Diffuse idiopathic skeletal hyperostosis (DISH) and ankylosing spondylitis (AS) are 2 clearly different disease entities having in common the involvement of the axial skeleton and the peripheral entheses1,2. Both diseases produce bone proliferation in the spine and at the extraspinal entheseal sites in the later phases of their course. Although the aspects of the bone proliferations of the 2 diseases are dissimilar, confusion of radiographic differential diagnosis between the 2 diseases exists, partly as a consequence of a lack of awareness of their respective characteristic features2,3. It has been pointed out that the differential diagnosis between DISH and longstanding advanced AS is not limited to the radiologic findings. The determination of which disease the patient has extends to the clinical field since patients with DISH can occasionally have severe limitation of spinal mobility, along with postural abnormalities that resemble longstanding advanced AS4. We describe a case, emphasizing that the clinical differential diagnosis between the 2 diseases is based on peripheral enthesopathy.
The patient, a 48-year-old man, came to us because of postural abnormalities, which at first suggested the diagnosis of longstanding advanced AS, but further evaluation led to the correct diagnosis of DISH. The observation of his heel revealed a diffuse swelling at the insertion of both Achilles tendons, resembling the typical fusiform soft tissue swelling of Achilles enthesitis of spondyloarthritis5 (Figure 1). However, palpation of the region did not reveal any inflammatory findings of enthesitis but did reveal bone proliferation due to large spurs, a condition confirmed by radiographs (Figure 2). A sacroiliac joint computed tomography (CT) scan showed the normal aspect of joint space and bony margins together with the presence of capsular ossifications (Figure 3).