Infected brain cells, white blood cells, live and dead fungi, bacteria, and parasites all accumulate in a brain abscess. A membrane forms around the area, and this creates a mass.
As the swelling grows, it puts increasing pressure on brain tissue. Since the skull is not flexible and does not expand, the swelling can create pressure on the surrounding brain. This can lead to the destruction of delicate brain tissue.
A number of factors and conditions can lead to a brain abscess.
Here are some key points about the cause of a brain abscess. More detail is in the main article.
- A brain abscess can result from an infection around the head or face or elsewhere in the body, or as a result of a traumatic blow or puncture wound.
- The infection may be a bacterial, viral, or parasitic.
- Congenital heart disease, meningitis, and some chronic infections increase the risk in children.
- People with a weakened immune system have a higher chance of developing an abscess.
- Often, the cause remains unclear, but identifying it increases the chance of effective treatment.
How does infection enter the brain?
Pinpointing the cause of a brain abscess will lead to more effective treatment.
A brain abscess can occur as a result of direct infection, or when infection spreads from another part of the body.
The infection enters the brain from three main routes.
The bloodstream carries blood-borne infections from another part of the body. This accounts for between 15 and 30 percent of cases.
Normally, the blood-brain barrier stops infection from entering the brain, but sometimes an infection can cross the barrier and infect the brain. Exactly how or why this happens remains unclear.
If an infection from another part of the body travels through the bloodstream to the central nervous system (CNS) or somehow bypasses the blood-brain barrier, and reaches the brain, it can cause an infection and an abscess.
When a bacterial brain abscesses comes from a lesion somewhere else in the body, the bacteria are normally Streptococci.
The most common sources of blood-borne infections that cause a brain abscess are:
Cyanotic heart disease, where a heart defect that is present from birth results in low blood oxygen levels.
Pneumonia and other lung infections and conditions.
Bronchiectasis, a permanent widening, or dilatation, of the the bronchi, large air tubes which begin at the bottom of the trachea and branch into the lungs. This can result in recurrent respiratory infections, as well as other serious illnesses.
Peritonitis, or inflammation of the tissue layer of cells lining the inner wall of the abdomen and pelvis and other abdominal infections.
Cystitis, an inflammation of the bladder, and and other infections located in the pelvis.
Bacterial endocarditis, an infection of the heart tissues.
Other reported causes of brain abscess include liver abscess, tongue piercing, and gastrointestinal conditions and procedures.
When treating the abscess, it is important to know where the original infection came from. Treating the first infection site can prevent another brain abscess from arising.
Direct contagion happens when an infection starts in the skull, perhaps in the nose or ear, and spreads into the central nervous system, and from there into the brain.
- otitis media, or middle ear infection
- mastoiditis, an infection of the bone behind the eye
- osteomyelitis, an infection of the bone
This type of infection accounts for 25 to 50 percent of brain abscesses.
This can cause a bacterial infection to occur directly at the site of the abscess, for example, after surgery.
- If a blow to the head causes a compound skull fracture, it may push fragments of bone into the brain. This can cause a brain abscess.
- A penetrating head wound, cause, for example, by a bullet or some other foreign body may also be a source of infection, especially if it is not removed.
- A brain abscess may also be a complication of surgery.
In such cases, identifying the cause is relatively easy.
Risk factors in children
A child is more likely to develop a brain abscess if they have:
- congenital heart disease, or one that is present from birth
- a chronic, or long-term, otitis or sinusitis
- infections of the teeth, the jaw, the face, or the scalp
- traction, when pins or screws are used to hold the head and neck still
- an infection that comes from using a shunt, a device used to drain cerebral fluid
Brain abscesses are two to three times more common in males than in females, with children and older people being most at risk.
Other risk factors
People who living in a country with poor levels of sanitation and less access to health care are at higher risk, as are those with a weakened immune system.
In developing countries, 8 percent of masses that develop in the brain are abscesses, compared with 1 percent in western countries.
Weakened immune system
People with a weakened immune system have a higher risk of developing a brain abscess caused by a blood borne infection, especially one caused by fungi or parasites.
- people with HIV or AIDS
- infants under the age of 6 months
- patients receiving chemotherapy
- individuals on long-term steroid use
- organ transplant recipients who take immunosuppressant drugs to prevent organ rejection
A person with HIV may be at higher risk of infection with fungi, such as Aspergillus, and protozoa, for example, Toxoplasma gondii.
Aspergillus causes aspergillosis, an infection that can affect the lungs and other organs, and Toxoplasma gondii causes toxoplasmosis.
Poliovirus, Taoxoplasma gondii, and Cryptococcus neoformans most frequently cause brain abscesses in patients with AIDS.
Identifying the cause
Where the abscess develops can offer a clue to the cause.
- A middle-ear infection may result in lesions in the middle and posterior cranial fossae.
- Congenital heart disease with right-to-left shunts may result in abscesses in the distribution of the middle cerebral artery.
- Frontal and ethmoid sinus infections often result in abscesses in the subdural sinuses.
Brain infections are fairly uncommon, but when they happen they can be life-threatening. If there is any possibility of a brain abscess, the person should see a doctor at once.
Knowing what caused the abscess can enable more effective treatment, but studies show that in up to 40 percent of cases, the cause may not be clear.
In recent years, the rate of fatality from a brain abscess has fallen from 50 percent to 20 percent, mainly because CT scanning techniques can now enable more accurate and effective diagnosis.