A Novel Magnetic Resonance Imaging Scoring System in Making Specific Diagnosis of Tubercular Spondylitis in Advanced Infection

INTRODUCTION

Magnetic resonance imaging (MRI) can make a diagnosis of infective spondylodiscitis if there is an alteration of signal in the disc and adjacent vertebrae with or without the presence of collections. However, by imaging alone, it is often difficult to ascribe to a particular causative pathogen for spondylodiscitis. Many radiological features common to tuberculosis and pyogenic spondylodiscitis with varying percentages have been described in the literature with some overlapping features and characteristic findings.^[1]

We performed a review of the MRI features of pathologically proven cases of tubercular and pyogenic spondylodiscitis. We have attempted to postulate a scoring system to help in the confident prediction of tuberculous (TB) etiology.

MATERIALS AND METHODS

The study was carried out at our hospital from January 2014 to December 2017. All patients who had a clinical suspicion of spinal infection, MRI findings depicting features of spinal infection and operated/biopsied for tissue culture confirmation were included in the study. A retrospective observational analysis was performed on 125 such biopsy- proven infective spondylitis patients which included 70 tubercular (Group A) and 55 pyogenic (Group B) patients, the infection was confirmed by tissue gene expert test for tuberculosis, histopathology or culture results.

The imaging was performed on 1.5 T Siemens Symphony MRI machine with eight channels and equipped with dedicated spine coil. Pulse sequences used included T1W and T2W sagittal sections, T2 STIR sagittal and Coronal, T2W SE axial, and T2W SE whole spine screening. Section thickness was 4 mm and the intersection gap was 1 mm. The MRI parameters for diagnosing spondylitis based on literature were identified and compared.^[2,3]

The variables were identified if they had the patterns of involvement as described in literature.^[2,3] The various variables studied include (1) contiguous involvement (> two adjacent vertebrae),^[4] (2) presence of para or intraosseous abscess, (3) subligamentous spread of infection, (4) presence of vertebral collapse, (5) large collection (>1.5 cm) with thin abscess wall (<2 mm), (6) presence of T2 hypointense debris, (7) presence of non-contiguous skip lesions,^[5-7] and (8) intervertebral disc involvement.

Less often recognized pattern is the presence of T2 hypointense debris which has been described in literature, however, is not discussed extensively.^[8] It can be seen within vertebral bodies or in the collections on T2W images; they correlate to the fragmentation of bone caused by the slowly progressing infection, appearing as hypointense areas and correspond to bony fragmentation sign on computed tomography (CT) scans.

Imaging findings on MRI of both groups were then compared, analyzed and features helping in differentiation were tabulated. An attempt to devise a novel MRI score to differentiate the two groups was done and results statistically analyzed.

RESULTS

The mean age of the study population was 52 years for tubercular and 54 years for pyogenic group with a minimum age of 12 and maximum of 87 years. No statistically significant age differentiation for the groups was observed.

The positive predictive value with the prevalence of each MRI parameter within both the groups and relationship of each parameter with tubercular group compared against pyogenic group was assessed. The Pearson’s Chi-square test and univariate logistic regression analysis have shown significant differentiation between the two pathologies with the first six MRI features [Table 1]. No significant statistical correlation for differentiation was observed for the presence of skip lesions and degree of intervertebral disc involvement.

Among the six statistically significant MRI parameters, the independent predictors for tubercular involvement turned out to be (1) sub-ligamentous infectious spread (adjusted odds ratios [AOR] = 21.6, 95% confidence interval [CI] = 2.19–59.9, P < 0.05), (2) presence of vertebral collapse (AOR = 8.68, 95% CI = 1.25–40.6, P < 0.05), (3) large collection with thin abscess wall (AOR = 43.07, 95% CI = 5.7–74.6, P < 0.001), and (4) presence of T2 hypointense debris (AOR = 14.56, 95% CI = 2.94–55.6, P < 0.05) which was obtained by multivariate regression analysis.

A score of 2 for each of the four significant independent predictors of tubercular spondylitis was given, and a score of 1 was given for the two parameters including the presence of contiguous involvement and para or intraosseous abscess which were statistically significant but not significant independent predictors. Thus, a total score of 10 was devised [Table 2].

Scoring for both group A and group B was tabulated and analyzed [Table 3].

Distribution of scores among the subjects aged 53.4 ± 17 years showed P < 0.001 (t-test and Mann–Whitney U-test) with mean of 7.4 for tubercular and 2.9 for pyogenic group (SD of 1.9). A score of 6 or above suggested tuberculosis and score below 6 suggested pyogenic infection (sensitivity – 94.2%, specificity – 89%, positive predictive value – 94.2%, and Chi-square value of 87.67, P < 0.00001) [Table 4 and Figure 1].

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Figure 1:

Comparison of scores between the groups.

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DISCUSSION

Osteoarticular tuberculosis involves 2–5% of all tubercular lesions in the body, of which 50% affect the spine.^[3] In 2015, 1.4 million people died from TB, including 0.4 million among people with HIV. Tuberculosis of the spine, if not diagnosed and treated in time can lead to serious complications such as deformity, instability, and neurological deterioration.

Imaging plays a key role in the detection of spinal infections as it provides valuable information regarding the detection and location of infection, the severity of disease, associated morphological changes and also might provide clue in detecting causative organism.

The diagnosis of etiology is important for many reasons. The treatment is entirely different for tubercular and pyogenic spinal infections. The prognosis of the disease differs for both. For tuberculosis with no neurological complications, the prognosis is good with early detection and starting antitubercular medical therapy. Cases with neurological deficit require additional surgical management for stabilization.

The rates for isolating the causative organism are <50% in TB spondylitis^[9,10] and 60–80% for pyogenic spondylitis.^[5-7] Moreover, even histopathologic evaluation of biopsy material does not always provide a specific diagnosis. In general, it is reported that the sensitivity and specificity of MRI for diagnosis of spinal TB are 100% and 88.2%, respectively.^[11]

Usually, two contiguous vertebrae are involved in both pyogenic and tubercular infections. It is reported in literature that typical TB spondylitis is seen affecting the single region in approximately 65%, multiple contiguous level infections are seen in 20% [Figure 2], and multiple non-contiguous skip levels of involvement are seen in about 10% of cases.^[12] Hence, in our protocol, we screen the whole spine with sagittal T2WI MRI sequence when investigating a spinal TB patient to identify non-contiguous (skip) lesions before surgical planning [Figure 3].^[13,14] Contiguous vertebral involvement of more than two levels is traditionally described for tuberculosis, in our study, although we found it to be specific for tubercular, it is less sensitive and we did not find it statistically significant as an independent predictor of tuberculosis; hence, a score of only one was given. The reason could be due to a few atypical cases included in our group including brucellosis, polymicrobial including fungal spondylitis infections, which also can involve more than two vertebrae.

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Figure 2:

Multiplanar T2, T1, and T2Fat sat magnetic resonance images depict contiguous vertebral involvement, anterior subligamentous spread with intraosseous and paravertebral collections, epidural abscess with cord compression, collapse of vertebral bodies, T2 hypointense areas and thin abscess wall. Features suggest tubercular etiology with a score of 10.

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Figure 3:

Multiplanar T2,T1 and T2Fat sat MR images depict skip lesions, contiguous involvement, subligamentous spread with intraosseous collections and thin abscess wall. Features sugggesting Tubercular etiology with score of 7.

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Atypical lesions are also described in tuberculosis which is rare and includes vertebra plana (common in children), ivory vertebra, isolated involvement of neural arch, solid soft tissue component, and non-contiguous bony lesions.^[15]

Soft tissue component can be either phlegmon or abscess in epidural, paravertebral or paraspinal regions. They appear as loculated and well-encapsulated collections with smooth walls. Our study showed that though the presence of abscess was more common in tubercular cases, it is not an independent predictor for tuberculosis as it is seen in both the pathologies; hence, no scoring was given. The size of the collection and thickness of the abscess wall showed a significant difference among both groups. Larger sized abscesses (size >1.5 cm) with thinner walls (<2 mm) were seen in the tubercular group while smaller abscesses with thick irregular wall were frequently seen in pyogenic infections [Figure 4]; hence, a score of 2 was given for larger abscesses.

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Figure 4:

Multiplanar T2 and T2Fat sat MR images depict absence of contiguous involvement, collapse and subligamentous spread, presence of intraosseous and paravertebral collections with thick abscess wall. Features sugggesting Pyogenic etiology with score of 1.

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The avascular nature of the intervertebral disc favors it to be the primary site of pyogenic infection [Figure 5]. On the contrary, due to hematogenous spread of tubercular bacilli, vertebra is primarily involved, and disc shows late involvement. In our study, involvement or sparing of disc was the least specific (3.6%) and not statistically significant between the two groups; hence, no score was given. We presume that being a tertiary referral center; our group had more advanced and delayed presentation cases wherein tubercular cases also had secondary involvement of disc. Presence of vertebral body destruction more than half suggesting collapse was highly sensitive (88.5%) and seen in cases of tuberculosis; hence, score of 2 was given.

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Figure 5:

Multiplanar T2,T1 and T2Fat sat MR images depict presence of disc involvement, intraosseous and paravertebral small collections with thick abscess wall. Features sugggesting Pyogenic etiology with score of 1.

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A relatively less discussed but yet important imaging finding is the presence of T2 hypointense debris corresponding to the fragmented appearance due to bone destruction seen on CT images. It is said to be specific for tuberculosis due to the slow and continuous destruction caused by the tubercle bacilli. Even the presence of T2 hypointense rim around the collections/abscesses which correspond to marrow sclerosis has been described to be specific for tuberculosis. We have found this imaging feature to be quite sensitive (80%) and very useful for delineating the etiological diagnosis; hence, a score of 2 was assigned.

The use of contrast enhanced MRI studies for spinal infections has also been described for demonstrating heterogeneous vertebral body enhancement, enhancement of the inflamed meninges or rim enhancement around abscesses.^[7,11,16,17] In our study, we have not done contrast since all the findings included are detectable sufficiently by routine plain MRI scan.

The use of above-mentioned MRI parameters and scoring can lead to a confident differentiation of TB from pyogenic lesions, in case of doubt (or) for further confirmation of diagnosis, image-guided needle biopsy/aspiration or surgical biopsy may be done and samples of bone tissue or abscess are obtained to stain for AFB and isolate organisms for culture, antibiotic sensitivity, and histopathology.^[16] Biopsy is also recommended to evaluate for rifampicin sensitivity before the start of antitubercular therapy.

CONCLUSION

In our study, we have found that sub-ligamentous infectious spread, presence of vertebral collapse, large collection with thin abscess wall, and presence of T2 hypointense debris are independent predictors of tubercular involvement. MRI can thus be used for noninvasive and confident diagnosis of spinal tuberculosis. This scoring system can be used in all cases of spinal infections to identify the etiology wherein a score of 6 or above indicates advanced tuberculosis and <6 indicates pyogenic spondylitis. This will reduce the burden of additional invasive investigations, additional costs, and time delay for initiating early treatment.