BACKGROUND AND CLINICAL INFORMATION: Head

Age: Brain abscess can be seen in all age but most frequently seen in patients under 40 years old.

Etiology: Brain abscess are associated with lung infections (such as lung abscess, bronchiectasis, and empyema), congenital heart disease, infectious endocarditis, sinusitis, ottitis media, dental infection, penetrating wound, compromised immunity, and IVDA.

Subdural empyema:

Subdural empyemas represent an infectious process between the dura mater and arachnoid.
They are typically intracranial; spinal subdural empyemas are exceedingly rare. Usually a sigle agent is involved with aerobic and anaerobic streptococci as the most common etiologic agents.
In adults, the arachnoid is highly impermeable to bacterial infections occurring adjacent to it. Consequently, extension of subdural infection to the subarachnoid space is rare, although a sterile subarachnoid inflammatory exudate may develop.
In infants, subdural empyema may progress to leptomeningitis.

Microbiology: Most brain abscess not due to direct brain trauma are caused by anaerobic or microaerophilic bacteria, especially streptococci, and special care must be taken in the bacteriology laboratory to search properly for these fastidious pathogens.

Locations and prediposing factors:

Sinusitis and ear infection: Temporal lobe or cerebellar hemispheric abscess are usually due to infection of contiguous sites or primary infection such as ottitis media or mastoiditis. Frontal or temporal lobe abscess may also associate with frontoethmoidal sinusitis, and sphenoidal sinusitis. Predominantly caused by Streptococci, Bacteroides, Enterobacteriaceae, Staphylococcus aureus, and Haemophilus species.
Paranasal sinusitis is overwhelmingly predominant predisposing factor for subdural empyemas. Infection spreads from mucosal veins of the frontal sinus to emissary veins that link the facial and dural venous sinus systems. The same can happen with middle ear infection.
Dental infection: Frontal lobe abscess. Common infections agents include mixed Fusobacterium, Bacteroides, and Streptococcus species.
Penetrating wound or postsurgical infection: The location is related to the location of the wound or surgical site. Common infectious agents include Staphylococcus aureus, streptococci, Enterobacteriaceae, Clostridium species.
Distant site of primary infections: Multiple abscess cavities are seen and is most commonly seen along the distribution of the middle cerebral artery. Multiple abscesses due to congenital diseases are associted with Streptococcus viridans, anaerobic and microaerophilic streptococci, and Haemophilis species. Multiple abscesses associated with lung infections (such as lung abscess, empyema, and bronchiectases) are associated wtih Fusobacterilum, Actinomyces, Bacteroides, streptococci, Nocardia asteroides.

Immunecompromised host: Multiple abscess cavities are seen and is most commonly seen along the distribution of the middle cerebral artery.

Patients with abnormal cell-mediated immunity (such as AIDS) usually develop abscess due to Toxoplasma gondii, Nocardia asteroides, Cryptococcus neoformans, Listera monocytogenes, Mycobacterium species.
Patients with neutropenia or neutrophil defects are usually due to aerobic gram-negative bacteria, Aspergillus species, Zygonycetes (including Mucor ramosissimus), Candida species.
Transplantation: The incidence of brain abscess in solid organ recipients are related to the type of transplantation (which dictate the degree of immunosuppression)- overall (0.61%), liver (0.63%), kidney (0.36%), heart and heart-lung (1.17%). Nonfungal abscess (Nocardia and Toxoplasma sp) are seen in healthy graft recipients long after transplantation. Medical therapy is usually effective in these patients. Fungal abscess (Candida and Aspergillus sp) are more commonly seen in patients with complications including major subsequent operations, retransplantations, antirejection therapy, associated bacteremia or viremia, and multiorgan failure. [Arch Surg 1997 132:304-310]

Infected emboli: Infected emboli that bypass the filtration of the lungs and scattered throughout the body have a particular tendency to produce abscesses in the brain in preference to other organs, in particular if there is a preexisting damage in the brain.

GROSS PATHOLOGY: Head

Macroscopically, abscess has a necrotic or pus containing center and rimmed by firable brain parenchymal tissue. [Figure]

They are usually associated with a marked degree of edema. Such edema will show up in MRI or CT images.

Abscess due to metastatic infection of a primary site are most frequently located in the white matter, which is less vascular than the gray matter. Septic emboli may be trapped at the gray white junction where there is a sudden decrease in the caliber of vessels.

Abscess due to local infection such as ottitis media will have the cortex penetrated before the white matter is involved.