Diffuse Idiopathic Skeletal Hyperostosis (DISH) and Ankylosing Spondylitis (AS) are conditions affecting the spine and other joints, often causing stiffness and pain. While symptoms may overlap, their underlying causes and appearance on medical imaging are distinct. Radiography is central to differentiating these conditions for accurate diagnosis and management.
Understanding Diffuse Idiopathic Skeletal Hyperostosis (DISH)
Diffuse Idiopathic Skeletal Hyperostosis, commonly known as DISH, is a non-inflammatory condition characterized by the ossification, or hardening into bone, of ligaments and tendons, primarily along the spine. This ossification typically involves the anterior longitudinal ligament, which runs down the front of the vertebral bodies. While many individuals with DISH are asymptomatic, common symptoms can include spinal stiffness and pain, often in the back.
A hallmark radiological feature of DISH is the “flowing” ossification along the anterolateral aspect of at least four contiguous vertebrae. This appearance is often described as resembling “melted wax” or “dripping candle wax”. Another important finding is the preservation of intervertebral disc height in the affected segments, with a relative absence of significant degenerative changes within the discs themselves. DISH does not involve erosion or fusion of the sacroiliac joints, which helps distinguish it from other conditions. Beyond the spine, ossification can also occur at other entheses, which are sites where tendons and ligaments attach to bone, such as in the heels, elbows, or shoulders.
Understanding Ankylosing Spondylitis (AS)
Ankylosing Spondylitis, or AS, is a chronic inflammatory condition that primarily affects the spine and the sacroiliac joints. Individuals with AS often experience chronic back pain and stiffness, which tends to be worse in the morning and improves with exercise. This inflammatory process can lead to progressive stiffness and reduced mobility over time.
Radiological assessment is important in diagnosing AS, revealing characteristic changes. A primary finding is sacroiliitis, which involves inflammation, erosion, and eventual fusion of the sacroiliac joints. In the spine, AS is characterized by the formation of syndesmophytes, which are thin, vertical bony growths that bridge adjacent vertebral bodies. These syndesmophytes originate from the corners of the vertebral bodies. In advanced stages, widespread syndesmophyte formation can lead to a complete spinal fusion, resulting in the distinctive “bamboo spine” appearance on radiographs. Other spinal findings include erosions and squaring of the vertebral bodies, particularly at the anterior corners, known as Romanus lesions.
Key Radiological Distinctions
Differentiating between DISH and AS radiologically relies on several key distinctions in the nature and distribution of ossification and joint involvement.
Ossification Type and Distribution
The type of new bone formation is a primary discriminator. In DISH, the ossification is “flowing,” often appearing thick and coarse, running along the anterior longitudinal ligament and involving at least four contiguous vertebrae. This non-marginal ossification can resemble “candle wax” dripping down the spine. Conversely, AS presents with discrete, vertical bony growths called syndesmophytes, which are thin and delicate, bridging the corners of vertebral bodies. Over time, these can lead to the characteristic “bamboo spine” due to widespread spinal fusion.
Sacroiliac Joint Involvement
The sacroiliac joints provide another important differentiating factor. In DISH, there is an absence of sacroiliac joint erosion or fusion, maintaining their normal appearance. In contrast, sacroiliitis, characterized by inflammation, erosion, and eventual fusion of the sacroiliac joints, is often the earliest manifestation of AS. While some bony bridging around the sacroiliac joint may be seen in DISH, complete fusion is more common in AS.
Disc Spaces and Facet Joints
Regarding the disc spaces, DISH shows preservation of intervertebral disc height, indicating that the discs themselves are spared from significant degenerative changes. In AS, however, inflammatory changes can lead to erosions at the vertebral corners and eventual narrowing or complete fusion across the disc spaces. The apophyseal or facet joints, which are small joints connecting the vertebrae, are usually spared in DISH. In AS, these joints can be affected by inflammation and subsequent fusion, contributing to spinal rigidity.
Enthesopathy and Laterality
Enthesopathy, the involvement of tendon and ligament attachment sites, also differs. DISH shows robust enthesopathy in peripheral areas like the heels, elbows, and shoulders, representing non-inflammatory new bone formation. AS, on the other hand, is characterized by inflammatory enthesitis, which can lead to erosions and new bone formation at entheses. Finally, DISH often affects the right side of the thoracic spine more frequently than the left, a phenomenon thought to be due to the pulsating aorta on the left side preventing ossification. AS exhibits more symmetrical involvement of the spine and sacroiliac joints.
Why Accurate Diagnosis Matters
Accurately distinguishing between DISH and AS holds importance for patient care and long-term prognosis. A misdiagnosis can lead to inappropriate treatment or delayed care. AS, an inflammatory condition, requires specific anti-inflammatory treatments, including nonsteroidal anti-inflammatory drugs (NSAIDs) and, in severe cases, biologic medications.
In contrast, DISH is a non-inflammatory condition, and its management focuses on symptom relief, such as pain management and physical therapy to maintain mobility. The long-term implications and potential complications also differ. AS can lead to increased risk of spinal fractures due to the fused and brittle nature of the spine, even from minor trauma. DISH can cause complications like dysphagia (difficulty swallowing) if large anterior osteophytes develop in the cervical spine. Therefore, precise radiological diagnosis is important in clinical decision-making, ensuring patients receive the most appropriate and effective management for their specific condition.