DISH vs Ankylosing Spondylitis Radiology: Key Observations

Radiology plays a crucial role in distinguishing between Diffuse Idiopathic Skeletal Hyperostosis (DISH) and Ankylosing Spondylitis, two conditions that affect the spine differently. Accurate differentiation is essential for proper diagnosis and treatment, as both disorders significantly impact spinal function and patient quality of life.

Spinal Changes In DISH

Diffuse Idiopathic Skeletal Hyperostosis is characterized by unique spinal changes. Radiologically, DISH is identified by flowing calcifications and ossifications along the anterolateral aspects of at least four contiguous vertebral bodies, typically in the thoracic spine. This pattern, often described as “candle wax” drippings, is distinct because the intervertebral disc spaces and facet joints remain unaffected. The ossification of the anterior longitudinal ligament leads to these characteristic bony bridges, without the inflammation seen in other spinal disorders. The absence of sacroiliac joint involvement further differentiates DISH from other spondyloarthropathies. Studies indicate the prevalence of DISH increases with age, affecting up to 25% of individuals over 50, with a higher incidence in males.

Clinically, patients with DISH may experience stiffness and reduced spinal mobility, particularly in the thoracic region. The condition is often asymptomatic and discovered incidentally during imaging for other reasons. When symptoms occur, they include mild pain and discomfort, generally less severe than in inflammatory spinal conditions. A systematic review noted that while DISH can lead to significant spinal rigidity, it rarely results in severe deformities seen in other spinal disorders.

Spinal Changes In Ankylosing Spondylitis

Ankylosing Spondylitis (AS) manifests distinct spinal changes observable through radiological examination. These changes primarily involve the sacroiliac joints and the axial spine, with syndesmophytes being a hallmark feature. Syndesmophytes are thin, vertically oriented bony growths that contribute to the “bamboo spine” appearance in advanced cases, due to the fusion of vertebrae. Radiographic evidence is crucial for diagnosis and assessment of AS. Early in the disease, X-rays may reveal erosions and sclerosis at the sacroiliac joints, progressing to complete ankylosis. This pattern of joint involvement is a critical differentiator from DISH. Magnetic Resonance Imaging (MRI) is used to detect early inflammatory changes before they appear on X-rays. Studies emphasize the utility of MRI in identifying active inflammation, integral to early diagnosis and management of AS.

The progression of spinal changes in AS is influenced by genetic predispositions, notably the HLA-B27 antigen. This genetic marker is strongly associated with an increased risk of developing AS, guiding clinicians in their diagnostic approach. The severity of spinal fusion and the rate of progression vary among individuals, necessitating personalized monitoring and treatment. Clinical trials demonstrate that biologic therapies, particularly tumor necrosis factor (TNF) inhibitors, can significantly reduce inflammation and slow structural damage progression. These insights underline the importance of integrating radiological assessments with clinical and genetic data to optimize patient outcomes.

Sacroiliac Joint Patterns

The sacroiliac joints are integral in distinguishing between different spondyloarthropathies, particularly DISH and AS. Radiologically, the sacroiliac joints in AS present early signs of sacroiliitis, characterized by irregular joint space, subchondral sclerosis, and erosions, leading to complete ankylosis. In contrast, DISH spares these joints, maintaining their normal appearance on imaging studies, serving as a pivotal diagnostic criterion.

Advanced imaging techniques, such as MRI, enhance the ability to detect sacroiliitis in its nascent stages, identifying bone marrow edema and synovitis before structural changes become evident on X-rays. Early detection is vital for timely intervention, as studies have shown that early treatment can significantly slow disease progression and improve patient outcomes. MRI findings of active inflammation can guide the use of targeted therapies, effective in reducing symptoms and preventing further joint damage.

The sacroiliac joint patterns also reveal the chronic nature of AS, with progression linked to genetic predispositions and environmental factors. The presence of the HLA-B27 antigen is a strong predictor of sacroiliac joint involvement and disease severity in AS patients, underscoring the genetic divergence between the two conditions. Understanding these patterns allows clinicians to tailor their diagnostic approach, incorporating genetic testing alongside radiological assessments.

Ligament And Enthesis Observations

The involvement of ligaments and entheses offers a distinct perspective in differentiating DISH from AS. In DISH, ossification of the anterior longitudinal ligament is a defining feature, with radiographs displaying flowing calcifications spanning several vertebrae. This process does not involve inflammation, setting it apart from AS, where enthesitis, or inflammation at ligament and tendon attachment sites, is prominent.

In AS, enthesitis contributes significantly to pain and structural changes. The inflammation at these attachment sites can lead to new bone formation, resulting in the fusion of spinal segments. This process is not limited to the spine; peripheral joints can also be affected, further distinguishing AS from DISH. Enthesitis in AS can often be detected before radiographic changes are evident, highlighting the importance of clinical evaluation and early intervention.