Still’s disease and rheumatoid arthritis (RA) are two disorders that cause inflammation in the body. Both diseases can cause joint pain, joint swelling, and fever.

Still’s disease causes inflammation that can affect the entire body, resulting in:

RA commonly affects a person’s joints, also causing joint pain and swelling, and restricted movement across the joints.

Read on to learn more about Still’s disease and RA, including their causes, symptoms, diagnosis, and treatment.

Still’s disease is an inflammatory disorder affecting the joints. Experts refer to Still’s disease as adult-onset Still’s disease (AOSD).

Still’s disease is rare, occurring in 1–34 of every 1 million people. Most cases of the disease begin between ages 16 and 35, but people outside this age range can also develop the disease.

RA is another inflammatory disease. It occurs when a person’s immune system incorrectly attacks the lining of the joints, causing joint inflammation.

RA most commonly affects a person’s:

  • hands
  • knees
  • ankles

It often occurs in the same joint on both sides of the body. However, it may sometimes affect other body parts, causing problems in areas such as the:

  • eyes
  • heart
  • circulatory system
  • lungs

Approximately 1.5 million people in the United States have RA.

Females are three times more likely than males to develop the disease between the ages of 30 and 60. Experts consider it rare in males under 45.

Key similarities and differences

Still’s disease and RA are both inflammatory diseases. They cause inflammation in the body and share some symptoms, such as joint pain and fever.

However, these two diseases also have some key differences.

For example, Still’s disease is idiopathic, which means medical professionals do not know its cause.

In contrast, doctors know that RA is an autoimmune disease — it occurs when a person’s immune system mistakenly attacks healthy cells.

Some scientists believe that the immune system may also play a role in Still’s disease, though more research is necessary to determine whether this is true.

The causes of the two conditions tend to vary, but there may be some overlap. Keep reading to find out more.

Causes of Still’s disease

Medical professionals do not know the cause of Still’s disease.

Some scientists believe that Still’s disease may be a reactive syndrome, meaning that various infectious agents may trigger the symptoms of the disease in people who have a genetic predisposition to it.

These genetic factors, alongside environmental factors such as viruses and bacteria, may play a role in the disease. However, more research is necessary to determine whether this is the case.

Other researchers believe that Still’s disease is an autoinflammatory syndrome.

Autoinflammatory syndromes are disorders that occur as a result of an abnormality in the innate immune system. This is the body’s first line of defense. It cannot build memory and is nonspecific to individual pathogens.

This is not the same as an autoimmune disorder, which occurs when a person’s adaptive immune system mistakenly attacks their healthy tissues. The adaptive immune system is the body’s second line of defense and can build a memory to create cells that target specific pathogens.

Once again, researchers need to investigate further to determine whether Still’s disease is an autoinflammatory syndrome.

Causes of RA

RA is an autoimmune disease. When a person has RA, their immune system attacks the synovium — the tissue that lines the edges of their joints. The synovium produces a fluid that helps the joint move smoothly.

RA causes the synovium to become inflamed and thickened. As a result, the area around the joint swells and appears red. It may also feel painful, and a person may have difficulty moving the joint.

Researchers do not fully understand why people develop RA. As with Still’s disease, some researchers believe that certain environmental triggers can activate specific genes, causing RA to develop.

Suggested triggers include:

  • viruses
  • bacteria
  • physical stress
  • emotional stress

Doctors commonly associate certain features with each condition, but there are some clear similarities because both affect the joints. Learn more about the symptoms of each.

Symptoms of Still’s disease

The primary clinical symptoms of Still’s disease are:

These symptoms occur in 75–95% of people with Still’s disease.

Other common symptoms of Still’s disease include:

Less common symptoms of Still’s disease include:

Symptoms of RA

RA symptoms come and go. The symptoms appear during a flare, and a person may experience no symptoms at all during remission.

Flares can last for:

  • days
  • weeks
  • months

During a flare, the most common symptoms of RA are joint pain, stiffness, and swelling, especially in the morning.

Other symptoms of RA include:

  • swelling or tenderness in more than one joint
  • fever
  • fatigue

The diagnostic process may involve a range of tests and medical history checks. Read more about the differences below.

Diagnosing Still’s disease

It can be difficult for a doctor to diagnose Still’s disease because no specific test or laboratory outcome can differentiate it from similar conditions.

A doctor will often make a diagnosis by:

  • performing a thorough clinical evaluation
  • taking a detailed medical history
  • analyzing a person’s symptoms

Imaging and blood tests

Imaging and blood tests may be useful and may reveal:

  • changes in the bones or joints
  • enlargement of the spleen
  • enlargement of the liver

A doctor may also use an echocardiogram. This test uses sound waves to create a picture of the heart. An echocardiogram may reveal inflammation of the pericardium or myocardium, which may indicate Still’s disease.

Blood test results may also reveal certain signs that could lead to a diagnosis, such as:

  • high levels of white blood cells
  • high levels of platelets
  • low levels of red blood cells

However, these abnormalities are often present in various other conditions.

The Yamaguchi criteria

Some medical professionals may also use the Yamaguchi criteria to diagnose Still’s disease. For a doctor to diagnose Still’s disease this way, a person must meet at least five criteria, including at least two of the major criteria.

The criteria are below, broken down into major and minor.

Major criteria:

  • a fever of at least 102.2°F (39°C) that lasts at least 1 week
  • joint pain or arthritis that lasts at least 2 weeks
  • the appearance of a rash when the fever spikes
  • high levels of white blood cells

Minor criteria:

Diagnosing RA

Doctors will often use the following steps to diagnose RA:

  • medical history
  • physical examination
  • blood tests

Medical history

During the medical history check, the doctor may ask the person:

  • what symptoms they are having
  • when the symptoms began
  • whether symptoms come and go
  • how severe their symptoms are
  • what actions make their symptoms better or worse
  • whether any family members have RA or another autoimmune disease

Physical examination

During the physical examination, the doctor may look for:

  • joint tenderness, swelling, warmth, or pain
  • limited joint movement
  • bumps under the skin
  • a low grade fever

A doctor may also order blood tests to look for inflammation and blood proteins that may be a sign of RA.

Treatment is available for both conditions, and a person may be able to manage symptoms at home.

Treating Still’s disease

The goal of treatment for Still’s disease is to reduce symptoms and physical indications of inflammation.

Doctors may use nonsteroidal anti-inflammatory drugs (NSAIDs) to reduce inflammation.

Some examples of NSAIDs are:

Doctors may also use corticosteroids to treat some systemic symptoms. However, a person should not use corticosteroids long-term as they may cause serious side effects.

Biological response modifiers are another possible treatment. Medical professionals sometimes refer to these medications as immunomodulatory. These medications can block the activity of cytokines, which are proteins involved in the body’s inflammatory response.

Further research is necessary to provide more conclusive evidence on whether these medications effectively treat Still’s disease.

Treating RA

A person can effectively treat and manage RA with medications. These medications aim to slow down the disease and prevent joint deformity.

These medications are called disease-modifying antirheumatic drugs (DMARDs). They work by blocking inflammation. This can stop or slow the disease process in most inflammatory forms of arthritis, including RA.

A person with RA may also take biological response modifiers as a second-line treatment.

Additionally, self-management strategies may help a person reduce their RA symptoms. These can include:

  • being physically active
  • maintaining a moderate weight
  • avoiding activities that may cause joint injuries
  • seeing their doctor regularly to receive suitable diagnosis and treatment

Still’s disease causes inflammation in the body. Common symptoms include fever, rash, and joint pain.

RA also causes inflammation and affects a person’s joints. Symptoms include joint pain and stiffness, especially in the morning.

Doctors may treat both conditions. A person may treat Still’s disease with NSAIDs or biological response modifiers. Common treatments for RA include DMARDs and biological response modifiers.