Causes of a brain abscess

Abrain abscess is usually caused by infection with either bacteria or fungi.

If the immune systemis unable to killan infection, it will try to limit its spread byusing healthy tissue to form an Abscess ,to stop the pus infecting other tissue.

Infections of the brain are rare because the body has evolveda number of defences to protect this vital organ. One of these is the blood-brain barrier, a thick membrane that filters out impurities from blood before allowing it into your brain.

However, in some cases, germs can get through these defences and infect the brain.

Although the exactlocation of the original infection can't always be identified, the most common sources are described below.

Infection in the skull

In up to half of cases, the brain abscess occurs as a complication of a nearby infection in the skull, such as:

• a persistent middle ear infection ( otitis media )
• sinusitis an infection of the sinuses, the air-filled cavities inside the cheekbones and forehead
• mastoiditis infection of the bone behind the eye

This used to be a major cause of brain abscesses, but because of improved treatments for infections, a brain abscess is now a rare complication of these kinds of infection.

Infectionthrough the bloodstream

Infections spread through the blood are thought toaccount for around one in four casesof brain abscesses.

People with a weakened immune system have a higher risk of developing a brain abscess from a blood-borne infection. This is because their immune system may not be capable of fighting off the initial infection.

You may have a weakened immune system if you:

• have a medical condition that weakens your immune systemsuch as HIV or AIDS
• receive medical treatmentknown to weaken the immune systemsuch as chemotherapy
• have an organ transplant and take immunosuppressant drugs to prevent your body rejecting the new organ

The most commonly reported infections and health conditions that may cause a brain abscess are:

• cyanotic heart disease a type of congenital heart disease (a heart defect present at birth) where the heart is unable to carry enough oxygen around the body;this lack of a regular oxygen supply makes the body more vulnerable to infection
• pulmonary arteriovenous fistula a rare condition in which abnormal connections develop between blood vessels inside the lungs; this can allow bacteria to get into the blood and,eventually, the brain
• a dental abscess or treatment for tooth decay
• lung infectionssuch as pneumonia or bronchiectasis
• infections of the heartsuch as endocarditis
• skin infections
• infections of the abdomensuch as peritonitis (an infection of the bowel lining)
• pelvic infectionssuch as infection of the bladder lining ( cystitis )

Infectionafter a head injury

Direct trauma to the skull can also lead to a brain abscess and is thought to beresponsible for 1 in 10 cases.

The most commonly reported causes include:

• a skull fracture caused by penetrating injury tothe head
• a gunshot or shrapnel wound

In rare cases, a brain abscess can develop as a complication of neurosurgery.