Infections of the Spine
Infections of the spinal column is a very broad topic and includes those diseases that arise spontaneously and those that are secondary to some inciting event. True infections are uncommon, particularly in the industrialized countries of the world. The estimated annual frequency is 0.037 for disc space infection, 0.037 for bacterial vertebral osteomyelitis and 0.037 epidural abscesses. Post-operative wound infections range from 1% after a simple discectomy to 6-8% after attempted fusion with hardware.
A certain group of people are at risk. These are the smokers, the obese, the malnourished, the immunosuppressed, either from acquired immunodeficiency syndrome or medical treatment for tumors; arthritis; organ transplantation; drug addicts; diabetics; or those who have undergone recent urinary tract instrumentation.
The symptoms vary with the particular disease but constant back pain without a history of trauma is worrisome. Often there is a delay in diagnosis because of the subtle presentation, the failure to appreciate unrelenting pain, and the absence of systemic signs such as temperature elevation. The laboratory study may be misleading, normal white blood cell counts are common, radiographs often show no abnormalities early in the course of the illness, and even more sensitive diagnostic tests such as bone scans may not become positive for a week. When the diagnosis is suspected, the MRI now seems to be the most reliable early confirmatory test, while elevations of the erythrocyte sedimentation rate are a valuable screening test.
Discitis as seen on X-ray
Discitis, or disc space infection, is an inflammatory lesion of the intervertebral disc that occurs in adults but more commonly in children. Its cause has been the subject of debate, although most authors believe it to be infectious. The infection probably begins in one of the continguous end plates, and the disc is infected secondarily. Severe back pain that begins insidiously is characteristic of the disease. Although most children will continue to walk in spite of the pain, young children may refuse to ambulate. The characteristic finding is extension of the spine and the child's complete refusal to flex the spine. Children with discitis usually are not systemically ill. They rarely have an elevated temperature and their white blood cell count is frequently normal. However the erythrocyte sedimentation rate is usually increased.
Lateral radiographs of the spine usually will reveal disc space narrowing with erosion of the vertebral end plates of the contiguous vertebrae. bone scanning may be helpful in localizing a lesion that is difficult to diagnose clinically. Some bone scans are falsely negative, so the diagnosis of disc space infection should not be excluded simply because the bone scan is normal. Magnetic resonance imaging (MRI) seems to be helpful in identifying a disc space infection. The appropriate treatment of these lesions has been the subject of controversy. Most authors recommend plaster cast immobilization, a treatment that seems to be effective by itself in many cases. Some authors think that antibiotics also should be given because the condition most likely is an infection of the disc (the organism involved is frequently Staphylococcus aureus). In treating the lesion in children, a biopsy is not usually necessary. A biopsy may be indicated in adolescents or adults, especially if drug abuse is suspected, because of the possibility of organisms other than Staphylococcus aureus being present.
Discitis seen on MRI
Vertebral osteomyelitis is infection in the vertebral body itself. It may be caused by either a bacteria or a fungus. Bacterial or pyogenic vertebral osteomyelitis is more common. Its presentation is different than a disc space infection. It may represent infection elsewhere in the body that has seeded the spine through the blood stream. These patients are systemically ill, exhibiting increased temperature, white blood cell and erythrocyte sedimentation rate. The average time from onset of symptoms to definitive diagnosis has been reported to range from 8 weeks to 3 months. The onset is usually insidious, with back pain the most common symptom. The pain is localized at first to the level of the involved area, with a gradual increase in intensity. The pain eventually becomes so severe it is not relieved by complete bedrest.
Usually neurologic signs are not present until late in the disease course when there can be destruction and collapse of the vertebral body. Other symptoms variably present include chills, weight loss, dysuria, photophobia, and drainage from a wound or incision if there has been prior surgery. The causative agent is usually Staphylococcus aureus. Long term antibiotic therapy is required, usually six weeks of intravenous antibiotics sometimes followed by oral antibiotics for another six week period. Surgery may be indicated, particularly when ongoing vertebral destruction is identified. Although rare, typical diseases such as tuberculosis of the spine should be considered when faced with spinal infections. This is more common in underdeveloped countries.
Osteomyelitis as seen on X-ray
Epidural abscesses are infections that form in the space around the dura, the tissue envelope that surrounds the spinal cord and nerve root. These pockets of purulent fluid may surround the spinal cord and/or the nerve roots and generate enough pressure to affect neurological function. The symptoms can be subtle with (paresthesis - a sensation like pins and needles) or mild weakness. Back examination should be performed for evaluation of asymmetry, paravertebral swelling and tender vertebrae and careful neurological examination should be performed. The same diagnostic tests are employed with erythrocyte sedimentation rate being the most consistent abnormal laboratory value. The white blood cell count and blood cultures are useful as baseline parameters. Plain radiographs are not helpful. Magnetic resonance imaging (MRI) is the study of choice for further evaluation. This single study will define spinal cord compression and the status of the spinal cord, determine the presence and extent of the abscess, provide a diagnosis of co-existing osteomyelitis, and exclude a drainable paraspinal fluid collection. The indications for surgical decompression of the spinal cord would be an increasing neurological deficit, persistent severe pain, or increasing temperature and white blood cell count.
Finally, the frequency of post-operative wound infections have increased with increased usage of spinal instrumentation. The advantages of rigidly fixing a spine with instrumentation in order to increase fusion rate and decrease post-operative external immobilization have become apparent. Infection is a major disadvantage of implanting foreign bodies in the spine. The use of prophylactic antibiotics helps but a small percentage cannot be avoided. It has recently been shown that in particular, smokers are a very high risk group for infection. The diagnosis of post-operative infections is usually based on drainage from the wound or extraordinary pain. Other signs are elevated temperature, hematoma formation, erythema and edema at the wound site, elevated erythrocyte sedimentation rate, and increased white blood cell count. Once an infection is suspected, prompt exploration of the wound is indicated in the operating room with the patient under general anesthesia. Facilities must be available for aggressive debridement of the entire wound. Imaging studies are of limited value. Treatment of infected wounds includes debridement, systemic antibiotics, and either closed suction, suction irrigation or open treatment. Multiple debridements may be required. If the infection persists despite multiple debridements, removal of hardware and all bone graft is indicated. The most common bacteria is Staphylococcus aureus. Duration of the intravenous antibiotics is variable depending upon the infection. The cost of a post-operative infection can be considerable and the results less than optimal.
Glossary of Terms
Discitis - Infection of the disc space; an inflammatory lesion of the intervertebral disc that occurs in adults but more commonly in children.
Osteomyelitis - Infection of the bone. Vertebral osteomyelitis means infection of the vertebral bones of the spinal column.
Epidural Abscess - Epidural means space above the dura (covering of the spinal cord and nerve roots). Abscess is the collection of purulent fluid or "pus". Thus, this is an accumulation of purulent fluid around the covering of the spinal cord and nerve roots.
Erythrocyte Sedimentation Rate - Laboratory test measuring rate at which red blood cells settle in a test tube - this can be a measure of inflammation or infection in the body.
Bone Scan - Radiologic study where patient is injected with a radioactive tracer (usually technetium) that is absorbed by the skeleton. Increased activity is seen in arthritis, cancers and infection. A very sensitive test.
MRI (Magnetic Resonance Imaging) - Non-invasive radiological test providing excellent image of soft tissue and bones.
Staphylococcus Aureus - Common bacteria in skin infections. The most common bacteria in bone bone infections.
Biopsy - Obtaining a small amount of tissue or cells for examination.
Dysuria - Pain with urination.
Photophobia - Extreme sensitivity to bright lights.
Debridements - Removal of infected or dead tissue.
References and Suggested Readings
1. Baker, et al. To decompress or not to decompress - Spinal epidural abscess. Clinical Infectious Diseases. 15:28-9, 1992.
2. Frymoyer JW, et al. The Adult Spine: Principles and Practice. Raven Press 1991. New York. Chapters 38, 39, 71.
3. Green NE. Thoracolumbar Spine. Pediatric Aspects. Orthopaedic Knowledge Update 2, American Academy of Orthopaedic Surgeons. Park Ridge, IL p. 288, 1987.
4. Holt RT, Senter BS. Post-operative spinal infectious spine: State of the Art Reviews. Vol. 6, No. 2, p. 389-394, May, 1992.
5. Massie JB, et al. Postoperative posterior spinal wound infections. Clinical Orthopaedics and Related Research. (284) p. 99-108, 1992.
6. Thalgott JS, Arther HB, Sasso RC, et al. Postoperative infections with spinal implants: Classification and Analysis: A multicenter study. Spine, 16:981 1984, 1991.
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