Diffuse idiopathic skeletal hyperostosis (DISH) and ankylosing spondylitis (AS) both affect the spine, but they’re different conditions — and their differences often show up on an X-ray. DISH, also called Forestier’s disease or ankylosing hyperostosis, is a common form of arthritis that mainly affects the thoracic spine (upper back). AS, on the other hand, is a type of spondyloarthritis, a form of arthritis that causes inflammation in the spine and nearby joints.

The exact cause of AS isn’t fully understood, but it’s known to be an autoimmune disease, meaning the immune system mistakenly attacks healthy tissues. A specific gene, HLA-B27, is linked to a higher risk of AS, but not everyone with this gene develops the condition, and some people with AS don’t have it. Still, testing for HLA-B27 can help doctors diagnose AS. AS mainly affects the lower spine, including the sacroiliac joints, which connect the spine to the pelvis and help with movement.

Because these two conditions have similarities, it’s easy to confuse them. “I was diagnosed a year ago with ankylosing spondylitis,” one MySpondylitisTeam member said. “I recently moved and have a new rheumatologist who thinks I actually have DISH — the X-rays and symptoms support the new diagnosis.”

Here’s what to know about these two conditions.

What Is the Difference Between Ankylosing Spondylitis and DISH?

Unlike AS, DISH isn’t a form of inflammatory arthritis. But DISH is sometimes misdiagnosed as AS because the conditions share similar signs and symptoms. The usual age of onset for AS is between 15 and 30 years old. DISH usually develops after 50. StatPearls, a medical resource that provides reviewed health information, reports that DISH affects about 15 percent of women and 25 percent of men over the age of 50. But less than 1 percent of the general population is living with AS.

According to BMC Rheumatology, the prevalence (how common a condition is in a group of people) of AS is increasing in women, while DISH remains twice as likely to affect men. Keep in mind, however, that prevalence rates may differ by study and location.

DISH Signs, Symptoms, and Complications

DISH is a type of osteoarthritis that causes calcification (abnormal calcium buildup). Calcification in DISH affects the ligaments and connections between the vertebrae (bones in the spine) called the entheses, the connective tissues where tendons and ligaments attach to bones.

The buildup of calcium causes bone spurs (growths of bone) along the edges of the vertebrae, most commonly in the cervical spine (neck). In some cases, severe ossification in the front of the spine (anterior longitudinal ligament) can lead to dysphagia (difficulty swallowing).

Spinal stiffness and pain are typical symptoms of DISH. These symptoms can also affect other areas, like the elbows, knees, heels, and shoulders. Morning stiffness and a reduced range of motion, particularly in the back, are also common.

Ankylosing Spondylitis Signs, Symptoms, and Complications

AS also affects the entheses of the spine. When AS inflammation erodes the connective tissues and bone, the immune system scrambles to repair it with new bone formation. This is called syndesmophytes (a type of bony growth in a ligament). These growths can lead to fusion of the spine, called ankylosis.

AS usually causes pain, inflammation, and stiffness in the buttocks and lower back. In some cases, AS affects the spine in the neck and upper back, also causing cervical myelopathy. Up to 62 percent of people living with AS will also develop osteoporosis, a condition that weakens bones and makes them more prone to fractures, especially in the spine.

AS pain, inflammation, and reduced range of motion can affect many areas of the body. This includes the hips, shoulders, and some peripheral joints (joints that are farther from the center of the body). In some cases, AS can also cause signs and symptoms in the gastrointestinal tract, eyes, or skin.

Differential Diagnosis: DISH or AS?

AS and DISH have overlapping symptoms, which can sometimes lead to misdiagnosis. However, they are different conditions with distinct causes and progression. When both diseases are in advanced stages, they have a lot in common. AS and DISH are both forms of arthritis that can produce new bone formation in the spine. Both conditions can involve the tendons, ligaments, and entheses. They can also both cause back pain, a reduced range of motion, myelopathies, and morning stiffness.

People usually notice AS symptoms before doctors can see signs of the disease on X-rays or other imaging tests. But DISH is different — it often doesn’t cause symptoms early on. In many cases, doctors find DISH by accident when an X-ray taken for another reason shows changes in the spine.

Keep reading to find out how doctors tell these two conditions apart.

Medical History: Risk Factors for DISH

Your family and personal medical history can be helpful to differentiate DISH from AS. The cause of DISH is still unknown, but advanced age and obesity are risk factors. A diagnosis of DISH is also more likely if you have a medical history of metabolic disorders such as diabetes. The use of certain acne medications, such as isotretinoin (Claravis), is also known to increase the risk of developing DISH.

Risk Factors for Ankylosing Spondylitis

One major risk factor for AS is the HLA-B27 gene, though not everyone with AS has it, and not everyone with the gene develops AS. A diagnosis of AS is also more likely when a person has a family history of the condition.

Physical Exam and Blood Tests

Physical examinations, radiographic imaging, and blood tests are all part of diagnosing AS and DISH. A rheumatologist will typically perform a physical exam to check for decreased range of motion and other abnormalities. They’ll usually conduct blood tests to look for inflammatory markers in the blood to support a diagnosis of AS.

X-Ray Images

The pelvic and spinal abnormalities that appear on X-ray images differ between AS and DISH. New bone formations may show up in similar locations on images of both conditions. The following are some differences between DISH vs. AS on X-ray:

• In people with DISH, the discs in the spine keep their normal height and space. But in AS, bone growth causes the discs to become more squared in shape.
• In AS, vertebrae fuse through the middle. In DISH, the connection develops around the outside of the joint.
• Changes in the sacroiliac joints — such as inflammation, hardening, wear and tear, or fusion — can help doctors diagnose AS.
• Flowing ossification at four or more vertebral levels usually appears on X-ray images of DISH.
• Bone changes look like dripping candle wax in those with DISH, while they’re smoother in people with AS.

Thanks to X-ray imaging, DISH and AS can be identified separately, even if someone has both conditions. Another important tool is MRI, which can be used to look for early changes in sacroiliac joints.

Treatments for DISH and AS

Early diagnosis and treatment are key to maintaining a relatively normal and active lifestyle with DISH and AS.

Because these conditions are so different, it’s important to get a personalized treatment recommendation from your healthcare team. They’ll work with you to determine which treatment or combination of treatments will best manage your condition.

Physical and Occupational Therapy

Physical therapy, stretching, and exercise can help preserve range of motion, reduce pain, and improve strength and balance. Occupational therapy and prescribed technical supports such as a brace may also help someone with AS or DISH perform everyday tasks.

Medications

There are many medications available to treat AS. Over-the-counter nonsteroidal anti-inflammatory drugs (NSAIDs) can help manage pain and stiffness from both conditions.

For both conditions, corticosteroid injections are also helpful for reducing pain in the short term.

Diet

There isn’t one diet recommended for people with DISH or AS. Some foods can make symptoms worse for certain people, so it’s important to adjust your diet based on what works best for you. The Mediterranean diet is considered a helpful anti-inflammatory diet for people living with inflammatory arthritis such as AS. Part of managing DISH involves maintaining a healthy weight and blood sugar level through a healthy diet.

Hot and Cold Therapy

Both heat and cold can help ease pain, stiffness, and swelling associated with DISH and AS. Heat helps with pain and tension by loosening the muscles around the joints and increasing flexibility and circulation, while cold packs may reduce swelling and pain. Cleveland Clinic recommends using heat therapy for 20 minutes, whether through a warm bath, heating pad, or a moist towel heated in the microwave.

Stress Reduction

Meditation, mindfulness, and other stress management practices may help keep stress levels low. This may help prevent symptoms from worsening during flares.

Surgery

Surgery is not often needed for AS or DISH. You may need surgery if DISH causes problems with the structure of your spine. Your rheumatology team can advise you on whether surgery is the right option for you.

Find Your Team

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Do you have DISH or ankylosing spondylitis? How did your doctor differentiate DISH vs. AS? Share your story in the comments below or by posting on your Activities page.