Spinal Epidural Abscess: A basic primer

This post was authored by Theresa Neumann and posted to The Eye Opener on March 11th, 2011.

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Epidural abscess compressing the spinal cord -courtesy of aafp.org

In a previous blog, I introduced the topic of neck and back pain which can have a host of causes, most of which are mechanical.  This blog attempts to explore an infectious etiology of neck and back pain that can be potentially devastating, resulting in paralysis and even death.

The spine is a complicated structure involving bones, discs, ligaments, muscles, blood vessels and nerves.  It’s two main functions are to provide axial support for the upright stature of the human body and fluid movement of the body parts while also protecting or housing a critical component of the central nervous system, the spinal cord. Oversimplified, the spinal cord is a conglomeration of nerve fibers that act as the “information highway” between the peripheral nerves supplying sensory and motor function to the body parts and the brain. The spinal cord transmits chemical messages from the brain, telling the body what to do and how to function, even functions we are not conscious of doing (digestion, breathing, etc.), and it receives input from all of our senses and interprets the data.  Without the spinal cord or if the spinal cord is affected by an injury, there is disconnect; we lose feeling and movement as well as control of some of our normal unconscious body functions.  The location of the spinal cord damage dictates the level at which the disconnect occurs.  To help you understand the anatomy of the spine, here’s a short video describing the basic anatomy of the spine.

httpv://www.youtube.com/watch?v=Zeo0Im7h4Go

 

An epidural abscess is a collection of pus that occurs as the result of an infectious process involving any part of the  spinal cord from the base of the head to the tailbone; the abscess is located within the protective boney compartment housing the spinal cord, the spinal canal, and the thick outer covering of the spinal cord, the dura.  The dura is comprised of 3 layers, the outer one being very tough, the middle one being very vascular, and the inner one being very “tender.”

Signs and Symptoms:

In the early stages of the infection, a patient will often complain of neck or back pain very specific to the location of the infection, but the pain can be referred due to nerve root irritation.  As the infection grows, it spreads along the axial plane of the spinal canal, but the pressure and swelling of the purulent collection also tends to compress the spinal cord, resulting in numbness, tingling and functional loss below the level of the compression.  This progression can be indolent or rapid, depending on both the virulence of the pathogen and the person’s immune system.  Without emergent treatment, the pus collection can “choke off” the spinal cord and its blood supply, leading to permanent spinal cord injury and paralysis.

How does the infection get there?

Patients who have undergone spinal surgery are at an increased risk of these types of infections, especially during the immediate post-operative period.  Surgical wounds can become infected allowing bacteria to track deep into the tissues and the spine through the operative plane.  If hardware (spinal instrumentation) has been used, these man-made devices become reservoirs or fomites for attachment of the bacteria, and it is extremely difficult to eradicate bacterial pathogens from the hardware.

The bloodstream is another source of migration for bacterial pathogens from peripheral sites (infected gums, endocarditis, bladder infection, skin abscesses/boils) to the spine.  Individuals particularly at risk are those with depleted immune systems (e.g. diabetics, patients with auto-immune diseases on chronic steroids, HIV, etc.) and IV drug abusers (directly inject materials into veins).  Having spinal hardware from a previous spine surgery will increase the risk of seeding to that instrumented site should bacteria become blood-borne.

Direct inoculation can occur if  poor technique is utilized during epidural spinal injections or epidural anaesthesia.  There can also be contiguous spread from adjacent infected tissues (e.g. diskitis, osteomyelitis).

What are the most common pathogens?

Staph aureus, a common skin pathogen, is the most common cause.  It is known to cause skin abscesses/boils, wound infections, sinus infections, bladder infections and even pneumonia!  The relatively recent incidence of MRSA (a very resistent variety of Staph aureus) in the community has changed the way medicine treats common skin ailments; its effect on the incidence and treatment of epidural abscesses has yet to be determined.  If an epidural abscess is suspected, antibiotic coverage for MRSA is now automatically included in the initial treatment due to the bacterial virulence and resistance to treatment.

E. coli ( a common bowel pathogen and cause of bladder infection), fungi (like yeast), and even Mycobacterium tuberculosis are also causes of epidural abscess.  One can also contract mixed infections with aerobic and anaerobic bacteria, depending on the source of the infection (intra-abdominal abscess, perforated appendix).

How is an epidural abscess diagnosed?

The clinician must have a high index of suspicion and keep an open mind.  A thorough history often leads to clues such as recent fevers, a recent skin abscess or cellulitis, IV drug abuse, recent dental extraction or procedure, and neck or back pain without a specific inciting incident.  Physical examination of the patient often reveals point tenderness directly over the affected area of the spine, worse with percussion or tapping on the boney prominences, and often worse in the recumbent position.

Visualization of the spine is best accomplished with an MRI of the spine (above, below and including the tender area); it is non-invasive and very detailed regarding the soft tissues.  Patient weight can be a factor in accessing these machines; they often have a maximum weight limit of 300 lbs.  Many morbidly obese patients, who often have type II diabetes, are at risk for epidural abscesses; they often have to be transported to external facilities for “open MRI” studies.  Claustrophobia can also be a restricting factor, often requiring patient sedation or anaesthesia.  Excruciating pain while lying flat can also be prohibitive.  An alternative study to visualize the spinal cord is a CT-myelogram during which the epidural space is accessed with a spinal needle and dye is injected for visualization under computed tomography.  The CT-myelogram is a higher-risk study and can also be limited by a patient’s weight and sensitivity to contrast dye.  A lumbar puncture should NOT be done since it can lead to spinal cord herniation and permanent spinal injury.

What is the treatment for an epidural abscess?

There are two schools of thought regarding treatment.  One school favors emergent surgical debridement of the abscess along with intravenous antibiotics; this also allows for identification and sensitivity testing of the organism.  The other school suggests that intravenous antibiotics alone can be sufficient if no signs of spinal cord impingement are present; if symptoms progress to the development of neurologic symptoms, then surgery becomes more urgent.

What is the prognosis in epidural abscess?

Prognosis depends on the patient’s underlying medical condition and the degree of spinal cord involvement at the time of diagnosis/intervention.  Obviously, the earlier the intervention and treatment, the better the prognosis; hence, I favor surgical debridement as soon as possible.  Delays in diagnosis often lead to permanent and life-altering neurologic damage and functional loss or even death.  These delays and the permanent neurologic sequellae suffered often become the basis for medical malpractice litigation.

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