What is a Brain Abscess?

A Brain Abscess is a localized pocket of infection.  Just like when a surface wound becomes infected and swells, an infection in the brain causes swelling and forms an abscess.  Abscesses form quickly.  As the infection enters the brain it creates a round pocket that grows as the infection spreads.  Often there is an area of fluid or pus in the center of the abscess which can expand produce pressure on the surrounding brain tissue causing the intracranial pressure to increase. The increasing pressure may cause certain brain activities to malfunction.  The brain is a delicate machine and must maintain a constant internal pressure to function properly, so as the area of infection swells, the increased intracranial pressure (ICP) threatens the normal functioning of the brain and can lead to severe brain damage. With the use of MRIs and the development of effective antibiotics, abscesses are rarely fatal if they are diagnosed promptly.

To enter the brain an infection needs a clear pathway to its interior.  After neurosurgery, or any penetrating head injury, there is always a chance that infection will enter the brain.  Brain abscesses are rare and are also seen in people with suppressed immune systems, like organ or bone marrow transplant recipients, or patients with immunosuppressant diseases.  Infections have been known to reach the brain from other parts of the body through the bloodstream, but more commonly pick a direct route through the sinuses or ear canal.  Another means of entry is osteomyelitis, which is an infection of the bone.  If the skull becomes infected, the infection can pass through the skull and into the brain.  

There are three major classifications of brain abscesses that deal mainly with where the abscess forms.  Cerebral abscess (discussed above) is located in brain tissue, while Cranial and Spinal Subdural Empyemas are abscesses that develop between the brain and the dura mater (which is a protective layer toward the outside of the brain), or in rarer cases in the same area of the spinal cord.   Subdural  empyemas, or abscesses, are typically the result of a sinus infection which travels through the sagittal sinus and spreads the infection into the outer layers of the brain or spinal cord. While empyemas can impair normal brain functioning by swelling and raising ICP producing impaired function of the brain or spinal cord.  However, because of the proximity to the outer layers of the brain and the deeper brain tissue, it is necessary to identify the empyema right away to prevent the possible spread of infection deeper into the brain.

If an epidural abscess is found, the most important objective is to remove the abscess, stop the infection, and relieve the pressure on the brain or spine as soon as possible.  Spinal epidural abscesses are most often linked to the spread of bacteria through the bloodstream.  Cranial epidural abscesses are typically caused by an infected wound, a local infection, or by an area of osteomyelitis that spreads into the epidural space.

Symptoms

The infection will produce inflammation in the brain tissue in the first 1-4 days, and will cause headache or other mild symptoms to appear.  Persistent headache is the most common symptom of a brain abscess. The headache can present in one specific area, or be a more generalized headache, and will not respond to medication.  After day four, with the formation of the abscess capsule, symptoms will become more severe. Hydrocephalus and neurological deficit set in, and by two weeks, a well-developed abscess leads to a noticeable decrease in brain function.  Mild confusion and coma are also associated with brain abscesses.  The symptoms of subdural empyemas progress quickly and most commonly present with sinusitis, fever and neurological deficit.  Other symptoms are:

• Headache

• Hydrocephalus (see Hydrocephalus)

• Edema (an area of swelling, pus and dead tissue)

• Fever

• Vomiting

• Mild Confusion/disorientation and Coma

• Focal neurological deficit – in which a specific area of the brain is affected and causes difficulty in the person’s functioning

• Seizures  (usually the result of increased intracranial pressure)

Diagnosis

Because symptoms develop so quickly, abscesses distinguish themselves clinically from other brain maladies. Magnetic resonance imaging (MRI) is the preferred method of initial diagnosis, and abscesses typically show up as a ring-shaped area.  With the use of magnetic resonance imaging it is possible to confirm the presence of a cerebral abscess and its location.  Cranial epidural abscesses and subdural empyemas are also diagnosed with the use of magnetic resonance imaging.  Lumbar puncture to procure a spinal fluid sample for analysis is strongly discouraged as a means of diagnosis, and may lead to serious complications.  Specifically, sinusitis, fever, and neurological deficit appearing together in a patient indicate the presence of a subdural empyema and should be regarded as highly suspicious.

Treatment

Treatment is the same for all abscesses. Once the abscess has been located, the type of infection must be identified so it can be treated with antibiotics.  Usually a sample of the abscess is obtained for analysis, or a sample of pus from the infected area is used.  Once the type of infection is determined antibiotic treatment should begin immediately. Intravenous treatment with antibiotics should continue for 3-6 weeks, then another 3 weeks of oral treatment should follow, with close monitoring for the disappearance of any sign of infection.  During antibiotic treatment, MRIs are taken frequently to monitor the abscess.  If the abscess does not appear to be shrinking then the decision must be made to re-aspirate the abscess or remove the abscess surgically.  Aspiration is also recommended in lieu of surgery if the abscess is in a sensitive area where damage to the surrounding tissue would have lasting effects, or if the abscess is deeply lodged in the brain where total removal would be difficult.

Surgical Treatment

Small brain abscess may be treated with antibiotics over several weeks.  If the brain abscess is large with significant brain swelling, surgical drainage of the abscess is indicated to accelerate healing and relieve increased intracranial pressure.

Post-operative Management

It is crucial that the patient continues taking the prescribed antibiotic medication for the full length of time recommended by their physician, and that any alteration in the patient’s condition following surgery is reported quickly.  Follow up MRIs should be taken after surgery while antibiotic treatment continues (6-8 weeks), and also one after treatment has ended.  An additional MRI should be given one - two months later to confirm the patient’s complete recovery.