Intracranial abscesses are uncommon, serious, life-threatening infections. They include brain abscess and subdural or extradural empyema and are classified according to the anatomical location or the etiologic agent. The term brain abscess is used in this article to represent all types of intracranial abscesses. 
Intracranial abscesses can originate from infection of contiguous structures (eg, otitis media, dental infection, mastoiditis, sinusitis) secondary to hematogenous spread from a remote site (especially in patients with cyanotic congenital heart disease), after skull trauma or surgery, and, rarely, following meningitis. In at least 15% of cases, no source can be identified. 
In recent years, the complex array of etiologic agents that cause brain abscess has become better understood.
Brain abscess is caused by intracranial inflammation with subsequent abscess formation. The most frequent intracranial locations (in descending order of frequency) are frontal-temporal, frontal-parietal, partial, cerebellar, and occipital lobes.  In at least 15% of cases, the source of the infection is unknown (cryptogenic). 
Infection may enter the intracranial compartment directly or indirectly via 3 routes.
Contiguous suppurative focus (45-50% of cases)
Direct extension usually causes a single brain abscess and may occur from necrotic areas of osteomyelitis in the posterior wall of the frontal sinus, the sphenoid and ethmoid sinuses, mandibular dental infections, as well as from subacute and chronic otitis media and mastoiditis.  This direct route of intracranial extension is more commonly associated with subacute and chronic otitic infection and mastoiditis than with sinusitis. 
Subacute and chronic otitis media and mastoiditis generally spread to the inferior temporal lobe and cerebellum. Frontal or ethmoid sinus spread to the frontal lobes. Odontogenic infections can spread to the intracranial space via direct extension or a hematogenous route. Mandibular odontogenic infections also generally spread to the frontal lobe.
The frequency of brain abscesses resulting from ear infections has declined in developed countries. However, abscesses complicating sinusitis has not decreased in frequency.  Contiguous spread could extend to various sites in the central nervous system, causing cavernous sinus thrombosis; retrograde meningitis; and epidural, subdural, and brain abscess.
The valveless venous network that interconnects the intracranial venous system and the vasculature of the sinus mucosa provides an alternative route of intracranial bacterial entry. Thrombophlebitis originating in the mucosal veins progressively involves the emissary veins of the skull, the dural venous sinuses, the subdural veins, and, finally, the cerebral veins. By this mode, the subdural space may be selectively infected without contamination of the intermediary structure; a subdural empyema can exist without evidence of extradural infection or osteomyelitis.
Intracranial extension of the infection by the venous route is common in paranasal sinus disease, especially in acute exacerbation of chronic inflammation. Chronic otitis media and mastoiditis generally spread to the inferior temporal lobe and cerebellum, causing frontal or ethmoid sinus infection and dental infection of the frontal lobe. 
Trauma (10% of cases)
Trauma that causes an open skull fracture allows organisms to seed directly in the brain. Brain abscess can also occur as a complication of intracranial surgery, and foreign body, such as pencil tip, lawn dart, bullets, and shrapnel. Occasionally brain abscess can develop after trauma to the face.
Hematogenous spread from a distant focus (25% of cases)
These abscesses are more commonly multiple and multiloculated and are frequently found in the distribution of the middle cerebral artery. The most common effected lobes (in descending frequency) are the fontal, temporal, parietal, cerebellar, and occipital. 
Hematogenous spread is associated with cyanotic heart disease (mostly in children), pulmonary arteriovenous malformations, endocarditis, chronic lung infections (eg, abscess, empyema, bronchiectasis), skin infections, abdominal and pelvic infections, neutropenia, transplantation,  esophageal dilatation, injection drug use,  and HIV infection.
Before the emergence of the AIDS pandemic, brain abscesses were estimated to account for 1 per 10,000 hospital admissions, or 1500-2500 cases annually.  The prevalence of brain abscess in patients with AIDS is higher, so the overall rate has thus increased.  The frequency of fungal brain abscess has increased because of the frequent administration of broad-spectrum antimicrobials, immunosuppressive agents, and corticosteroids.
Brain abscesses are rare in developed countries but are a significant problem in developing countries. The predisposing factors vary in different parts of the world.
With the introduction of antimicrobics and the increasing availability of imaging studies, such as CT scanning and MRI, the mortality rate has decreased to less than 5-15%. Rupture of a brain abscess, however, is associated with a high mortality rate (up to 80%).
The frequency of neurological sequelae in persons who survive the infection varies from 20-79% and is predicated on how quickly the diagnosis is reached and antibiotics administered. 
Brain abscesses are more common in males than in females.
Brain abscesses occur more frequently in the first 4 decades of life. Because the main predisposing cause of subdural empyema in young children is bacterial meningitis, a decrease in meningitis due to the Haemophilus influenzae vaccine has reduced the prevalence in young children.
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