Background
An epidural abscess is a rare but potentially life-threatening condition that necessitates early detection and prompt management. It is characterized by a collection of pus between the dura (the outer membrane covering the brain and spinal cord) and the bones of the skull or spine. The two types of epidural abscess are spinal epidural abscess (SEA) [1] and intracranial epidural abscess (IEA) [2] , differentiated by their location within the central nervous system (CNS) and variations in risk factors and symptoms (see Pathophysiology).
The loose association between the dura and vertebral bodies allows SEAs to extend across multiple levels, often leading to significant neurologic findings and the need for multiple laminectomies. The lumbar and thoracic regions are more frequently affected than the cervical spine.
In IEAs, the dura's tight adherence to the skull restricts their expansion, which can lead to dangerously increased intracranial pressure, constituting a neurosurgical emergency.
Timely recognition of these conditions and consultation with a neurosurgeon and infectious disease specialist are crucial for optimizing neurologic outcomes. [3]
Microbiologic causes
Spinal epidural abscess
The microbiologic causes of spinal epidural abscess (SEA) include the following:
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S aureus (60%; up to 25% may be methicillin-resistant S aureus [MRSA]) [4]
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Enteric gram-negative bacilli, especially Escherichia coli (10%); their incidence is rising in some series
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Coagulase-negative staphylococci (3%-5%), primarily involving spinal instrumentation or epidural anesthesia/injections
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Bacteroides species and other anaerobes (2%) [18]
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Pseudomonas species (2%)
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Streptococci, including Streptococcus viridans, group B streptococci, and pneumococcus (10%)
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Pasteurella multocida - few case reports [19]
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Mycobacteria, usually Mycobacterium tuberculosis (< 1% in Western countries but much more common in developing countries)
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Polymicrobial (possibly 5%-10%)
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Unknown (6%-10%)
Intracranial epidural abscess
In intracranial epidural abscess (IEA), upper-respiratory bacterial pathogens predominate in sinus-associated disease, whereas nosocomial pathogens are of concern in cases that develop after craniotomy. The most common causative organisms include the following:
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Staphylococci, both coagulase-positive and coagulase-negative
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Streptococci, including anaerobic and microaerophilic species
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Aerobic gram-negative bacilli
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Propionibacterium acnes
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Other anaerobes
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Can be polymicrobial
Other causes
Various factors can lead to spinal epidural abscesses. Direct extension of local infections, such as vertebral osteomyelitis or psoas abscess, accounts for 10%-30% of cases. [4] Hematogenous seeding, often originating from soft-tissue processes or conditions that cause bacteremia—including endocarditis, urinary tract infections, and intravenous drug use—represents approximately half of the cases. [5] Invasive procedures, including spinal surgery and epidural anesthesia, are linked to 15%-22% of SEA, with infection rates varying based on the duration of catheter placement. [6] In some instances, the source of the abscess remains unidentified.
Pathophysiology
Spinal epidural abscess
Risk factors
Common risk factors include diabetes mellitus, spinal trauma or surgery, intravenous drug abuse, alcoholism, renal insufficiency, immunosuppression, and the presence of central lines or implantable devices. [3, 4, 8, 9, 10] The prevalence of intravenous drug use as a risk factor is increasing.
Anatomy
Most abscesses occur posteriorly, whereas anterior locations are often associated with vertebral osteomyelitis or psoas abscess. The thoracic and lumbar regions are the most common sites, with the cervical spine accounting for about 20% of cases. [11] Abscesses can spread to multiple vertebral levels as they extend along the spinal dural sheath.
Mechanism of injury
Significant factors contributing to injury include direct compression of the spinal cord and vascular occlusion due to septic thrombophlebitis or vasculitis. Additionally, infarction of the cord from loss of arterial blood flow may contribute, although the exact mechanism remains debated. [6]
Intracranial epidural abscess
Causes
An IEA is a suppurative infection of the epidural space, which refers to the area between the dura mater and the inner table of the skull. Due to the dura's tight adherence to the skull, IEAs typically appear well-circumscribed on radiologic imaging and may progress subacutely. There is often associated osteomyelitis of the overlying bone. [12]
Intracranial epidural abscesses most commonly arise from direct, contiguous spread of infection from the paranasal sinuses (eg, frontal sinus), middle ear (eg, from otitis media), orbit, or mastoids (eg, from mastoiditis). They may also occur as complications of neurosurgical procedures (iatrogenic IEAs) or as sequelae of head trauma. Approximately 1%–3% of craniotomies result in an IEA, particularly those involving breaches of the frontal sinus. [45] In about 10% of cases, an IEA is associated with a subdural empyema, a pyogenic infection located between the dura and arachnoid mater, which progresses more rapidly and carries a poorer prognosis. Autopsy studies have shown that 81% of patients with IEAs exhibit evidence of subdural extension of the infection, with 35% having meningitis and 17% presenting with intraparenchymal abscesses.
Causative organisms
The most common causative organisms in non-neurosurgical IEAs are streptococci and anaerobes, particularly in cases associated with sinusitis. In contrast, staphylococci (especially S aureus) are the predominant pathogens in traumatic and neurosurgical-associated IEAs, with gram-negative bacteria occurring less frequently.
Mechanism of injury
Once the organism enters the epidural space, hyperemia and fibrin deposition occur, leading to the accumulation of purulent material and the development of chronic granulation and fibrous tissue. Dural attachments, particularly at sutures, help contain the infection. However, if this barrier is compromised due to trauma or surgery, further spread of the infection may occur, resulting in complications such as osteomyelitis of the bone flap, subdural empyema, dural sinus or cortical vein thrombosis, purulent leptomeningitis, and intraparenchymal brain abscess. The virulence of the organism, the immune resistance of the host (eg, in immunocompromised patients), and the promptness of treatment significantly influence the outcome of this condition. [6]
Epidemiology
Frequency
In the United States, the annual incidence of SEA has increased from 0.2-1 cases per 10,000 hospital admissions in the 1980s and 1990s to 2.5-5.1 per 10,000 admissions. [4] This rise is attributed to increased injection drug use, more invasive spinal procedures, and improved diagnostic capabilities (eg, MRI). The incidence of IEA is recognized to be much lower than that of SEA.
Mortality/morbidity
Historically, SEA had a high mortality rate, but this has decreased significantly due to advancements in diagnostics and treatment. Current mortality rates range from 2%-20%, with higher risks in patients with severe comorbidities or uncontrolled sepsis. The prognosis is not influenced by the etiology of the abscess. Early diagnosis is critical to prevent permanent neurologic deficits and mortality.
Intracranial epidural abscess has a favorable prognosis with an attributable mortality rate of less than 10%. The patient's neurologic status at diagnosis is the best predictor of outcomes, with increased morbidity associated with delayed surgery. [8, 13] Comorbidities also affect prognosis.
Sex and age
Epidural abscesses are more common in males than females. Spinal epidural abscess can occur at any age, with a median onset around age 50-60 years. Intracranial epidural abscess is more prevalent in boys and men, particularly in younger populations due to higher rates of otologic infections and sinusitis. [14] Iatrogenic IEAs can occur at any age, especially in the elderly with comorbidities affecting immune response and wound healing. [15]
Prognosis
The degree of neurologic recovery after surgery correlates with the duration and initial severity of the neurologic defect. Spinal epidural abscess carries a mortality rate of 2%-20%. Recurrences occur in about 6.6% of cases, particularly in those with a history of intravenous drug use or local spinal wound infection. [16] Poor outcomes are more likely in patients with multiple medical problems, prior spinal surgery, or significant thecal sac compression. The neurologic status of the patient at the time of diagnosis is the best predictor of neurologic outcome, and morbidity is increased in both conditions when indicated surgery is delayed.
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CT scan showing a lenticular-shaped intracranial epidural abscess.
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