Transient synovitis and septic arthritis both affect the legs or hips and typically occur in children. One resolves on its own, but the other can be life threatening.

Transient synovitis, also called irritable hip or toxic synovitis, involves inflammation of the lining of the hip joint. It usually occurs in children. It often goes away within a couple of weeks.

Septic arthritis is an infection that develops in joint tissue and fluid. It also most commonly affects children but is a more severe condition that can be life threatening.

Read on to learn more about how to differentiate these two conditions.

Transient synovitisSeptic arthritis
Most common joint affectedhipknee and other large joints
Peak age3–10 years2–3 years
Severityusually goes away on its own without complicationscan be life threatening

Here’s a look at the symptoms of transient synovitis and septic arthritis.

Transient synovitis symptoms

Symptoms of transient synovitis generally improve within 24–48 hours. They completely resolve within 1–2 weeks in about 75% of cases.

Symptoms include:

  • hip pain on one side
  • subtle limp
  • refusal to bear weight
  • restricted range of motion
  • in babies, increased agitation or more frequent crying

Transient synovitis seems to frequently occur after respiratory infections. Before hip pain develops, your child may have symptoms like:

Septic arthritis symptoms

Septic arthritis usually develops in one large joint, like the knee or hip. It may develop in multiple small joints. About 5–10% of people have symptoms in multiple joints.

Symptoms can include:

Here are the causes of transient synovitis and septic arthritis.

Transient synovitis causes

The cause of transient synovitis is not known. However, many children have a respiratory infection or recent trauma to their hip before it develops. It most frequently occurs in children 3–10 years old.

Researchers continue to investigate potential viral causes, like parvovirus B19 and human herpesvirus 6.

Septic arthritis causes

Septic arthritis is a serious joint infection that most commonly occurs from bacteria. Viruses and fungi can also cause it.

Septic arthritis is most common in children, although it can occur at any age. It develops in 2–6 people per 100,000 per year.

Septic arthritis may develop after:

  • injury to a joint
  • microorganisms spread into your blood and travel to your joint
  • joint surgery

Some factors can increase the risk of septic arthritis. They include:

Reach out to your child’s doctor right away if they have leg pain or start limping. Many different conditions can cause these symptoms, and some, like septic arthritis or a broken bone, can be serious and need prompt treatment.

Generally, a doctor starts the diagnostic process by:

  • asking about symptoms
  • reviewing personal and family medical history
  • performing a physical exam

If they suspect septic arthritis, the next step is often arthrocentesis. This test takes a sample of joint fluid for lab analysis.

Your doctor may also order blood tests, such as:

They may also order imaging tests to look at the extent of joint damage and help rule out other conditions. These imaging tests may include:

To make a diagnosis of transient synovitis, a doctor rules out other conditions. Many of the same tests for septic arthritis may be used.

Here’s a look at the treatment options for transient synovitis and septic arthritis.

Transient synovitis treatment

Transient synovitis usually resolves itself within a couple of weeks without complications. However, in about 20–25% of people, it comes back.

Treatment options include:

Septic arthritis treatment

Prompt treatment is needed to limit joint damage and reduce the risk of death. Mortality rates are as high as 7–15%.

The first treatment for septic arthritis caused by bacteria is antibiotics. Doctors may prescribe antifungal medications if a fungus is the underlying cause.

Antibiotics are often delivered intravenously (through an IV), but you may also receive oral antibiotics.

Doctors may drain fluid from your affected joint to ease pain and swelling.

Surgery is often needed to remove damaged sections of the joint after the infection is treated.

Steps you can take to prevent septic arthritis include:

Since the cause of transient synovitis is not known, it is not clear how to prevent it.

Here are some frequently asked questions people have about transient synovitis and septic arthritis.

Is synovitis the same as septic arthritis?

“Synovitis” refers to inflammation and swelling of a joint’s lining. Septic arthritis is a joint infection.

How do you differentiate between septic arthritis and osteomyelitis?

Osteomyelitis is an infection of a bone. Septic arthritis is an infection of joint tissue and fluid.

Septic arthritis usually causes pain in the knee. Osteomyelitis most commonly develops in the spine.

Transient synovitis commonly develops in children and causes sudden pain in one hip. Septic arthritis is also more common in children but can develop at any age. It can cause pain in your knee, hip, or other joints.

Transient synovitis usually goes away on its own, but septic arthritis can be life threatening. It’s important to visit a doctor right away if you develop potential symptoms of septic arthritis, like severe joint pain and a fever.