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 ear
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.
Infection through the bloodstream
Infections spread through the blood are thought to account for around 1 in 4 cases of 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 system – such as HIV or AIDS
- receive medical treatment known to weaken the immune system – such as chemotherapy
- have an organ transplant and take immunosuppressant medicines 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 infections – such as pneumonia or bronchiectasis
- infections of the heart – such as endocarditis
- skin infections
- infections of the abdomen – such as peritonitis (an infection of the bowel lining)
- pelvic infections – such as infection of the bladder lining (cystitis)
Infection after a head injury
Direct trauma to the skull can also lead to a brain abscess and is thought to be responsible for 1 in 10 cases.
The most commonly reported causes include:
- a skull fracture caused by a penetrating injury to the head
- a gunshot or shrapnel wound
In rare cases, a brain abscess can develop as a complication of neurosurgery.