An epidural abscess is a rare but potentially life-threatening disease that requires early detection and prompt management. It is defined as an inflammation that involves a collection of pus between the dura (the outer membrane that covers the brain and spinal cord) and the bones of the skull or spine. Spinal epidural abscess (SEA) and intracranial epidural abscess (IEA) are the two types of epidural abscess, and the difference is based on where they develop within the CNS and some variations in risk factors (see Pathophysiology) and symptoms (see History).
A loose association between the dura and vertebral bodies enables extension of spinal epidural abscess to numerous levels, frequently resulting in extensive neurological findings and often necessitating multiple laminectomies. The lumbar and thoracic spine are more commonly affected than the cervical spine.
Tight adherence of the dura to the skull limits expansion of intracranial epidural abscess, often resulting in dangerously increased intracranial pressure, which is a neurosurgical emergency.
Early recognition of these diseases and timely consultation with a neurosurgeon and infectious disease specialist is vital to optimizing the neurological outcome.
Spinal epidural abscess
Causes of spinal epidural abscess
Ten to thirty percent of spinal epidural abscesses result from direct extension of local infection, usually vertebral osteomyelitis, psoas abscess, or contiguous soft-tissue infection. [1, 2, 3, 4, 5, 6]
About half are due to hematogenous seeding. The most likely source is a soft-tissue process, but anything capable of causing bacteremia can result in spinal epidural abscess (endocarditis, urinary tract infection, respiratory tract infections, intravenous drug use, vascular access devices). Hematogenous seeding of the spinal epidural abscess can result in multilevel noncontiguous spinal epidural abscess.
Fifteen to twenty-two percent of spinal epidural abscesses are due to invasive procedures or instrumentation. Spinal surgery, epidural anesthesia, steroid and pain-relieving injections, and placement of pain pumps are all associated with spinal epidural abscess. Short-term epidural anesthesia is much less risky than a catheter left in place for days or permanently implanted. Rates of infection after intraoperative epidural block are about 1 in 2,000, while longer-duration (days) epidural pain catheter placement may be associated with rates of infection as high as 4.3%. Simple epidural injections rarely cause infection; the risk has been estimated at 1 in 10,000 to 1 in 60,000 injections.
In some cases (up to 30% in some series), the source of the spinal epidural abscess is not identified.
Risk factors for spinal epidural abscess
The most common risk factor for spinal epidural abscess is diabetes mellitus, followed by spinal trauma (may be remote) or surgery, intravenous drug abuse, alcoholism, renal insufficiency, immunosuppression (including infection, steroid use, cirrhosis, and malignancy), pregnancy, and spinal/epidural anesthesia or injections. [1, 2, 7, 8]
Intravenous drug use seems to represent an increasing risk factor in many series.
Anatomy of spinal epidural abscess
The thoracic and lumbar areas are the most likely sites of involvement, with the cervical spine accounting for approximately 20% of cases. 
Spread to multiple vertebral levels is common and occurs as the abscess extends up and down the spinal dural sheath. In some cases, this process involves most or all of the spine.
Mechanism of injury
Direct compression of the cord is clearly a major factor. 
Vascular occlusion due septic thrombophlebitis and/or vasculitis is also a factor
The exact mechanism of injury remains controversial.
Intracranial epidural abscess
The annual incidence of spinal epidural abscess has risen in the past 2-3 decades from 0.2-1 cases per 10,000 hospital admissions to 2.5-3 per 10,000 admissions.  The rising incidence of spinal epidural abscess has been attributed to the increasing prevalence of injection drug use, as well as to an increased performance of invasive spinal procedures.
The annual incidence of intracranial epidural abscess is difficult to determine but is recognized to be much less common than spinal epidural abscess.
Few data on epidural abscesses are available outside the United States, but the frequency appears to be similar to that in the United States.
Spinal epidural abscess: At the beginning of the 20th century, almost all individuals with spinal epidural abscess died. However, associated mortality rates have dropped significantly over the past 50 years, likely because of better diagnostic modalities. Nonetheless, despite advances in imaging and surgical care, the current mortality rate ranges from 2%-20%. [1, 2, 8] Not surprisingly, the mortality risk is greater in those with severe underlying comorbidities or uncontrolled sepsis. Differences in etiology (ie, iatrogenic vs noniatrogenic) do not affect the prognosis. The essential problem of spinal epidural abscess lies in the necessity of early diagnosis, as permanent neurological deficits and possible mortality can be avoided or reduced only with timely treatment.
Intracranial epidural abscess: With antibiotic and surgical management, intracranial epidural abscess carries a good prognosis, with an attributable mortality rate of less than 10%.
The neurological status of the patient at the time of diagnosis is the best predictor of neurological outcome, and morbidity is increased in both conditions when indicated surgery is delayed. [1, 2, 10] Comorbidities also often impact the outcome.
Most studies report that epidural abscess is more common in males than in females.
Spinal epidural abscess can occur at any age. The median age of onset of spinal epidural abscess is approximately 50-60 years.
Intracranial epidural abscess is most common in the second and third decades of life.
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