Brain Abscess -

Brain Abscess

Definition

A brain abscess is a focal, intracerebral infection that begins as a localized area of cerebritis (inflammation of brain parenchyma) and evolves into a collection of pus surrounded by a vascularized capsule.

Pathophysiology

Stages of evolution (histopathologic):

Stage	Time (approx)	Pathology	Key Feature
1. Early cerebritis	1–3 days	Localized inflammation, neutrophilic infiltration	No capsule
2. Late cerebritis	4–9 days	Central necrosis, macrophage infiltration	Beginning of capsule formation
3. Early capsule	10–13 days	Fibroblast proliferation, collagen capsule forms	Thin capsule, edema persists
4. Late capsule	≥14 days	Mature collagen capsule, surrounding gliosis	Organized abscess

Mechanism:

Local infection → cerebritis → necrosis → liquefaction → pus formation → capsule formation.

Etiology

1. Source of Infection

Route	Mechanism	Common organisms
Contiguous spread (45–50%)	From otitis media, mastoiditis, sinusitis, dental infection	Streptococcus milleri group, anaerobes, Bacteroides
Hematogenous spread(25%)	From lung abscess, endocarditis, cyanotic CHD	Streptococcus, Staphylococcus aureus
Direct inoculation	Trauma, neurosurgery	S. aureus, Gram-negative bacilli
Cryptogenic	No source found (15–20%)	Mixed flora

Common Causative Organisms

Setting / Predisposition	Common Organisms
Otitis / Mastoiditis / Sinusitis	Streptococcus milleri, anaerobes (Bacteroides, Peptostreptococcus)
Dental infection	Fusobacterium, Prevotella, Streptococcus
Post-traumatic / Post-surgical	S. aureus, Gram-negative bacilli (Enterobacteriaceae, Pseudomonas)
Cyanotic congenital heart disease	S. viridans, S. intermedius
Immunocompromised (esp. HIV)	Nocardia, Toxoplasma gondii, Aspergillus, Candida
Neutropenia / Transplant	Fungi (Aspergillus, Mucor), Nocardia

Location Correlation

Source	Typical Location
Otitis media / Mastoiditis	Temporal lobe, cerebellum
Sinusitis	Frontal lobe
Dental infection	Frontal lobe
Hematogenous spread	Multiple abscesses at gray-white junction

Clinical Features

1. General Symptoms

Headache (most common)
Fever (only 50% of cases)
Nausea, vomiting
Lethargy or altered sensorium
Raised ICP symptoms (papilledema)

2. Focal Neurologic Deficits

Hemiparesis, aphasia, visual field defects depending on site

3. Seizures

Present in ~25–35% (often focal)

4. Signs of Infection

Fever, leukocytosis (may be absent in immunocompromised)

Complications

Rupture into ventricle → ventriculitis → rapid deterioration, high mortality.
Rupture into subarachnoid space → meningitis.
Increased ICP, herniation.
Seizure disorder (late sequela).

Diagnosis

1. Neuroimaging

MRI (Preferred)

High sensitivity for early cerebritis.
Ring-enhancing lesion with surrounding vasogenic edema.
Central restricted diffusion on DWI (helps differentiate from tumor or necrosis).

CT Brain (with contrast)

Shows ring-enhancing lesion with hypodense center.
Multiple lesions suggest hematogenous spread.

Differential diagnosis on imaging:

Condition	DWI finding	Capsule	Typical context
Brain abscess	Restricted diffusion (bright core)	Smooth, thin	Infective
Metastasis	No restricted diffusion	Irregular	Cancer
Glioblastoma	Heterogeneous	Irregular	Older patient
Toxoplasma (HIV)	Multiple lesions	Variable	AIDS, CD4<100

2. Laboratory Studies

Blood cultures (positive in ~10–30%)
ESR/CRP elevated
Avoid LP (risk of herniation due to ↑ICP)

3. Aspiration / Stereotactic biopsy

For microbiologic diagnosis (gram stain, culture, anaerobic culture, fungal/AFB).

Management

1. Empiric Antibiotic Therapy (IDSA Guidelines)

Source / Setting	Empiric Antibiotic
Otogenic / Sinus / Dental	Ceftriaxone or Cefotaxime + Metronidazole
Post-traumatic / Neurosurgical	Vancomycin + Cefepime / Meropenem
Hematogenous (no source)	Ceftriaxone + Metronidazole ± Vancomycin
Immunocompromised	Meropenem + Vancomycin ± Voriconazole ± TMP-SMX (for Nocardia)

Duration:

6–8 weeks IV therapy, guided by clinical and radiologic response.

2. Surgical Intervention

Indications:

Abscess >2.5 cm in diameter
Mass effect or ↑ICP
Failure of medical therapy
Diagnostic uncertainty
Multiloculated or posterior fossa lesion

Options:

Stereotactic aspiration (preferred)
Craniotomy and excision (if superficial, multiloculated, or ruptured)

3. Supportive Management

ICP control: Elevate head, mannitol, hypertonic saline
Seizure control: AEDs (levetiracetam or phenytoin)
Steroids: Only if significant mass effect or edema causing herniation risk; otherwise avoided (may impair capsule formation).

Prognosis

Factor	Outcome
Early diagnosis + appropriate therapy	~90% recovery
Delay in therapy / rupture	↑ Mortality (up to 80%)
Immunocompromised / multiple abscesses	Poor prognosis

Residual neurologic sequelae (20–30%): seizures, focal deficits.

Special Types

1. Nocardia brain abscess

Occurs in immunocompromised (esp. transplant, steroid use)
Multiple abscesses common
Diagnosis: Modified acid-fast stain
Treatment: TMP-SMX ± Imipenem

2. Toxoplasma abscess

Common in HIV (CD4 <100)
Multiple lesions, basal ganglia/thalamus
Empiric therapy: Pyrimethamine + Sulfadiazine + Leucovorin

3. Fungal abscess (Aspergillus, Mucor)

Seen in neutropenia, diabetes, transplant
Hemorrhagic lesions, angioinvasive
Treatment: Voriconazole (Aspergillus), Liposomal Amphotericin B (Mucor)

Key Radiologic Differentiation Table

Feature	Brain Abscess	Tumor (Metastasis/Glioma)	Toxoplasmosis
DWI	Restricted (bright core)	No restriction	Restricted
Enhancement	Smooth ring	Irregular ring	Thin ring
Center	Pus (liquid)	Necrosis	Pus
Location	Gray-white junction	Variable	Basal ganglia, thalamus

Complications

Ventricular rupture (high mortality)
Subdural empyema
Hydrocephalus
Persistent epilepsy
Residual neurological deficit

Special Situations

A. Brain Abscess in Cyanotic CHD

Due to right-to-left shunt
Often parietal lobe
Streptococcal

B. HIV Patient

Multiple ring lesions → think:

Toxoplasmosis
Primary CNS lymphoma