DISH is generally diagnosed using the Resnick criteria 7,8, which are defined by swelling ossification of the anterior longitudinal ligament 1 and absence of apophyseal joint ankylosis and SIJ erosion, sclerosis, or intraarticular osseous fusion 8. Conversely, AS is often diagnosed using the modified New York criteria 14,21. AS is characterised by annulus fibrosus ossification and adjacent vertebral body bridging anteriorly and laterally known as a ‘bamboo spine’ 1 and SIJ sclerosis, joint space narrowing, erosion, or fusion 1. Some reports indicate that DISH and AS can be differentiated based on these radiological characteristics 1. However, we sometimes encounter patients who do not have typical radiological findings defined in the abovementioned criteria and are difficult to differentiate between these two diseases. Therefore, this current study compared the detailed radiological characteristics of DISH and AS on spinopelvic CT images.
Previous literature has described that in the later phase of AS, SIJ shows sclerosis, joint space narrowing, erosion, or osseous fusion 1. Conversely, in the SIJ of patients with DISH, only ligamentous area obliteration and mild synovial area narrowing can occur; however, SIJ fusion is not observed 1. However, some other studies have reported that 23% of DISH is associated with fusion 15,19, even though the DISH diagnosis does not require this involvement based on the Resnick criteria 8. A previous study suggested that SIJ fusion occurring both in patients with DISH and AS may be possibly due to similar developmental pathways, leading to inflammation-associated enthesitis in younger patients in AS and more mechanistically associated enthesopathy in older patients with DISH 19. In this study, as the percentage of partial and complete SIJ fusion was higher in AS than in DISH, up to 63% of patients with DISH had partial or complete SIJ fusion. The SIJ fusion rate in this study was larger than that in previous studies possibly due to the evaluation method in which we included partial SIJ fusion. However, this study demonstrated that SIJ fusion was frequently observed not only in patients with AS but in those with DISH and that might not be the necessary criteria to differentiate DISH from AS.
In the radiological evaluation of anterior and posterior bridging around the SIJ, previous studies have reported that patients with DISH have high anterior and posterior bridging rates around the SIJ. In their reports, 71.6% 15, 48% 19 and 30% 22 of patients with DISH had anterior bony bridging and 5.4% 15, 20% 19 and 17% 22 of patients with DISH had posterior bony bridging. In this study, we found that anterior and posterior bony bridging around the SIJ was significantly more common in DISH than in AS. We separately evaluated one-side and both-side bridging on the SIJ and found that both-side bridging was much higher in DISH than in AS either in anterior or posterior bridging. This finding may be useful for differentiating AS from DISH.
In the anterior spinal bony bridging of the vertebrae, inflammation occurs at the attachment of the annulus fibrosus, and the healing process results in AS syndesmophytes 1. Conversely, bridging in DISH results from an ossification process involving the anterior longitudinal ligament 1. Although both AS and DISH are characterised by anterior bony bridging, that of AS is generally characterised by smooth bridging ‘bamboo spine’ and that of DISH is by ossification of candle-wax-type ‘flowing mantles’ 1. We defined both of them as anterior bony bridging. The number of anterior spinal bony bridging was greater in AS than in DISH, especially in the lumbar spine. These results are similar after age and sex matching. As previously reported, DISH mainly affects the thoracic spine 18,23. However, our study showed that AS tends to affect the whole spine, including the lumbar spine, which may also be one of the differentiating points between DISH and AS.
In analysing the shape of spinal bridging, as expected, the rate of candle-wax-type was much higher in DISH than in AS. This result is consistent with that of a previous report 1. However, a certain percentage of patients with DISH had surprisingly smooth-type bridging (0–30% candle-wax-type: 10.8%). Conversely, some patients with AS had candle-wax-type bridging (30–70% candle-wax-type: 20.8%). To the best of our knowledge, this is the first study to reveal that anterior bony bridging in patients with DISH occasionally demonstrated ‘AS-like’ bony bridging. Our findings refute that of the previously reported 1,5,8, i.e. the typical appearance of bony bridging in DISH. Physicians should consider that candle-wax-type bridging is not always present in DISH, and this appearance alone cannot prove its diagnosis.
In evaluating the facet joint fusion, the DISH diagnosis requires the absence of joint fusion 8. A previous study showed that facet joint ankylosis in AS is more common than that in DISH, and a small number of cervical ones with DISH were observed in whole-body magnetic resonance imaging 20. They showed no thoracic and lumbar facet ankylosis in DISH. Another study reported that thoracic and lumbar facets in patients with AS had more inflammatory lesions 24. We also found that both-side facet fusions of the thoracic, lumbar and whole spine were more common in AS than in DISH, and results were similar after age and sex matching (Table 4). Interestingly, 61% of patients with DISH had at least one facet fusion in our study. We first demonstrated that facet fusions occur at a high rate not only in AS but also in DISH. For the differential diagnosis of DISH and AS, spinal facet fusion exists should be considered in a certain number of patients with DISH in addition to SIJ fusion.
This study has some limitations. First, the sample size of patients with AS was small because AS is relatively uncommon in our country. However, many important comparisons reached statistical significance. Second, the number of bony bridging and fusion depends on the patient’s age. However, results after age matching showed a similar tendency.
In conclusion, both sides of complete SIJ fusion are common in patients with AS, and anterior/posterior bridging around the SIJ is common in patients with DISH. However, a considerable number of patients with DISH have SIJ fusion. In the anterior spinal bridging, patients with AS are characterised by smooth bridging, which commonly occurs in the lumbar spine of patients with AS when compared to those with DISH. Conversely, patients with DISH are characterised by candle-wax-type bridging, which commonly occurs in the thoracic spine. However, interestingly, a certain percentage of patients with DISH had smooth-type bridging and some patients with AS had candle-wax-type bridging. Furthermore, a considerable number of patients with DISH showed spinal facet fusion. These facts should be considered when making a diagnosis for AS or DISH.