the dominant or largest lesion first, followed by staged aspirations.
 

Penetrating trauma abscess: Debridement of necrotic brain and removal of foreign body, followed by prolonged antibiotics.

 

Q. What are the key points in postoperative management?

 

Serial CT or MRI every 48–72 h after aspiration.
Continue IV antibiotics for minimum 8 weeks or longer until full radiologic resolution.
Continue AEDs for 6 months (seizure prophylaxis).
Manage ICP; if refractory, consider decompressive craniectomy.
Treat and eradicate the primary focus (sinus, ear, dental, or cardiac source).

 

Q. What are the complications of cerebral abscess?

 

Rupture into ventricles (fatal in 80 %).
Subdural or epidural empyema.
Hydrocephalus.
Seizures and focal deficits.
Recurrence, especially with incomplete capsule removal or immunosuppression.

 

Q. What is the prognosis?

 

Mortality 10–20 %, prognosis is best with early diagnosis and adequate therapy.
Neurological sequelae (hemiparesis, epilepsy) in up to 50 %.
Fungal and multiple abscesses have poorer prognosis.