An epidural abscess is caused by infection in the area between the bones of the skull or spine, and the outer meninges (the membranes covering the brain and spinal cord). This infection is classified as an intracranial epidural abscess if it is located in the skull area, or as a spinal epidural abscess if it is found in the spine area. The infection is usually caused by bacteria (staphylococcus is common), but some may be caused by fungus. Infected material (pus) frequently includes destroyed tissue cells, white blood cells, and live or dead microorganisms which may wall off into an abscess. There is often inflammation of the tissues around the abscess in response to the infection. Most symptoms are due to enlargement of the abscess and surrounding inflammation, which can lead to tissue compression in the brain and spinal cord. The infection can result from the spread of nearby infections or it may be caused by microorganisms that spread from distant locations via the bloodstream. However, in up to one-third of patients, there is no identified source of infection. In the case of an intracranial epidural abscess (within the skull), risk factors include: chronic ear infections, mastoiditis, chronic sinusitis, in addition to head injury and recent neurosurgery. A spinal epidural abscess may be seen in patients with bone infections of the vertebral column (vertebral osteomyelitis), skin infections such as boils, bloodstream infections, and after back surgery or other invasive procedures involving the spine. People who inject drugs are also at increased risk. Epidural abscess is a rare disorder. Nine out of ten cases are located in the spine (spinal epidural abscess). The infection may spread into the bones of the spine or skull (osteomyelitis). It may also spread into the spinal fluid and cause meningitis, or lead to a brain abscess, or a spinal cord abscess.
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