Pyogenic infections of the spine are relatively uncommon. However, with the recent rise in diabetics, immunosuppressed patients, intravenous drug use, and the number of invasive spinal procedures, the overall incidence of spinal infections is rising with reports as high as 10 per 10,000 admissions per year. The incidence of these infections is typically bimodal. Discitis is most commonly responsible for the peak incidence between the ages of 10 and 20. The second peak, which corresponds to a larger proportion of pyogenic infections, occurs in the adult population who are in the fifth decade or older. This patient population is more likely to develop epidural abscesses due to existing medical comorbidities and/or previous invasive spinal procedures. For unclear reason, males are twice as likely to be affected as females.

Predisposing factors include advanced age, poor nutritional status, diabetes mellitus, renal failure, septicemia, distant infections, intravenous drug use, alcohol abuse, smoking, HIV, malignancy, an otherwise immunocompromised state, rheumatoid arthritis, chronic steroid use, recent spine surgery, trauma, and presence of intravascular or intraspinal devices (Table 12.1).

Osteomyelitis in the spine accounts for 2% to 7% of all cases of osteomyelitis. Due to the rich blood supply of the vertebral body, this region is involved approximately 95% of the time, while the posterior spinal elements are involved in only 5% of cases. Over half of reported cases
of osteomyelitis occur in the lumbar spine. The thoracic spine is the next most commonly affected, followed by the cervical spine. Sacral infections are quite rare (Table 12.2).

Approximately half of all pyogenic spine infections are caused by Staphylococcus aureus. Less common causative organisms include Escherichia coli, S. epidermidis, Haemophilus influenzae, Psuedomonas species, Enterococcus species, Aeruginosus bacillus, Salmonella species, and Group A and B Streptococcus (Table 12.3). Patients with sickle cell anemia are at higher risk for Salmonella infection, but S. aureus is still the most common pathogen in this population. Likewise, Pseudomonas species is characteristic of IV drug abuse but the most common organism encountered in this population is still Staphylococcus. Diabetic patients have a higher incidence of inoculation with anaerobic bacteria. In the scenario of an indolent infection, consideration should be given to slower-growing organisms such as Streptococcus viridans and S. epidermidis. More recently, Propionibacterium acnes have been cultured with high frequency from disk material from patients undergoing spinal surgery, raising the possibility of low back pain and disk degeneration from an infectious etiology.

Pyogenic spine infections are generally classified according to location. Most commonly affected regions are the epidural space (epidural abscess), disks (discitis), vertebral bodies (osteomyelitis), and disk–vertebral body complexes (spondylodiscitis). Frequently, infections occur simultaneously in multiple regions at the same time. Less commonly, the facet joints can develop a septic arthritis (Fig. 12.2). Infections in the lumbar spine may extend into the psoas muscle, causing a psoas abscess.