Diffuse Idiopathic Skeletal Hyperostosis (DISH) and Ankylosing Spondylitis (AS) are chronic conditions that primarily affect the spine. While both disorders involve the formation of new bone that can lead to spinal fusion, they are fundamentally distinct diseases arising from different underlying processes. DISH, sometimes called Forestier’s disease, is a non-inflammatory disorder characterized by widespread ligament calcification. AS, conversely, is an autoimmune, inflammatory form of spondyloarthritis. The differences between these two conditions involve the core pathology, the patients they affect, and the resulting therapeutic strategies.
The Underlying Disease Mechanism
Ankylosing Spondylitis (AS) is a systemic, autoimmune disorder driven by chronic inflammation. This inflammation primarily targets the entheses, the sites where ligaments and tendons attach to bone, occurring mainly in the lower spine and sacroiliac (SI) joints. Over time, the inflammatory process leads to bone erosion, which is subsequently replaced by new bone formation, eventually causing spinal segments to fuse together in a process called ankylosis. This inflammatory cascade is closely associated with the immune system, involving cytokines such as Tumor Necrosis Factor-alpha (TNF-α) and Interleukin-17 (IL-17). The fusion results from the formation of thin, vertical bony bridges called syndesmophytes, which create the characteristic “bamboo spine” appearance in advanced stages.
Diffuse Idiopathic Skeletal Hyperostosis (DISH), conversely, is considered a non-inflammatory disorder defined by excessive new bone formation, or hyperostosis. This new bone forms along the anterior and lateral aspects of the spine through the ossification of the anterior longitudinal ligament and other surrounding soft tissues. The resulting bony bridges are thick, flowing, and often described as having the appearance of “dripping candle wax.” The ossification in DISH is often linked to metabolic factors, with a strong association observed in patients who have conditions such as obesity, type 2 diabetes mellitus, and hyperlipidemia.
Unlike AS, DISH typically spares the synovial joints of the spine, notably the facet joints and the true articular portion of the sacroiliac joints. The new bone growth in DISH is thought to be a response to various mechanical, metabolic, and growth factors, rather than a reaction to chronic immune-mediated inflammation.
Contrasting Symptoms and Demographics
Ankylosing Spondylitis typically manifests in young adulthood, with symptoms generally appearing before the age of 45, and shows a clear male predominance. The hallmark symptom is inflammatory back pain, which is worse with rest or inactivity, particularly at night and in the early morning. This morning stiffness often lasts for more than 30 minutes and improves significantly with physical activity and exercise. AS is frequently accompanied by extra-articular manifestations, indicating its systemic inflammatory nature.
These related conditions can include:
- Acute anterior uveitis (eye inflammation)
- Inflammatory bowel disease (IBD)
- Crohn’s disease or ulcerative colitis
- Psoriasis
Diffuse Idiopathic Skeletal Hyperostosis (DISH), by contrast, is a disorder of aging, with the onset typically occurring in older adults, usually after the age of 50. It also has a higher prevalence in men, affecting up to one-quarter of men over the age of 50. The pain and stiffness associated with DISH are generally mechanical, meaning they often worsen with activity and may not exhibit the prolonged morning stiffness characteristic of inflammation.
A significant portion of individuals with DISH remain asymptomatic, often discovered incidentally during imaging for other reasons. When symptoms do occur, they are generally related to the mechanical restriction of the spine, leading to stiffness, especially in the thoracic spine. A unique and concerning complication of DISH is dysphagia, or difficulty swallowing, which occurs when large osteophytes in the cervical spine compress the esophagus.
How Imaging and Lab Tests Differentiate the Conditions
Diagnosis of DISH is primarily based on radiographic findings that describe the non-inflammatory bone growth pattern. Imaging requires the presence of flowing calcification or ossification along the anterolateral aspect of at least four contiguous vertebral bodies. Crucially, these criteria also stipulate the preservation of intervertebral disc height and the absence of bony ankylosis or erosion in the sacroiliac and facet joints. The appearance of the bone bridging in DISH is thick and irregular, often lifting away from the vertebral body, and it most commonly affects the right side of the thoracic spine.
Laboratory markers of inflammation, such as the C-reactive protein (CRP) and Erythrocyte Sedimentation Rate (ESR), are typically within the normal range in individuals with DISH. The genetic marker HLA-B27, strongly associated with inflammatory spondyloarthritis, is not linked to DISH.
In contrast, Ankylosing Spondylitis (AS) diagnosis is supported by findings of inflammation and bony changes in the sacroiliac joints and spine. Imaging, often through X-ray or MRI, reveals sacroiliitis, which is inflammation and eventual fusion of the SI joints. The syndesmophytes in AS are thin and vertical, rising from the edge of the vertebral body and giving the spine a smooth, classic “bamboo” appearance in advanced stages.
Laboratory testing holds significant weight in diagnosing AS, particularly the testing for the genetic marker Human Leukocyte Antigen B27 (HLA-B27). While not strictly diagnostic, the HLA-B27 gene is present in 80% to 90% of individuals with AS. Additionally, inflammation in active AS often results in elevated levels of acute-phase reactants like CRP and ESR, which help confirm the underlying inflammatory nature of the disease.
Treatment and Long-Term Management
Since the pathology of Ankylosing Spondylitis (AS) is driven by chronic inflammation, the primary goal of treatment is to suppress the immune response and halt the progression of joint damage. Nonsteroidal anti-inflammatory drugs (NSAIDs) are a first-line therapy for pain and stiffness, with many patients experiencing significant relief. For more severe or progressive disease, targeted biologic therapies are used to block specific inflammatory pathways.
These advanced treatments include TNF-alpha inhibitors and Interleukin-17 (IL-17) inhibitors, which effectively reduce systemic inflammation and disease activity. Physical therapy and exercise are also major components of long-term management, aiming to maintain spinal mobility and prevent the irreversible fusion of the vertebrae. Surgical intervention is generally reserved for correcting severe spinal deformity or managing unstable fractures that can occur in the fused, rigid spine.
The management of Diffuse Idiopathic Skeletal Hyperostosis (DISH) is focused on addressing symptoms, as there is no therapy that can reverse or halt the non-inflammatory bone growth. Pain relief is typically achieved through the use of over-the-counter pain relievers or NSAIDs, although the response to anti-inflammatory medications may be less pronounced than in AS. Physical therapy is recommended to help maintain flexibility and range of motion, which can mitigate the stiffness caused by the ossification.
Surgery in DISH is only considered when the excessive bone growth leads to severe complications. This includes surgical removal of osteophytes that cause dysphagia by compressing the esophagus. Surgery may also be necessary to stabilize the spine following a fracture, as the rigid, fused spine is highly susceptible to unstable fractures even from low-impact trauma. Furthermore, managing associated metabolic conditions, such as diabetes and obesity, is an important part of the long-term care plan for DISH.