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Brain Abscess

Brain abscess is a bacterial infection within the brain. Brain abscess is a free or encapsulated collection of pus that usually occurs in the temporal lobe, cerebellum, or frontal lobes. It can vary in size and may present singly or multilocularly. Brain abscess has a relatively low occurrence. Although it can occur at any age, it's most common in people ages 10 to 35 and is rare in older adults.

An untreated brain abscess is usually fatal; with treatment, the prognosis is only fair. About 30% of patients develop focal seizures. Multiple metastatic abscesses secondary to systemic or other infections have the poorest prognosis.

Causes

A brain abscess usually occurs secondary to some other infection, especially otitis media, sinusitis, dental abscess, and mastoiditis. Other causes include subdural empyema; bacterial endocarditis; human immunodeficiency virus infection; bacteremia; pulmonary or pleural infection; pelvic, abdominal, and skin infections; and cranial trauma, such as a penetrating head wound or compound skull fracture.

This condition also occurs in about 2% of children with congenital heart disease, possibly because the hypoxic brain is a good culture medium for bacteria. Common infecting organisms are pyogenic bacteria, such as Staphylococcus aureus and Streptococcus viridans. Penetrating head trauma or bacteremia usually leads to staphylococcal infection; pulmonary disease, to streptococcal infection.

Pathophysiology

A brain abscess usually begins with localized inflammatory necrosis and edema, septic thrombosis of vessels, and suppurative encephalitis. This is followed by thick encapsulation of accumulated pus, and adjacent meningeal infiltration by neutrophils, lymphocytes, and plasma cells.

Risk factors for developing a brain abscess include the following:

• congenital (present at birth) heart disease
• meningitis
• chronic middle ear and sinus infections
• dental or jaw infections
• infections of the face or scalp
• head injury or skull fracture
• traction - a medical device that uses pins/screws that are placed around the head to hold the head and neck areas still; used in patients with broken necks or for specific surgeries that require the head and neck to be immobilized.
• shunt (devices used to drain excess amounts of cerebral spinal fluid) infections

Signs and symptoms

Onset varies according to cause and location. Early symptoms are characteristic of a bacterial infection and include headache, chills, fever, malaise, confusion, and drowsiness. The white blood cell count is elevated with a differential indicating infection. As the lesion enlarges, it produces clinical effects similar to those of a brain tumor. At this time, symptoms correlate with a disturbance of function in the invaded lobe.

other features differ with the site of the abscess:

• temporal lobe abscess: auditory receptive dysphasia, central facial weakness, hemiparesis
• cerebellar abscess: dizziness, coarse nystagmus, gaze weakness on the lesion side, tremor, ataxia
• frontal lobe abscess: expressive dysphasia, hemiparesis with unilateral motor seizure, drowsiness, inattention, mental function impairment, seizures.

Diagnosis

Diagnosis of brain abscess is performed by using a computed tomography scan (CT) or a magnetic resonance imaging (MRI) scan to determine the site of infection. Tissue removal (biopsy) is usually performed as well. A biopsy is performed to determine the type of bacterium involved. Biopsies can also be used to rule out tumor or other noninfectious localized lesions, which may look the same on the scans.

Other tests are performed to determine the source of the infection. These tests include blood cultures, x rays of the chest, and a physical exam of the ears, sinuses, and teeth. A test for human immunodeficiency virus (HIV) is usually also performed.

Treatment

Therapy consists of antibiotics to combat the underlying infection and surgical aspiration or drainage of the abscess. However, surgery is delayed until the abscess becomes encapsulated (a CT scan helps determine this) and is contraindicated in patients with congenital heart disease or another debilitating cardiac condition. Administration of a penicillinase-resistant antibiotic, such as nafcillin or methicillin, for at least 2 weeks before surgery can reduce the risk of spreading infection.

Other treatments during the acute phase are palliative and supportive; they include mechanical ventilation and administration of I. V. fluids with diuretics (urea, mannitol) and glucocorticoids (dexamethasone) to combat increased ICP and cerebral edema. Anticonvulsants, such as phenytoin and phenobarbital, help prevent seizures.

Prevention

The risk of developing a cerebral abscess may be reduced by treating any disorders that can cause them. Such treatment should include a follow-up examination after infections are treated.

Preventive antibiotics given for people with congenital or rheumatic heart disorders prior to dental or urologic procedures may reduce the risk.