Infections of the spine can occur in several adjacent yet anatomically distinct locations: 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 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 that are susceptible to infection include the vertebral bodies, the intervertebral disc, and the epidural space. These distinct anatomic regions have different blood supplies and therefore different capacities to spread infections. Of the three, the epidural space is most vascular, followed by the bone; the intervertebral disc is the least vascular.
The term vertebral osteomyelitis refers to an infection localized to the bones of the vertebral column. Vertebral osteomyelitis is usually seeded from a hematogenous source, that is, pathogens circulating in the blood which spread to the bones of the spine. If the vertebral bodies become infected, they can lose their structural integrity and collapse, leading to deformity.
Discitis refers to an infection of the intervertebral disc space. If both the disc space and the adjacent vertebral body are 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 can further 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 overlap when there is a progression of a single infectious focus. For example, osteomyelitis can extend posteriorly to form an epidural abscess or it might 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).
The age of the patient influences the specific location prone to infection, as the blood supply to the spine changes with age. In children, the nucleus pulposus of each intervertebral disc is well-vascularized. Thus, in children, infection from a distal origin will spread to the nucleus pulposus preferentially. In adults, the nucleus pulposus is avascular, whereas the vertebral bodies and end plates have a relatively higher volume/low velocity blood flow. Therefore, in the adult, streaming bacteria are more apt to land in the bone.
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 might be the source of contiguous spread. Contiguous spread may also be the result of a lung infection or psoas muscle abscess which, if left untreated, can extend into the nearby vertebral bodies.
Vertebral osteomyelitis affects the lumbar spine in a majority of cases. The thoracic spine is affected in about 35% 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% or more of cases; S.epidermis 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 (as seen in patients with 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 diseases 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. Any decline in motor or sensory function requires further diagnostic imaging and laboratory studies.
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 include 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 or less of cases. The absence of fever can be misleading, as patients are often taking NSAID analgesics that may suppress fever as well.
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 complete triad is found concurrently in only a fraction 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 or exam findings. Nonetheless, it is important to inquire about numbness, paresthesia, bowel, bladder and sexual dysfunction, and lower extremity motor weakness.
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 tests 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 positive in only about 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 to identify the causative organism and antibiotic sensitivities. When the causative organism is not identified by blood culture, a vertebral body biopsy may be needed.
Plain radiographs are routinely obtained, though they will often not show any changes until at least 2 weeks after 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 other items on the differential diagnosis list such as 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 identifying the organism 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 reveals a higher 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 an additional biopsy is recommended as it increases the likelihood of identifying an infectious agent. As a last resort diagnostically, 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. However, the incidence is increasing, likely as a consequence of the rising prevalence of intravascular devices and surgical procedures (usually spinal surgery), immunosuppressive medications, chronic diseases (e.g., diabetes), and the aging population.
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 suggesting involvement of the spinal cord or any hemodynamic instability or severe systemic symptoms concerning for sepsis. Therefore, a thorough neurological exam is warranted in all patients that exhibit these signs and symptoms since it helps increase our index of suspicion.
Treatment Options and Outcomes
Because one of the complications of infection includes hemodynamic instability, fluid resuscitation and intravenous broad-spectrum antibiotics (such as vancomycin and a 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 all cases of 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) 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. Needless to say, in cases that do not meet those criteria, or fail to respond to antibiotic treatment, surgical drainage with debridement of the infected tissues is recommended, especially given the morbidity associated with incompletely treated epidural infections.
After surgical drainage of an abscess, a post-operative course of antibiotics for 2 to 4 weeks is thought to be sufficient if there is no bony involvement. On the other hand, 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 direct 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, uncommon in the USA, is more prevalent in parts of the world where people drink unpasteurized sheep, goat, or camel milk. Brucella, like other bacteria, may be identified by routine blood cultures and is not readily apparent on most radiographic tests unless signs of osteomyelitis are already present.
Postoperative spine infections are relatively rare but can have catastrophic sequelae. Thus, they 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. Identify the appropriate labs and imaging studies that will help confirm the diagnosis of spinal infection.