Epidural abscess pathophysiology

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: João André Alves Silva, M.D. [2]

Overview

According to the location of the epidural abscess, its pathophysiology will differ, particularly in the origin of the infection but also in symptoms, commonly causing organism, progression of the condition and therefore the treatment required. In the case of intracranial epidural abscess, it surges most frequently as a complication of cranial surgical procedures and sinusitis, particularly paranasal sinusitis, as the infection progresses intracranially. On the other hand, spinal epidual abscess happens most frequently due to spinal instrumentation, vascular access and injection drug use.

Pathophysiology

Intracranial Epidural Abscess

Although less common than spinal epidural abscess and with a more indolent evolution, it also requires prompt diagnosis and treatment to avoid severe outcomes. Since intracranial dura mater is adherent to the inner table of the skull, the so-called epidural space is actually a virtual space, which can become a real space by increasing pressure from a liquid, such as pus or blood or a solid mass, such as a tumor. This tight adherence contributes to the slow progression and typical round-shape appearance of the abscess. Because the dura mater is tightly attached to the skull, in the area of the foramen magnum, this abscess is usually restricted to the cranial cavity. On the periphery of the pus collection is created a wall of inflammation, which may calcify and therefore be identified in imaging studies.

• Etiologies:
  • Paranasal sinusitis, particularly of the frontal sinuses.
  • Osteomyelitis of the skull (possible association with Pott's Puffy Tumor.
  • Extension of infection from otitis, mastoiditis or orbit.
  • Direct inoculation, during a surgical procedure or trauma.
  • In pediatric population: scalp venous catheter.
  • Remote infection sites, through haematological spread, such as respiratory and urinary tract infections.
  • Possible association with exposure to expanded polytetrafluethylene.

• In the case of an intracranial epidural abscess originated in an infection site, such as sinusitis, the organisms may reach the virtual epidural space by several means:
  • thrombophlebitis of diploic veins, which is facilitated by the valveless nature of these vessels, allowing for retrograde flow of bacteria.
  • direct extension from osteomyelitis of the skull.
  • following trauma.
  • following surgery.
  • congenital defect, such as communication between sinuses and areas of lesion.

Spinal Epidural Abscess

Unlike the virtual intracranial epidural space, the spinal epidural space is a real space, more specifically in the areas posterior and lateral to the spinal cord, extending down the length of the spinal canal. This epidural space, which is larger at the sacral region, contains fat, arteries and venous plexus. Because the dura mater is more adherent to the bony surface of the vertebral bodies, from the foramen magnum down to the level of L1, the majority of spinal epidural abscesses are located posteriorly, extending to multiple levels.

• With the progression of the inflammation, the spinal cord may be damaged in several ways:

• Thrombosis and thrombophlebitis of adjacent veins;
• Direct compression from the inflammatory mass;
• Ischemia;
• Bacterial toxins;
• Inflammatory response and its mediators.

• Bacteria gain access to the epidural space by:

• Hematogenous dissemination Important to notice that, just as the spinal epidural space might be infected by bacteria from elsewhere, so does bacteria infecting the spinal epidural space, might travel through the blood stream to infect other tissues.
• Direct inoculation

• This access is facilitated by several predisposing factors, such as:

• underlying disease (HIV infection, alcoholism or diabetes)
• immunosuppressive therapy
• spinal condition (degenerative joint disease)
• neurosurgical procedure
• local or systemic source of infection (osteomyelitis, UTI, soft-tissue infections, IV drug use, sepsis, infected pressure sore).

The more risk factors the patient has, bigger the chance of developing an epidural abscess.

• Etiology - considering that nowadays, infection from neurosurgical procedures or trauma, is one of the most common causes of infection of the epidural space, skin flora will be an important source of pathogenic organisms. Hence, Staphylococcus aureus is responsible for almost two thirds of the reported cases. Due to the generalised use of antibiotics through the years, the number of reported cases of SEA due to MRSA has increased exponentially, reaching up to 40% of the cases in some institutions, particularly in patients with spinal or vascular implanted devices. Other less common, but still important organisms are:

• Staphylococcus epidermidis
• Escherichia coli
• Pseudomonas aeruginosa

Other rare organisms are:

• Anaerobic bacteria
• Mycobacteria
• Fungi
• Parasites

As the inflammation progresses, the extension of the abscess also increases, to the average extent of 3 to 5 spinal cord segments. Nevertheless, certain situations, in which the whole spinal cord is involved may arise, in which case it's called panspinal infection. However, the degree of damage, associated neurologic symptoms and sequelae, are not directly related to the extension of the abscess, since even small abscesses may cause severe sequelae. Relating to the content of the abscess it may be essentially pus, particularly in acute situations, or may be granulation tissue which is often times observed during the surgical drainage, especially in abscesses with more than 2 weeks.