Atypical infections encompass a broad range of organisms including bacteria, spirochetes, fungi, parasites, and yeast (Table 11.1). Risk factors for an atypical spinal infection include recent surgery, immunocompromised state, diabetes, recent genitourinary surgery, and older age. Fungal infections are typically seen in the immunocompromised including those with HIV. Candida is especially seen in immunocompromised patients. Additionally, uncontrolled diabetes is a risk factor for fungal infection, especially aspergillosis and mucormycosis.

Although most cases of tuberculosis (TB) are caused by Mycobacterium tuberculosis, M. africanum and M. bovis infection also may result in “tuberculosis,” as we know it. Tuberculous spondylitis is most common within the thoracic spine, followed by the lumbar and cervical regions. Risk factors for tuberculous infection include HIV disease, homelessness, alcoholism, intravenous drug abuse, and immunocompromised states.

Infections of the vertebral column with or without involvement of the disk space represent less than 2% to 4% of all cases of osteomyelitis. The relative incidence of infectious spondylitis is more common in men than women, with a relative ratio between 1.5:1 and 3:1. It has a relative peak incidence in the sixth decade of life. The lumbar spine is the most common location for spondylodiscitis in 50% of cases, followed by the thoracic spine in 33% of cases, with the cervical spine being the least common location at 3% to 10% of cases.

An overall increase in incidence of atypical infections within the United States can be attributed to immunocompromised states. Nearly one-third of the world’s population is infected with M. tuberculosis, and approximately 10% to 15% of infections disseminate to extrapulmonary sites. Only about 5% of these patients have spinal involvement (approximately 0.5% of the world’s population, or 30 million people). Approximately one-third of patients with active TB of the spine develop neurologic impairment. Approximately 50% of extrapulmonary bone involvement is found in the spine. Tuberculous infection of the spine is becoming less prevalent with the advent of effective antibiotics for treating pulmonary TB. Nevertheless, local resurgence can be due to increasing immigration, poor compliance to antimicrobial treatment, and the growing homeless population. Fungal disease has been increasing in prevalence due to immunosuppression and the use of hyperalimentation.

Exposure to endemic areas that are known to carry the relevant organism is a major risk factor. Coccidioidomycosis is endemic in the San Joaquin Valley in California and Central and South America. Histoplasmosis is endemic in the Ohio River Valley. Blastomycosis is endemic in areas bordering the Mississippi and Ohio Rivers, the Great Lakes, the St. Lawrence River, and in Central and South America, Africa, and the Middle East. Brucellosis is found more commonly in areas that do not pasteurize milk. M. avium-intracellulare also is associated with poorly processed milk.

There are several pathophysiologic mechanisms that can lead to a spinal infection. The most common route is
through hematogenous seeding from septicemia directly to the vertebral bodies. Hematogenous spread occurs at the end arterioles at the site adjacent to the end plates posterior to the anterior longitudinal ligament, which can then extend superiorly and inferiorly. Inoculation of organism is within the vertebral bodies. Alternatively, venous spread through Batson’s plexus, which forms the epidural venous plexus within the central canal, can occur due the valveless veins that extend the length of the spinal canal. Direct inoculation of the spine can occur in cases of penetrating trauma or direct exposure from skin breakdown or open wounds. Direct infection of the spinal cord can result in infectious myelitis. Epidural, subdural, and paravertebral abscesses are all foci of infection that can compress the spinal cord and cause neurologic findings.

For atypical infections, another common route of infection is inhalation. Many fungal and granulomatous organisms are contracted from aerosolized particles. After primary pulmonary involvement, many individuals develop secondary or disseminated disease in which extrapulmonary metastasis occurs. Involvement of the vertebral body begins with deposition of bacilli in the vertebral body, then accumulation of monocytes, epithelioid cells, and Langerhans’ cells as part of a delayed-type hypersensitivity reaction. The continued host immune response of the human body generates the enlarging masses and subsequent damage to surrounding tissues. Yet another source is from the lymphatic system, which is especially relevant for mycobacterial spread via lung or pleural drainage. For these types of infections, direct spread is a less common route.

Tuberculous infection begins within the vertebral bodies, with the anterior-superior aspect the most common location. Because mycobacteria lack the proteolytic enzymes to digest the disk, TB classically spares the disk. Instead, subligamentous spread is seen, with adjacent vertebral bodies or skip lesions involved. Granulomas of the vertebral body may originate in the metaphyseal area near the subchondral end plates, anteriorly or centrally, and typically cause collapse and deformity (Table 11.2).

Parasitic infection is rare but can affect the thecal sac and its contents. Neurocysticercosis can hematogenously spread within the subarachnoid space to the thecal sac. Echinococcal infection can similarly spread and is seen in the Middle East and Australia among sheepherders. Additionally, schistosomiasis can involve the spine. Inoculation is through ingestion of the flukes followed by venous channel spread from the pelvic veins to the paramedullary veins. Schistosomal infection may present as a myelitis of the cord with irregular enhancement and edema, or may present as masses within the thecal sac that represent the eggs with surrounding inflammatory reactive changes.