Infections of the spine can occur in several adjacent but anatomically distinct locations: namely, the vertebral bodies, the intervertebral discs and the epidural space (see Figure 1). Infection of the vertebral bodies is termed “osteomyelitis,” as it would be any other bone. Disc and epidural space infections are denoted as “discitis” and “spinal epidural abscess” respectively.
In general, the most commonly implicated organisms are bacterial, but spinal infection can also be caused by mycobacterium, fungi or, less commonly, viruses.
Spinal infections are relatively rare but are clinically very important as they may cause significant morbidity and mortality – especially damage to the spinal cord and nerve roots that manifest as pain and dysfunction. Accordingly, suspected spinal infection requires urgent medical attention.
Structure and Function
The primary anatomic regions of the spine which are susceptible to infection include the vertebral bodies, the intervertebral disc, and the epidural space.
The term vertebral osteomyelitis refers to an infection localized to the bones of the vertebral column. This usually starts from hematogenous source, that is, arrives to the spine via the blood stream, but can spread directly from a nearby source.
Discitis refers to an infection of the intervertebral disc space. If both the disc space and the adjacent vertebral body is infected, as is common with bacterial infections, the term “spondylodiscitis” may be used.
An epidural abscess is a contained collection of purulent fluid in the area between the dura mater and the vertebral bone (see Figure 2). The epidural space contains arteries and veins which can be compressed by the abscess. An abscess can cause septic thrombophlebitis, which will also impede blood flow. A larger abscess can compress the spinal cord and nerve roots directly.
Although osteomyelitis, discitis and spinal epidural abscesses refer to discrete infection patterns, these conditions can occur in tandem, usually as a progression of a single infectious focus. For example, osteomyelitis can extend posteriorly to form an epidural abscess; or osteomyelitis can spread to the adjacent intervertebral disc and vertebral body, causing spondylodiscitis.
The etiology of most spinal infections is usually seeding of the spine from a distant source of infection via the bloodstream, so-called hematogenous spread.
Because there are age-dependent differences in the blood supply of the spine, the age of the patient determines the specific location prone to infections. In children, the nucleus pulposis of the intervertebral discs are well-vascularized. As such, in children, infection from a distal origin will spread to the nucleus pulposus preferentially. In adults, the nucleus pulposis is avascular, whereas the vertebral bodies and end plates have high volume/low velocity blood flow. Therefore, in the adult, streaming bacteria are more apt to land in the vertebral bodies.
Beyond arterial hematogenous spread, other modes of infection of the spine include direct inoculation of the spine and contiguous spread from adjacent soft tissues. Direct inoculation may occur due to iatrogenic injury during surgery or from penetrating trauma. An infected sacral decubitus ulcer or retropharyngeal/retroperitoneal abscess, among others, might be the source of contiguous spread. Contiguous spread may be the result of a lung infection or psoas muscle abscess which, left untreated, can extend into the nearby vertebral bodies.
Vertebral osteomyelitis affects the lumbar spine in a majority of cases, but can also affect in the thoracic spine in 30-40% of cases and the cervical spine in about 10% of cases. S.aureus is the most common bacteria responsible for vertebral osteomyelitis accounting for 50-65% of cases; staph epidermidis is the second most common organism. Other less common organisms include gram negative bacteria (often originating from septic genitourinary or respiratory tract infections), pseudomonas (most common in intravenous drug users), salmonella (sickle cell disease), and Brucella (common in countries where unpasteurized sheep, goat, or camel milk is consumed).
Vertebral osteomyelitis leads to abscess formation in about 20% of cases. Spinal epidural abscesses are almost always found due to progression of adjacent osteomyelitis or discitis.
The general complaints most often elicited on history from patients with an infectious disease of the spine include severe and debilitating back pain for more than a week, fever, and malaise.
In addition, it is important to elicit any relevant predisposing factors for spinal infection (see risk factors below), as this may help aid in forming the differential diagnosis.
The diagnosis of disc space infection in young children can be especially challenging given their limited ability to articulate their symptoms. Clinical clues in toddlers included a refusal to sit or walk and resistance to physical activity. In addition, painful limping, change in appetite, fever, and abdominal or back pain can be consistent with discitis.
In older children or adolescents, focal back pain and point tenderness are the usual (and non-specific) findings for spinal infection.
Vertebral osteomyelitis can present with an acute, subacute, or chronic disease course, most often in an adult 60 years of age or older. Back pain is the most common initial symptom and is present in over 85% of cases. Fever, however, is a less reliable finding, occurring in only about half of cases. The presence or absence of fever is also confounded by the fact that patients are often taking analgesics such as an NSAID – and these medications may suppress a fever too.
Patients with a spinal epidural abscess most often have one or more predisposing risk factors or conditions such as diabetes, immunodeficiency syndromes, intravenous drug use, prior spine surgery, malnutrition, and recent systemic infection.
Spinal epidural abscess can present with a ‘classic triad’ of fever, spinal pain, and neurologic deficits, but the entire triad exists in only a small proportion of patients. The most common symptom is severe back pain, which occurs in the majority of patients. Fever is present in only about 50% of patients. While neurologic deficits are the most feared complication of spinal epidural abscess, only about a third of patients will present with neurologic symptoms on exam findings. Nonetheless, it is important to inquire about saddle anesthesia or shooting paresthesias, bowel or bladder dysfunction, sexual dysfunction, and lower extremity loss motor function.
The diagnosis of spinal infection is challenging given the nonspecific signs and symptoms. Thus, spinal infection should always be included on the differential for any patient with known risk factors or with constitutional symptoms suggestive of infection.
Acute phase reactants, erythrocyte sedimentation rate (ESR), C-reactive protein (CRP), and procalcitonin (PCT) levels are elevated in >80% of patients with disc space infection or vertebral osteomyelitis. Note, however, these lab test could be elevated for a multitude of reasons. Thus, ESR, CRP and PCT are good screening tests in patients with symptoms concerning for spinal infection but do not establish the diagnosis themselves. Rather, elevated acute phase reactant values suggest the need for further investigation with advanced imaging.
Despite its low specificity, the CRP trend can be used to monitor for effectiveness of treatment: baseline levels are expected to decrease as the infection is cleared.
The white blood count (WBC) is only elevated in about half of patients with vertebral osteomyelitis or disc space infection.
Blood cultures and urine cultures are also important in identifying a potential infection source and thus guiding treatment, though cultures are only positive in 33-50% of patients. However, it is important to obtain cultures in patients suspected of having a spinal infection, as positive results would preclude the need for more invasive diagnostic procedures, such as vertebral body biopsies, since identifying the causative organism and antibiotic sensitivities is critical for determining optimal treatment.
Plain radiographs are typically the first-line imaging modality, though often will not show any changes until 2-4 weeks after the point of infection. Some of the early findings for spondylodiscitis are loss of vertebral body architecture and paraspinous soft tissue swelling. Later findings include disc space narrowing, endplate erosion or vertebral body collapse, which begin to appear 10-21 days after the onset of infection. CT scans (Figure 3) are similar to what plain radiographs may show, only they are more sensitive for earlier changes in the bone.
Radiographs have a very limited role in diagnosing an isolated spinal epidural abscess, as the collection usually cannot be visualized at all.
MRI (see Figure 4) with contrast is the gold standard imaging modality, with approximately 95% sensitivity and specificity for spondylodiscitis. Findings include a decreased signal intensity in the focus of infection within the vertebral bodies and discs on T1-weighted images, increased vertebral body enhancement on T2-weighted images, and contrast (often gadolinium) enhancement of paraspinal and epidural processes.
MRI is particularly useful for visualizing the epidural space
(Figure 5). Epidural abscesses tend to have low signal on T1-weighted images
and high or intermediate intensity on T2-weighted ones.
Importantly, MRI can help rule out the other possible diagnosis listed in the differential diagnosis, especially malignancy.
For patients who cannot undergo MRI, technetium bone scan is a reasonable alternative for the diagnosis of spondylodiscitis, but it is less sensitive and far less specific.
More invasive diagnostic procedures may be required in patients
with clinical and radiographic evidence of osteomyelitis with negative
blood/urine cultures, as this information is critical in choosing the
appropriate antibiotic. The gold standard for these patients is a CT-guided
biopsy with culture and histopathology (see Figure 6). This method of obtaining
a culture has a better diagnostic yield relative to blood culture, but is
obviously more invasive with a higher rate of complications. Histopathology of
the biopsy specimen may provide evidence of more rare infections (brucellosis
or tuberculosis) characterized by granuloma formation. If the first biopsy is
non-conclusive, a repeat biopsy may be warranted as it increases the likelihood
of identifying an infectious agent. As a last resort, an open biopsy would be
indicated in patients with a negative CT-guided biopsy in the setting of a very
high suspicion for infection.
Spinal infections are relatively rare conditions. Discitis is estimated to occur in 3/100,000 pediatric patients annually, most often in children younger than 5 years old, males more than females.
Isolated disc space infections in adults are rare; the most common cause of discitis is progression of adjacent vertebral osteomyelitis (spondylodiscitis).
The incidence of vertebral osteomyelitis is approximately 2/100,000 per year but is likely to rise. The rising incidence of spinal surgical procedures with implants, the use of immunosuppressive medications, the prevalence of chronic diseases such as diabetes, and the aging population all make vertebral osteomyelitis more likely.
Spinal epidural abscess is seen in fewer than 1/100,000 people per year.
The differential diagnosis for spinal infection includes malignancy (primary or metastatic), inflammatory arthritis, vertebral compression fracture and visceral infection/abscess. Ordinary degenerative spondylosis and disc disease, owing to their prevalence, must also be considered.
While there are features on history and physical exam that can help sort this differential diagnosis list, the nonspecific nature of the symptoms in spinal infection often leads to a reliance on objective findings such as imaging and laboratory tests.
The most feared complications of spinal infections are permanent neurologic damage and sepsis. Thus, the major red flag symptoms include any neurologic deficits suggested involvement of the spinal cord or any hemodynamic instability or severe systemic symptoms concerning for sepsis. A thorough neurological exam is essential in all patients that exhibit these signs and symptoms.
Treatment Options and Outcomes
Since one of the presenting signs of infection can be hemodynamic instability, resuscitation and immediate intravenous broad-spectrum antibiotics (vancomycin and third generation cephalosporin) should be given to any patient presenting with signs of sepsis, even prior to confirmation of the offending microbe.
Ideally, blood cultures are drawn before antibiotics are given, as a culture provides not only a general diagnosis but is critical for tailoring the appropriate antibiotic treatment. In the stable patient with a suspected disc space infection, vertebral osteomyelitis, or spondylodiscitis, antibiotics should ideally be withheld until a culture-proven diagnosis is made.
The specific antibiotic will depend upon the causative organism, but patients will typically require 6-12 weeks of antibiotics. Potentially useful non-operative adjuncts include bed rest to reduce stress on the spine, analgesics for pain, and bracing to help reduce pain and prevent potential spinal deformity.
The majority of cases of isolated spondylodiscitis without abscess formation do not require surgery: antibiotics and occasionally bracing will suffice.
If a spinal epidural abscess is present, urgent surgical decompression might be required, especially in the presence of any neurological deficits. Traditionally surgical decompression has been indicated for any and all epidural abscess formation. However, more recent data has suggested that it may be reasonable to treat patients with antibiotics alone if three main criteria are fulfilled: 1) that the abscess is small, 2) no neurologic deficits are present and 3) a microbiologic diagnosis can be made via blood culture or CT-guided aspiration. However, in cases that do not meet those criteria, or fail to respond to antibiotic treatment, surgical drainage with debridement of the infected tissues is still recommended given the morbidity associated with untreated epidural infections.
A post-operative course of antibiotics for 2-4 weeks is sufficient if there is no bony involvement, but if the bone is infected as well, the antibiotic treatment should be extended to at least 6 weeks and sometimes even to 12 weeks.
Complications of isolated spondylodiscitis without abscess formation include narrowing of the disc space, spinal deformity, and persistent back pain. Overall, there is an 80% success rate with non-operative treatment, but morbidity and mortality of infections increase with age. The mortality rate is relatively low at <5%.
The complication rate and prognosis are less favorable for spinal epidural abscess. There is an approximate 15% incidence of permanent neurological damage from either compression or infarction of the spinal cord. The mortality rate of spinal epidural abscesses is about 5%. Ultimately, the most important predictor of neurologic outcome is the patient’s neurologic status immediately prior to surgery, highlighting the importance of early and accurate diagnosis of this condition.
Risk Factors and Prevention
There are several risk factors associated with the development of a spinal infection. These include immunodeficiency or immunosuppression (diabetes, medications, primary immunodeficiency syndrome), intravenous drug use, increasing age, malignancy, trauma, existing degenerative spine disease, malnutrition, obesity, and recent systemic infection. Some of these risk factors are amenable to modification, often only with great effort and good luck.
Two other risk factors are worthy of special mention: tuberculosis (TB) and prior spine surgery.
Although tuberculosis is uncommon in the United States (with fewer than 3 cases per 100,000 persons), it is highly prevalent in other parts of the world. Thus, spinal tuberculosis should be considered and excluded in patients coming from countries where tuberculosis is endemic. Approximately 5% of all TB patients have spine involvement (see Figure 7). A chest x-ray should be ordered for any patients in which TB is a possibility, as late diagnosis is associated with severe and irreversible kyphosis (even after successful treatment). Brucellosis, although uncommon in the USA, is nonetheless prevalent in parts of the world where many people drink unpasteurized sheep, goat, or camel milk. Brucella, like other bacteria, may be identified by routine blood cultures and is not readily apparently on most radiographic tests unless signs of osteomyelitis are already present.
Postoperative spine infections are relatively rare but have the potential for catastrophic sequelae, and thus must be detected as quickly as possible. Post-operative patients, especially if there is implanted hardware, should be monitored closely with urgent treatment initiated when signs suggesting infection are present. Wound drainage is the most common presentation but often the presentation is more subtle. A good heuristic to detect a post-operative infection is to pay attention to the pattern of pain. In general, post-operative pain should decrease as time from surgery passes. Pain that increases with time is therefore suspicious.
Tuberculosis of the spine is known as Pott’s Disease (see Figure 8).
Vertebral osteomyelitis, disc space infection, discitis, spondylodiscitis, spinal epidural abscess
Understand the anatomic and pathophysiologic differences between vertebral osteomyelitis, discitis, and spinal epidural abscess. Perform a targeted history and physical exam with special attention to red flags concerning for spinal cord involvement or sepsis. Order appropriate labs and imaging studies which will help confirm the diagnosis of spinal infection. Understand which patients with spinal infection require non-operative vs. operative treatment
Under Red Flags, see red text. There should be some punctuation in here, or rewrite for clarity.