Infectious spinal disease is regarded as an infection by a specific organism that affects the vertebral body, intervertebral disc and adjacent perivertebral soft tissue27). In spite of medical development, its incidence seems to be increasing as a result of larger proportion of the older patients with chronic debilitating disease, the rise of intravenous drug abuser, and the increase in spinal procedure, instrumentation and surgery25,27).

In Korea, studies assessing infectious spinal disease are rare, and most of the researches have been focused to pyogenic spondylitis or tuberculous spondylitis, or radiological differentiation of them4,14,15,18,19,20,24,28).

The objectives of this study are to describe the epidemiology of all types of spinal infectious disease and their clinical and microbiological characteristics as well as to access the diagnostic methodology, and parameters related to the outcomes.

A retrospective study was performed in all infectious spinal disease cases presenting from January 2005 to April 2010 to three tertiary teaching hospitals within a city of 1.5 million in Korea. Cases fulfilling the following criteria were enrolled on the basis of clinical, radiological, laboratory, pathologic, and microbiological data.

The patients who refused any evaluation or treatment or the cases of superficial wound infection were excluded.

Patients without any positive results from the microbial studies were regarded as cases of pyogenic infection, if they showed characteristic radiologic findings consistent with spinal infection, clinical response to antimicrobial therapy, and their histology not showing granulomatous finding.

Patient demographics, risk factors, clinical features and outcomes were assessed. Risk factors entailed the presence of diabetes, chronic renal failure, liver cirrhosis, intake history of immunosuppressaant, remote infection, and underlying malignancy. Previous spinal surgery or procedure as well as the other types of treatment they received were documented.

SPSS version 21 was used for statistical analyses of the data. t-test was used to compare the continuous variables, and χ² tests and Mann-Whitney test were used to compare the non-parametric variables. The threshold for statistical significance was established at p<0.05.

Infectious spinal disease is not a common disease, but its incidence seems to be increasing. In spite of advanced medical science including surgical techniques making operations shorter and its wound smaller and various newly developed antibiotics, the incidence of infectious spinal disease is getting higher as a result of larger proportion of the older patients with chronic debilitating disease, the rise of intravenous drug abuser, and the increase in spinal surgery and instrumentation25,27). In Korea, like other eastern developing country, it has been thought that tuberculous disease is frequent and the incidence of TSI is higher than PSI. Lee et al.19) reported 85% of primary spinal infection were TSI in 1996, and another Lee et al.20) reported 71.0% of spondylitis were TSI in 1999. In 2001, Park and Kim24) reported that 13 patients of the 19 spondylitis patients who had been managed surgically were diagnosed as tuberculous spine infection. However, the incidence of TSI tends to decrease owing to vaccination of tuberculosis and anti-tuberculosis medication and the incidence of PSI seems to rise as results from increase of spinal procedures or surgeries and misuse of antibiotics12,16,27). Recently, in 2010, Kim et al.15) reported pyogenic spondylodiscitis (62.7%) is more frequent than tuberculous spondylodiscitis (37.3%). In our study, PSI (78.3%) had occurred three times more often than TSI (21.7%). Despite many spine-special hospitals manage patients with spinal diseases, the patients of infectious spinal disease tend to be transferred to tertiary hospitals because of the availability of various antibiotics. We think our study is more objective and reliable than any other studies, because we gathered and analyzed the data from three tertiary hospitals in one city.

The known risk factors of the infectious spinal disease are diabetes, liver cirrhosis, underlying malignant disease, end stage of renal disease, intravenous drug abuser, remote infection and any other immune compromised conditions7,11,29). We focused whether the previous spinal surgeries or procedures would be a risk factor of infection. Recently, it has been increasing not only the number of spinal surgeries and procedures, but also the incidence of spinal infectious disease. We included the history of previous spinal surgeries or procedures in the investigation lists to find out their correlation. In our study, 39.1% of all infectious patients have had spine surgery or invasive procedure which was proven as the most predisposing factor. We cannot state that all these surgeries and procedures affected infection, and cannot conclude that previous spinal surgery or procedure become a definite risk factor, because this study was retrospective design and we could not exclude the patients whose infectious condition had been prior to their spinal surgery or procedures. In comparison of both groups, however, the ratio of presence of previous spinal surgery or procedure in PSI is more than twice as high as that in TSI and it showed statistically significant difference (p=0.048). The previous spinal surgery and procedure can cause spinal infection by direct inoculation of bacteria. If we suppose that it is very difficult to inoculate Mycobacterium tuberculosis directly to the back when performing the procedures, and in addition, the ratio of presence of previous spinal surgery or procedure in PSI is significantly higher than that in TSI, it can be suggested that previous spinal surgeries and invasive procedures is one of the crucial risk factors in PSI.

WCC, ESR, and CRP are the important screening markers of infectious diseases in both making the diagnosis and assessing the treatment response. In most studies, values of inflammatory markers are higher in pyogenic infection than tuberculous infection3,15,29). These patterns help in differentiation of pyogenic and tuberculous spinal infection. Furthermore, decreasing pattern of CRP which had elevated initially helps us make decisions of the accurate diagnosis and the effectiveness of treatment, because CRP responses appropriately for effective treatment. In our study, all inflammatory markers were elevated in both groups, and all inflammatory markers were increased higher in PSI group than TSI group (Table 4). Among them, the differences between both groups were significant in initial WCC and initial CRP which are entirely consistent with the report of Yee et al.29).

There is consensus that the most common lesion of pyogenic spinal infection is lumbar spine1,7,15,16,21,29). According to the studies about tuberculous spinal infection, the frequency of involvement varies widely, with the peak levels being at the thoracolumbar junction and with decreasing frequencies at more rostral and caudal levels27). Some authors reported tuberculous spondylitis occurred more frequently in thoracic spine5,8,13,24,29). Meanwhile, much more studies investigated in Korea, including this study, showed that TSI occurred more frequently in lumbar spine4,14,17,18,28).

Infectious spinal disease may involve various skeletal structures such as vertebral body, intervertebral disc, epidural space and perispinal soft tissue. Among them, most common type of infection is spondylitis in both PSI and TSI. However the preference of coincidence is different according to the etiologic microorganisms. Generally, disc involvement of infection is occurred more frequently and earlier in PSI, in the other hand, the discs are often preserved in TSI6,26). We might conclude that discs are more easily invaded in PSI from the results of our analysis. Table 2 and 3 showed that the frequency of disc involvement is just slightly higher in PSI which did not meet statistically significance (p=0.197), but the number of patients with pyogenic spondylitis invading disc space without abscess (SD) is 9 (12.5%) in PSI, whereas no case in TSI (p=0.003). In addition, the proportion of spondylitis making the abscess without discitis (SA) in PSI is about a half of that of TSI (p=0.117).

The most important point for the treatment of spinal infection is the identification of etiologic microorganism. The procedures for this step include blood culture, percutaneous tissue biopsy and culture, and open biopsy and culture. Nowadays real-time polymerase chain reaction (RT-PCR) is used as one of the definite diagnostic tools for tuberculosis and this method has improved the diagnostic results9,23). The results of RT-PCR were not analyzed here, because we had performed it just in some cases in our study. When a spinal infection is suspicious, the blood culture is firstly performed, if neurologically stable, because it is the simplest. But it shows lower positive rates than any other procedures. When the result of blood culture is negative, the procedures for obtaining infected tissue should be considered before antibiotic treatment start6). Positive rate of open biopsy and culture is the highest and percutaneous culture and blood culture follows21). The reason of higher positive rate of open biopsy and culture is that accurate and enough specimens can be obtained as much as needed from the infected tissue. In our study, however, positive culture rate of open biopsy was lower than that of percutaneous culture. We thought these findings might come from the many cases in which antibiotic therapy started before open surgery. Seven cases of 11 showing the negative culture results were administered antibiotics within a month before open culture. We thought this also affected our overall positive culture rate, which was lower than the other reports. The other cause of low positive culture rate is thought to be the techniques obtaining the specimen. Percutaneous biopsies were usually fine needle bone marrow biopsies based on fluoroscopic technique. In order to get enough tissue more accurately from the pathologic area, percutaneous CT-guided needle biopsy is a good option to enhance the culture result6,10,25). The most common isolated etiologic microorganism causing PSI in this study is Staphylococcus aureus followed by E. coli, like in most of the reports1,7,15,16,21,22,24,27,29).

When we investigated the prevalent etiologic microorganisms in two groups of patients having predisposing factor of infection; previous spine procedures or diabetes mellitus (Table 8), we had expected that Staphylococcus aureus would be more frequent in group with medical history of spinal procedures, but unknown etiologic organism was the most. When the negative culture rate was respectively analyzed according to the type of previous spinal procedures to verify the reason, we found that negative rate was higher in the group of open surgeries than the group of percutaneous procedures (Table 9), which was thought to result also from use of prophylactic parenteral antibiotics we always uses for about a week in cases of open surgeries, that is, prior use of antibiotics makes identification of the etiologic organism difficult2).

This study has both advantages and limitations. We collected a lot of clinical data of spinal infectious diseases from three tertiary hospitals in one city, and expect this is the latest data representing South Korea. However, this study lacks the consistency of diagnostic tools and therapeutic options, because this was retrospective and the diagnosis and management were different between the hospitals.

The incidence of PSI has shown three times greater than that of TSI. Previous invasive spinal procedure is the most commonly noted risk factor, and its numerical increment is also thought to be associated with increasing incidence of PSI. Even though the lumbar spine is the most involved segments in both types of spinal infection, disc is more susceptible to PSI unlike abscess to TSI. Initial higher WCC and CRP could be one strong indicator of PSI rather than TSI, especially in early phase of infection in which etiologic microorganism has not yet been proven.