Brain Abscess

Definition, Etiology, PathogenesisTop

A brain abscess is a focal infection of the brain parenchyma. It spreads directly by continuity or via a hematogenous route (even from remote primary foci, such as the endocardium). An early inflammatory infiltrate disintegrates after ~2 weeks, forming a reservoir of purulent material enclosed in a thin, well-vascularized capsule that is surrounded by a zone of cerebral edema.

Etiology varies with the location of the primary site of infection as well as other risk factors:

1) Sinusitis: Aerobic or anaerobic streptococci, Haemophilus spp, Bacteroides spp, Fusobacterium spp, Streptococcus anginosus.

2) Otitis media or mastoiditis: Streptococci, aerobic gram-negative intestinal bacilli (particularly Proteus spp), Bacteroides spp, Pseudomonas aeruginosa.

3) Endocarditis: Viridans-group streptococci.

4) Trauma: Staphylococcus aureus.

5) Impaired cell-mediated immunity: Fungi Candida spp, Aspergillus spp, rarely Cryptococcus neoformans; in patients with AIDS predominantly Toxoplasma gondii, fungi (Cryptococcus neoformans).

6) Tuberculous brain abscess (tuberculoma) filled with caseous mass.

7) Other: Dental infections, surgery-related infections, bronchiectasis, pulmonary abscess, sepsis.

Clinical Features and Natural HistoryTop

Headache is the presenting symptom; it is often dull, generalized, and in 50% of patients accompanied by fever. Signs and symptoms of elevated intracranial pressure and cerebral edema develop gradually. Focal neurologic deficits are present in 50% of patients and include paresis, paralysis, aphasia, and seizures. Twenty-five percent of patients have papilledema.

Mortality rates are up to 25%. Permanent neurologic deficits (paresis, paralysis, seizures) develop in 30% to 50% of patients.

DiagnosisTop

1. Laboratory tests: Nonspecific findings in routine cerebrospinal fluid (CSF) analysis (elevated opening pressure; other parameters may be normal). Blood and CSF cultures are usually negative. Culture of needle-aspirated abscess material has the highest diagnostic yield (cultures should always include both aerobic and anaerobic bacteria and fungi). Due to the risk of brain herniation, consultation with a neurosurgeon and careful consideration of brain biopsy versus lumbar puncture after performing computed tomography (CT) and/or magnetic resonance imaging (MRI) of the head is needed.

2. CT and MRI: Characteristic features include a hypodense enhancing inflammatory infiltrate with a well-enhancing capsule and a peripherally located hypodense zone of cerebral edema. The adjacent brain structures are displaced and compressed (mass effect).

TreatmentTop

1. Surgical treatment is the method of choice. It includes needle aspiration or surgical resection (particularly in the case of abscesses located in the posterior cranial fossa and those of fungal and tuberculous etiology).

Contraindications: Multiple brain abscesses, a difficult surgical approach, small abscesses <2 cm in diameter.

2. Antimicrobial therapy: An IV third-generation cephalosporin + IV metronidazole for 8 weeks in combination with surgery or alone in patients in whom surgery is not feasible. Fungal CNS infections: see Meningitis.

3. Supportive treatment: see Meningitis.

4. Monitoring: In our practice, assuming clinical improvement, we repeat the radiologic assessment after 4 to 6 weeks. A deterioration or even lack of response may dictate the need to repeat the assessment earlier.

ComplicationsTop

Perforation of the abscess into the brain ventricles is the most serious complication. It manifests as a sudden deterioration of the patient’s general condition and is associated with mortality rates of 80% to 100%.