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A brain abscess is a localized collection of pus in the brain.
Brain abscesses are fairly uncommon. They can result from an infection that spreads from somewhere else in the head (such as a tooth, the nose, or an ear) or that spreads from another part of the body through the bloodstream to the brain. An abscess may form when bacteria enter after a head wound that penetrates the brain, including those that occur during brain surgery.
Many types of bacteria, including Staphylococcus aureus and Bacteroides fragilis can cause a brain abscess. Toxoplasma gondii (see Parasitic Infections: Toxoplasmosis), a protozoan, and fungi, such as aspergilli, are common causes of brain abscess in people who have a weakened immune system. The immune system may be weakened by disorders such as human immunodeficiency virus (HIV) infection, which leads to acquired immunodeficiency syndrome (AIDS), or by drugs that suppress the immune system. Such drugs may be used to prevent rejection of a transplanted organ or to treat cancer or autoimmune disorders.
Fluid collects around a brain abscess. As a result, the surrounding brain tissue swells, and pressure within the skull increases. The larger the abscess, the greater the swelling and the pressure. If the abscess leaks or breaks and the pus enters the cerebrospinal fluid, acute meningitis results.
Symptoms
A brain abscess can cause many different symptoms, depending on its location, its size, and the extent of inflammation and swelling around the abscess. People may have a headache, feel nauseated, vomit, become unusually drowsy, and then lapse into coma (which is often occurs when pressure within the brain continues to increase). Seizures may occur, one side of the body may become weak, or thinking may be impaired. Symptoms can develop over days or weeks. A fever and chills may occur at first but then disappear.
Diagnosis
The best test for diagnosing a suspected brain abscess is magnetic resonance imaging (MRI) that uses a substance called gadolinium. Gadolinium, a paramagnetic contrast agent that is injected intravenously, shows up in injured brain regions that have lost their blood-brain barrier—in other words, lost their ability to be highly selective in what molecules are permitted entry from blood into the brain. (Gadolinium is not harmful.) Alternatively, computed tomography (CT) that uses a dye visible on x-rays (radiopaque dye) can be done. MRI has higher resolution and can show early abnormalities better than CT. However, additional tests may be needed to establish the diagnosis because a brain tumor or damage due to a stroke can resemble an abscess. A specialized form of MRI, called magnetic resonance spectroscopy, can distinguish between an abscess (which contains dead or dying tissue) and a tumor (which contains living rapidly duplicating cells).
To identify the causative organism, doctors withdraw a sample of pus from the abscess with a needle. It is examined under a microscope and sent to a laboratory to grow (culture) bacteria in the fluid so that they can be identified. MRI or CT is used to guide the needle into the abscess. For this procedure (called stereotactic aspiration or biopsy), a frame is attached to the skull. The frame provides reference points that can be identified on the MRI or CT scan and enable doctors to guide the needle precisely into the abscess.
Treatment
A brain abscess is fatal unless treated with antibiotics and possibly surgery. The most commonly used antibiotics are cephalosporins (such as cefotaxime or ceftriaxone), vancomycin or nafcillin, and metronidazole. An antibiotic is usually given for 4 to 6 weeks, and MRI or CT is repeated every 2 weeks to monitor the response to treatment. If the abscess does not shrink, a surgeon may have to drain the abscess with a needle (using stereotactic techniques to guide placement of the needle) or perform open surgery to remove the entire abscess. Recovery may be quick or slow depending on how successful surgery is, how many abscesses are present, and how well the person's immune system is functioning. If people with a weakened immune system have an abscess due to Toxoplasma gondii or a fungus, they must take antibiotics for the rest of their life.
Doctors treat the swelling and increased pressure within the skull aggressively because these effects can permanently damage the brain. Corticosteroids, such as dexamethasone, and other drugs that reduce swelling and pressure (such as mannitol) may be used.
Anticonvulsants may be given to prevent seizures.
Last full review/revision May 2008 by Michael Jacewicz, MD
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