In the past, a brain abscess was "invariably fatal," but researchers writing in 2014 noted that progress in diagnosis and treatment have significantly increased the chances of survival.
The effects vary, depending on the size of the abscess and where it forms in the brain.
Between 1,500 and 2,500 cases occur each year in the United States. Brain abscesses are most likely to affect adult men aged under 30 years. Among children, they most commonly develop in those aged 4–7 years. Newborns are also at risk.
Vaccination programs have reduced the incidence of brain abscesses in young children.
The signs and symptoms of a brain abscess are as follows:
Headache is a common symptom of a brain abscess.
- a headache (69–70 percent of cases)
- a fever (45–53 percent)
- seizures (25–35 percent)
- nausea and vomiting (40 percent)
A seizure may be the first sign of an abscess. Nausea and vomiting tend to occur as pressure builds inside the brain.
Pain usually starts on the side of the abscess, and it may begin slowly or suddenly.
Changes in mental status occur in 65 percent of cases, and they may lead to:
- drowsiness and lethargy
- poor mental focus
- poor responsiveness
- slow thought processes
- coma (possibly)
Neurologic difficulties affect 50–65 percent of people with brain abscesses. These issues often follow a headache, appearing within days or weeks, and they can include:
- muscle weakness
- weakness or paralysis on one side of the body
- speech problems, such as slurred speech
- poor coordination
Other symptoms may include:
- a stiff neck, back, or shoulders
- blurred, double, or graying vision
The symptoms of a brain abscess result from a combination of infection, brain tissue damage, and pressure on the brain, as the abscess grows to take up more space.
If the headache suddenly becomes worse, it may mean that the abscess has burst.
In two-thirds of cases, symptoms are present for as long as 2 weeks. On average, doctors diagnose the issue 8 days after symptoms start.
A brain abscess is most likely to result from a bacterial or fungal infection in some part of the brain. Parasites can also cause an abscess.
When the bacteria, fungi, or parasites infect part of the brain, inflammation and swelling occur. In these cases, the abscess will consist of infected brain cells, active and dead white blood cells, and the organisms that cause the problem.
As the cells accumulate, a wall or membrane develops around the abscess. This helps to isolate the infection and keep it from spreading to healthy tissue.
If an abscess swells, it puts increasing pressure on surrounding brain tissue.
The skull is not flexible, and it cannot expand. The pressure from the abscess can block blood vessels, preventing oxygen from reaching the brain, and this results in damage or destruction of delicate brain tissue.
How infection enters the brain
Brain infections are fairly uncommon for several reasons.
One reason involves the blood-brain barrier, a protective network of blood vessels and cells. It blocks certain components from the blood that flows to the brain, but it allows others to pass through.
Sometimes, an infection can get through the blood-brain barrier. This can happen when inflammation damages the barrier, leading to gaps.
The infection enters the brain through three main routes.
- come through the blood from an infection in another part of the body
- spread from a nearby site, such as the ear
- result from a traumatic injury or surgery
Infection from another area of the body
If an infection occurs somewhere else in the body, the infectious organisms can travel through the bloodstream, bypass the blood-brain barrier, and enter and infect the brain.
Between 9 and 43 percent of abscesses result from pathogens that traveled from another part of the body.
Many bacterial brain abscesses stem from a lesion somewhere else in the body. It is crucial to find that primary lesion, or there may be a repeat infection in the future.
A person with a weakened immune system has a higher risk of developing a brain abscess from a bloodborne infection.
A person may have a weakened immune system if they:
- have HIV
- have AIDS
- are infants under the age of 6 months
- are receiving chemotherapy
- are using long-term steroid medication
- have had an organ transplant and take immunosuppressant drugs to prevent organ rejection
The most common infections known to cause brain abscesses are:
- endocarditis, an infection of the heart valve
- pneumonia, bronchiectasis, and other lung infections and conditions
- abdominal infections, such as peritonitis, an inflammation of the inner wall of the abdomen and pelvis
- cystitis, or inflammation of the bladder, and other pelvic infections
An untreated middle ear infection can lead to a brain abscess.
An infection can spread from a nearby area, and this accounts for 14–58 percent of brain abscesses.
If an infection starts inside the skull, for example in the nose or the ear, it can spread to the brain.
Infections that can trigger a brain abscess include:
- otitis media, or a middle ear infection
- mastoiditis, an infection of the bone behind the ear
The location of the abscess may depend on the site and type of the original infection.
A brain abscess can result from trauma, such as from neurological surgery or a penetrating brain injury.
An abscess can result from:
- a blow to the head that causes a compound skull fracture, in which fragments of bone are pushed into the brain
- the presence of a foreign body, such as a bullet, if someone does not remove it
- a complication of surgery, in rare cases
To diagnose a brain abscess, the doctor will evaluate signs and symptoms and look at the patient's recent medical and travel histories.
They will need to know whether the individual:
- has had an infection recently
- has a weakened immune system
Symptoms can be similar to those of other illnesses and conditions, so it may take time to confirm a diagnosis. The diagnosis will be more straightforward if the doctor can pinpoint exactly when symptoms started and how they progressed.
Tests may include:
- a blood test to check for high levels of white blood cells, which can indicate an infection
- imaging scans, such as an MRI or a CT scan, in which an abscess will show up as one or more spots
- a CT-guided aspiration, a type of needle biopsy, which involves taking a sample of pus for analysis
The number of fatalities from brain abscesses has fallen in recent decades, due to the increasingly routine use of CT and MRI scans in detection.
Treatment will generally involve surgery and medication.
Recent developments in medicine and technology mean that the chance of recovering from a brain abscess is much higher now than in the past.
Usually, if a doctor suspects a brain abscess, they will immediately prescribe wide-spectrum antibiotics, because an abscess can be life-threatening.
If tests show that an infection is viral rather than bacterial, the doctor will alter the treatment accordingly.
The effectiveness of the treatment will depend on:
- the size of the abscess
- how many abscesses there are
- the cause of the abscess
- the general state of the person's health
If the abscess is smaller than 1 inch across, the person will probably only receive intravenous antibiotic, antifungal, or antiviral medication. However, a doctor may need to drain a smaller abscess to determine which antibiotics will be best.
If an abscess is bigger than 1 inch across, a doctor will need to aspirate it, drain it, or cut it out.
If there are several abscesses, cutting them out may be too risky. The surgeon will recommend aspiration.
The person will also need treatment for any primary infection, for example in the lungs, abdomen, or nose.
A person may need surgery if:
- pressure in the brain continues to build
- the abscess does not respond to medication
- there is gas in the abscess
- there is a risk that the abscess might burst
A craniotomy is a procedure in which the surgeon makes an opening in the skull.
The steps are as follows:
- The surgeon will shave a small area of the scalp.
- They remove a small piece of bone, to gain access to the brain.
- They can either remove the abscess or drain the pus, possibly with the aid of a CT scan.
- They replace the bone and stitch the skin.
A short course of high-dosage corticosteroids may help if there is increased intracranial pressure and a risk of complications, such as meningitis.
However, doctors do not prescribe corticosteroids as a routine measure.
A doctor may prescribe anticonvulsants to prevent seizures, and a person who has had a brain abscess may need to take anticonvulsants for up to 5 years.
Studies show that 5–32 percent of brain abscesses are fatal.
Among those who survive, there may be lasting effects, depending on the extent of any damage. A brain abscess can lead to long-term neurological problems, involving issues with physical function, personality changes, and seizures.
It is important to recognize the possible symptoms of a brain abscess and to seek help immediately if they occur. The sooner a person receives treatment, the more likely it is to be effective, and the higher the chances of survival.
The outlook also depends on the characteristics of the abscess. Multiple abscesses and any located deep within the brain are more difficult to treat.
The doctor will monitor a person during recovery, as an abscess can recur. This may involve undergoing a weekly CT scan for at least 2 weeks. Also, an abscess can return after months or years, so long-term monitoring is essential.