An epidural abscess is a pus-filled region that is formed between the dura and the bone of the spine or skull. It can cause a variety of life-threatening complications, since it is located at close proximity to vital organs: the brain and spinal cord.
Spinal epidural abscess
The symptoms exhibited by patients are a direct result of the spinal cord compression and hypoperfusion, caused by the abscess. Initial symptoms almost always include back pain. Symptoms usually evolve quickly, within some hours or days, but may take up to several months in order to fully develop, depending on the pathogen.
Apart from back pain, an abscess that is left untreated will lead to mobility limitation, radicular irritation, paresthesias and sensory loss and ultimately paralysis. There is no specific clinical picture in the case of a spinal epidural abscess and many symptoms may not manifest.
Systemic symptoms are also present, including fever, tenderness at the location of the spine, accentuated reflexes that gradually give way to reflex absence and respiratory problems. Even though the triad of back pain, fever and neurological symptoms is considered to be typical of a SEA, it is present in only a minority of the patients.
Intracranial epidural abscess
Intracranial epidural abscesses often produce symptomatology that is difficult to distinguish from the one caused by the underlying condition. Symptoms manifest as a result of increased pressure to the brain and of the ongoing infection. Patients are often febrile, experience nausea, headaches, fatigue, emesis and exhibit a lethargic attitude. Depending on the patient's medical history, patients whose IEA is caused by an invasive procedure to the head usually exhibit no fever and neurological symptoms are not as alarming; the most significant finding is a wound infection. Patients who develop IEA without a prior invasive procedure to the head exhibit encephalopathy and focal neurological symptoms. Epileptic phenomena and a mental status impairment are also expected in cases of intracranial epidural abscesses.
Both types of abscesses should be promptly diagnosed and treated in order to avoid complications.
The first step is a complete blood count, which will show signs of inflammation (leukocytosis/ left shift), as well as a possible anemic tendency and low platelet counts. Blood cultures are mandatory in many cases of febrile patients, when a serious condition is suspected: more than half of the patients present with septicemia and the pathogen can be isolated via a blood culture. Accurate identification of the pathogen will contribute greatly to the administration of an effective antibiotic.
Imaging modalities also provide excellent information regarding the presence of a potential abscess. Depending on the symptoms presented by a patient, a magnetic resonance imaging (MRI) scan of the head or spine will reveal an epidural abscess with specificity and sensitivity that exceed 90%. A computerized tomography (CT) scan can also be performed in cases where it is not possible to perform an MRI. Once the location of a mass has been identified, CT-guided aspiration can help to harvest material for culturing purposes.
When an SEA or an IEA are possible diagnoses, a lumbar puncture should not be performed; in an SEA case, it can lead to dissemination of the pathogens to the subarachnoid space, and in an IEA case to a cerebellar tonsillar herniation, due to a sudden rise in intracranial pressure.
During the past years, the general recommendation for the treatment of SEAs includes both antibiotic treatment and surgical drainage of the epidural abscess. Material harvested from the abscess via a CT-guided aspiration will provide the identification of the pathogen and blood cultures can also help to detect the culprit.
There are several sub-categories of patients who may be treated solely with antibiotic therapy. They are patients for whom surgery is absolutely contraindicated due to high risk of mortality because of comorbidities, patients with no neurological symptoms and accurately detected pathogens, and those who exhibit paralysis for 2-3 days, which is deemed irreversible. A physician should bear in mind that any patient who is treated exclusively with antibiotics runs a high risk of sudden decline within the first 3 days of treatment and that, should this complication arise, even surgical intervention may not restore full function. Patients with coexistent conditions, such as diabetes, septicemia, age >65 and a detected methicillin resistant Staphylococcus aureus (MRSA) as the culprit also run a higher risk of detrimental complications if therapy relies on antimicrobial agents alone.
Antibiotics are administered initially empirically, because there is an immediate need for treatment. Medications include antibiotics used to cover for both gram (+) and gram (-) bacteria, such as 3rd and 4th generation cephalosporins. Vancomycin should be added if there is a suspicion for MRSA. As soon as the cultures identify the pathogen responsible for the infection, treatment should be adjusted, depending on the new data.
Intracranial epidural abscess
Intracranial epidural abscesses are also treated with surgical intervention and antibiotic treatment. The latter is the same as the one described above for SEAs; the only difference is that patients who present with IEA have usually been subjected to pathogens in the hospital, as this type of abscess usually results from invasive procedures to the skull area. Nosocomial pathogens should be considered and vancomycin should always be a part of the treatment regimen.
As for surgical intervention, patients greatly benefit from decompression procedures, in order to reduce the infectious bacterial load and the pressure exerted on the brain by the abscess.
Prognosis is better with early detection of the abscess, prompt and appropriate treatment. The onset of neurological symptomatology constitutes a poor prognostic marker and greatly increases the risk of permanent damage and demise.
Generally, intracranial abscesses are caused as a complication of sinusitis, otitis media, craniotomy or mastoiditis. The microorganisms that most commonly cause intracranial epidural abscesses are those connected to infections of the respiratory tract and, more specifically, those that cause sinusitis. Other pathogens may be contracted after brain surgery, since the protective barrier of the skull is temporarily removed and the brain is exposed to microorganisms which contaminate surgical rooms. The pathogens most commonly causing IEA are the following:
A wider range of microbes can cause spinal epidural abscesses. SEAs are generally a complication of vertebral osteomyelitis, infection of adjacent tissue that has expanded spinal instrumentation, injections or dissemination of pathogens from a distant location.    :
Present data in the United States estimate the yearly incidence of SEAs to amount to 3 in every 10,000 hospitalizations and its incidence underlines an increasing tendency, when compared to the past. This has been primarily attributed to medical advances leading to more invasive procedures to the spine, as well as a rise in the use of intravenous drugs. As for IEAs, their incidence has not been calculated; it is believed, however, that they are a rarer phenomenon than a spinal epidural abscess.
With regard to mortality rates, spinal epidural abscesses are still responsible for the death of up to 20% of the patients that are hospitalized for this condition   . Early detection is crucial and helps to avoid the establishment of deficits of neurological nature and mortality. Intracranial abscesses, on the other hand, seem to respond better to treatment with antimicrobials and surgery. Mortality rates have been estimated to no more than 10%. Similarly to SEAs, IEAs must be diagnosed and surgically treated as early as possible, since late detection leads to life-threatening and difficult-to-reverse complications . In both types of abscesses, coexistent diseases and poor general status can increase mortality and morbidity.
Lastly, epidural abscesses seem to affect men more often than women. Spinal epidural abscesses exhibit a frequency peak at the 5th decade of life, whereas intracranial epidural abscesses are commonly observed amongst patients in the age group of 20-30 years old.
Spinal epidural abscesses are located between the dura and the spine, most commonly in the thoracic and lumbar regions of the spine. Infectious material usually settles in this location following vertebral osteomyelitis or psoas abscess; infection is spread throughout the tissue, leading to a direct expansion to the spine. Another possible mechanism that accounts for the development of such an abscess is dissemination of pathogens from a distant location: septicemia can result in the translocation of microorganisms that secondarily infect the spine. Endocarditis, respiratory tract infections, IV drug use and infections of the urinary tract can all lead to the formation of a spinal epidural abscess if the complication of septicemia occurs. Additionally, invasive procedures that involve the spine are responsible for temporarily rendering this sensitive organ exposed to various pathogens: spinal instrumentation, epidural anesthesia and painkiller-releasing pumps are all potential culprits. Most abscesses are developed in the posterior surface of the spine and the infection can expand to include more than one vertebral bodies.
Conditions such as diabetes mellitus, renal failure, pregnancy and immunosuppression further contribute to the development of a spinal epidural abscess via various mechanisms and are therefore considered as risk factors.
The most common risk factor for spinal epidural abscess is diabetes mellitus. Other risk factors include alcoholism, renal insufficiency, immunosuppression, pregnancy, and spinal/epidural anesthesia or injections. Cancer, cirrhosis, infection and the use of steroids is also believed to contribute to the pathogenesis of the disease. IV drug use seems to be an increasing risk factor in many cases.
On the other hand, intracranial epidural abscesses are found between the dura and the skull bone. Any condition that renders this anatomical location exposed to the surroundings, such as a craniotomy or a serious head injury, can cause an IEA. Abscesses can also develop as a complication of sinusitis, mastoiditis and otitis: the infected regions are in close proximity to the brain and pathogens can invade the surrounding tissue, leading to an intracranial abscess. An equally serious complication is the subdural empyema.
An epidural abscess is an organized collection of pus, located in the area between the dura, i.e. the outermost of the three layers covering the spinal cord and brain, and the respective bones. Hence, there are two types of epidural abscesses: the intracranial epidural abscess (IEA) and the spinal epidural abscess (SEA). Their distinction lies primarily in the anatomical location where they appear (Brain or spine) and secondarily in some pathophysiological differences and distinct symptoms.
Epidural abscesses located on the spine have a high risk of expansion; the vertebrae and dura are not firmly connected to each other, a fact which allows for the abscess to extend its limits and ultimately encompass many vertebrae. On the other hand, abscesses located on the brain dura can lead to high intracranial pressure, precisely because the connection between the dura and skull is different: the two structures are tightly connected at that point, and the presence of an abscess exerts pressure to the brain parenchyma.
SEAs and IEAs are diagnosed with the aid of computerized tomography (CT) and magnetic resonance imaging (MRI)    .
A spinal epidural abscess is a pus-filled region that is found between the spine and the underlying membrane layer covering the spinal cord. An intracranial epidural abscess has the same structure, located between the skull and the outermost membrane layer that covers the brain. Abscesses form as a result of infections.
Spinal and intracranial epidural abscesses can develop when microbes gain access to the locations mentioned above. Possible causes include head surgery, intravenous drug use, head trauma, infections of the sinuses, osteomyelitis and infections on other locations, from which bacteria are transferred to the brain or spine through the blood.
Intracranial epidural abscesses can cause fever, lethargy, vomiting, nausea, headaches, mental status impairment and various other symptoms related to brain dysfunction. A spinal epidural abscess causes weakness in the muscles of the arms and legs, bowel dysfunction, lack of coordination, fever and paralysis.
Both types of abscesses must be surgically treated; at the same time patients should receive antibiotics as well. If an individual is not a candidate to undergo a surgical procedure as it is too risky for them, they may be treated exclusively with antibiotics, although the general recommendations do not suggest this as first and best option.