An abscess is a pus-filled swelling caused by infection with either bacteria or fungi.
The abscess is created by your immune system as a defence mechanism. If the immune system realises that it is unable to defeat an infection, it will try to limit its spread. Your immune system will use healthy tissue to form a wall around the source of infection to stop the pus infecting other tissue.
The routes for brain infection
Infections of the brain are very rare because the body has a number of defences to protect this vital organ. One of these is the blood-brain barrier, which is a thick membrane that filters out impurities from blood before allowing it into your brain.
However, in some people, for reasons that are not always entirely clear, germs can get through these defences and infect the brain.
The three most common routes for germs to enter the brain are as follows:
- Germs have already infected another part of the body, get into the bloodstream, bypass the blood-brain barrier and then infect the brain.
- Germs have already infected one of the nearby cavities in the skull (such as the ears or nose) and go on to infect the brain.
- Germs pass through the skull and enter the brain after the skull is damaged, for example after being hit by a blunt object or after a gunshot wound.
These are explained in more detail below.
In as many as one-third of cases, there is no apparent cause.
Germs invading the brain through the bloodstream
Infections that are spread through the blood are thought to be a major cause of brain abscesses.
People with a weakened immune system (who are immunocompromised) 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.
Known risk factors include:
- having a medical condition that weakens your immune system, such as HIV or AIDS
- receiving medical treatment that is known to weaken the immune system, such as chemotherapy
- having an organ transplant and taking immunosuppressants 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 congenital form of heart disease 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)
- lung infections, such as pneumonia or bronchiectasis
- skin infections
- infections of the abdomen (tummy), such as peritonitis (an infection of the lining of the bowel)
- pelvic infections such as infection of the lining of the bladder (cystitis)
Germs from another infection in the skull
In up to a quarter 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, which are the air-filled cavities inside the cheekbones and forehead)
- mastoiditis (an 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.
Germs invading the brain after a head injury
Direct trauma to the skull can also lead to a brain abscess.
The most commonly reported causes include:
- skull fracture caused by penetrating injury
- gunshot or shrapnel wound
In very rare cases, a brain abscess can develop as a complication of surgery.
