Osteomyelitis refers to an infection of the osseous aspects of the spinal column, whereas osteodiskitis refers to an infection of the intervertebral disk space. Because the vascular supply of the disk space is relatively limited with most of its blood supply originating from the vertebral body and occurring via diffusion across the end plate, infection of the disk space usually originates from the vertebral body. Osteomyelitis typically involves the anterior column and is rarely seen in the posterior column.¹ Significant morbidity and mortality can occur if this condition is left untreated. The incidence of osteomyelitis has been reported at 2.2 per 100,000 people per year and appears to be increasing to 5.8 per 100,000 people per year.²

The risk factors for osteomyelitis of the spine are similar to those found for osteomyelitis of the appendicular skeleton. Conditions that affect the immune system such as diabetes, smoking, HIV, and hepatitis C are among the more common risk factors associated with osteomyelitis. Other factors include the presence of another infection, previous spine surgery, and skin compromise. A systematic review of 14 studies with a total of 1,008 patients who had pyogenic vertebral osteomyelitis (PVO) was conducted. The authors found that the median age of the patients was 59 years, and 62% of affected individuals were male.³ Comorbidities in this study included diabetes mellitus in 24% and intravenous drug use in 11%.³ The lumbar spine was affected in 59% of patients followed by the thoracic spine in 30% and cervical spine in 11%³ (Figure 1).

Osteomyelitis and diskitis typically occur either from direct inoculation or hematogenous spread from another site.⁴ Hematogenous spread accounts for most cases as the multiple vascular supplies to the spine provide an avenue for bacteria to readily seed the vertebrae.¹,⁴ Direct inoculation typically requires skin compromise, such as following spinal surgery or in the setting of chronic ulcers.⁵ After bacteria have been introduced to the vertebral body, they may spread through diffusion and lead to diskitis of the adjacent disk spaces.

Staphylococcus aureus is the most common bacterial cause of osteomyelitis/osteodiskitis. The second most common pathogen isolated in cases of PVO is another gram-positive bacteria, Streptococcus. Gramnegative bacteria are also a frequent cause of spinal
infections, with the most common species including Escherichia coli and Klebsiella pneumoniae.⁶ In patients with a history of intravenous drug abuse, Pseudomonas aeruginosa also has been described as a common bacterial cause.⁷,⁸

In a 2020 study of 586 patients with PVO over a 12-year period, S aureus was found to be the most common pathogen at 43.5%, followed by gram-negative infection at 22.2% and Streptococcus at 20.1%.⁹ A total of 64% of patients underwent echocardiography and 11.2% of these patients had infective endocarditis. Gram-negative infections were found more commonly in older patients, females, and those with cirrhosis or a solid tumor. S aureus was more common in males and younger patients. MRSA was more common in those with chronic renal disease.⁹

Spinal epidural abscess (SEA) is an infection of the epidural space in the spinal canal. If left untreated, this infection is associated with high morbidity and mortality, and it can have devastating neurologic complications due to the proximity of the infection to the neural elements.¹,¹⁵ SEA is most common in males ages 50 to 70 years and rarely is seen in the pediatric population. The incidence of SEA has been reported to range from 2 to 5 cases per 10,000 hospital admissions.¹⁵,¹⁶,¹⁷

Risk factors for SEA include intravenous drug use, recent trauma, and alcohol use.¹⁸ Procedures involving direct inoculation such as spinal epidural or facet injections also increase the risk of SEA. Patients with
medical comorbidities including diabetes and immunocompromising conditions are also at increased risk.¹⁸ In a review of 128 patients with SEA, the most common risk factor was intravenous drug use (39.1%) followed by diabetes (21.9%).¹⁹

S aureus is the most common bacterial cause of SEA; methicillin-susceptible S aureus is more common than MRSA. Other pathogens that have been reported include coagulase-negative Staphylococcus species, Streptococcus species, and gram-negative bacteria.¹⁸ The lumbar spine has been found to be the most frequent location for SEA in multiple studies. One study reported that 54.7% of cases were in the lumbar spine, with 39.1% in the thoracic spine.¹⁹

SEA can result from either direct inoculation or hematogenous spread.¹ In a review of all SEA cases at one tertiary care hospital over 10 years, hematogenous spread was the most common source, with recent surgeries/procedures being the second most common.¹⁷

Neurologic dysfunction in the setting of SEA can occur due to either direct compression or secondary to spinal cord ischemia.¹⁵ This ischemia may be caused by mass effect of the abscess on the cord or through bacterial occlusion of the vasculature.¹⁵ The abscess may be located either ventrally or dorsally. Ventral SEA most commonly occurs in the setting of vertebral osteomyelitis/osteodiskitis, whereas dorsal SEA more commonly can be from a de novo process.¹