Brain Infection

What Are The Causes Of A Brain Abscess (Cerebral Abscess)?

The most common causes of a brain abscess, also known as a cerebral abscess, are bacteria or fungi infecting part of the brain. Parasites may also cause brain abscesses. The most common bacteria recovered from cultures are Streptococcus cocci.
 
When the bacteria, fungi or parasite infect part of the brain, inflammation (swelling) occurs. Infected brain cells, immune cells (white blood cells), live and dead organisms (fungi, bacteria, parasites) accumulate, a membrane forms around the area, creating a mass.
 
Our immune system creates the membrane to isolate the infection; however, sometimes our defense mechanism does more harm than good.
 
If the mass (abscess) swells, it places increasing pressure on brain tissue. The skull is not flexible; it does not expand. The mass can block blood vessels to the brain, resulting in the possible destruction of delicate brain tissue.  
 
Put simply:

  • Our immune system realizes it cannot overcome the infection.  
  • So it tries to curb its spread by forming a wall (membrane) around the source of infection, creating an abscess.  
  • It does this (create an abscess) in order to stop the pus from infecting healthy tissue.  
  • As the abscess grows it places pressure in delicate brain tissue, and sometimes blocks the supply of oxygen-rich-blood to that part of the brain.  
  • Delicate brain tissue can become damaged or destroyed.  

Brain infections are fairly uncommon for several reasons, one of which is the blood-brain barrier. This is a protective network of blood vessels and cells that filters out impurities from blood flowing to the brain.  
 
Sometimes an infection can get through the blood-brain barrier and infect the brain. The infection enters the brain from three main routes:  

The bloodstream (blood borne infections) - account for about 25% of cases. If an infection occurs somewhere else in the body, it can travel through the bloodstream, bypass the blood-brain barrier, and enter and infect the brain. Many bacterial brain abscesses come from a primary lesion somewhere else in the body. Locating that primary lesion is crucial, otherwise there is an increased risk of a relapse.  
 
People with weakened immune system have a higher risk of developing a brain abscess caused by a blood borne infection. Examples include people with HIV/AIDS, infants under the age of six months, patients receiving chemotherapy, individuals on long-term steroid use, and organ transplant recipients who take immunosuppressant drugs to prevent organ rejection.  
 
The most common blood-borne infections known to cause a brain abscess are:  
 

  • Cyanotic heart disease - Pneumonia, bronchiectasis and other lung infections and conditions.  
  • Peritonitis - and other abdominal infections.  
  • Cystitis - inflammation of the bladder; and other infections located in the pelvis  
  • Direct contagion - account for between 45% and 50% of brain abscesses. The infection starts off inside the skull, perhaps in the nose or ear and spreads into the brain. Examples include otitis media (middle ear infection), sinusitis, or mastoiditis (infection of the bone behind the eye).  
  • Direct trauma - account for about 10% of brain abscesses. A head trauma, in which a compound skull fracture may push fragments of bone into the brain, can also cause a brain abscess. In such cases identifying the cause is relatively easy. A bullet or some other foreign body which is left in place may also be a source of infection. In extremely rare cases a brain abscess may be a complication of surgery.  
  • The location of the abscess may help the doctor identify where the primary lesion is (where the original infection is):  
  • 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  

Fungi and parasites tend to cause brain abscesses in patients with weakened immune systems. Poliovirus, Taoxoplasma gondii, and Cryptococcus neoformans most frequently cause brain abscesses in patients with AIDS.