What is toxic synovitis

Toxic synovitis is also called transient synovitis, which is an acute non-specific, temporary inflammatory process affecting the hip joint synovium (lining of your joints) that can cause limping and pain in the hip and leg ¹. Toxic synovitis usually goes away within a week or two, and causes no long-term problems. Toxic synovitis cause is not known. Toxic synovitis of the hip is a common cause of hip pain in children aged 3-10 years or in children before puberty. Boys are affected more often than girls. A 2010 study from the Netherlands reported the mean age at presentation was 4.7 years ². While the majority of cases occur in pediatric patients between the ages of 3 and 10 years of age, medical literature does demonstrate rare case presentations in both younger infants and the adult population ³. The incidence rate in males is twice that of females, and about 1% to 4% of the time a patient may demonstrate bilateral involvement ⁴.

Toxic synovitis causes hip arthralgia (hip joint pain) and arthritis secondary to a transient inflammation of the synovium of the hip. While toxic synovitis is a benign, self-limiting process, doctors must recognize the critical importance of differentiating toxic synovitis from an acute infectious process. Ultrasonography demonstrates an effusion that causes bulging of the anterior joint capsule. Synovial fluid has increased proteoglycans.

Toxic synovitis treatment often includes limiting activity to make the child more comfortable. But, there is no danger with normal activities. Your health care provider may prescribe nonsteroidal anti-inflammatory drugs (NSAIDS) to reduce pain. The hip pain goes away within 7 to 10 days.

Toxic synovitis key points:

• Toxic synovitis is a self-limited inflammatory joint condition.
• Differentiating from septic arthritis is the primary concern.
• No single laboratory or imaging study will be definitive to confirm or exclude transient synovitis.

Is toxic synovitis contagious?

The exact cause of toxic synovitis cause is unknown. Toxic synovitis goes away on its own. There are no expected long-term complications.

Toxic synovitis vs Septic arthritis

Septic arthritis is an infection of a joint due to bacteria and other organisms that enter the bloodstream. The infection may affect one or more joints, resulting in serious inflammation, pain and damage to the tissues in and around the joint. Usually the knees are affected, however the hips, ankle, wrist, elbows and shoulder may also be affected.

Young children and older adults are most likely to develop septic arthritis. Strong antibiotics are needed to treat the infection and limit damage. Frequently, surgical drainage (via arthroscopy) is required to remove pus.

Prompt treatment with antibiotics usually resolves the infection. If treatment is delayed, however, the infection can quickly lead to joint degeneration — usually within five to seven days — and permanent damage.

A significant complication of septic arthritis is osteoarthritis and joint deformity. In severe cases, the joint may need to be surgically reconstructed. If the infection affects a prosthetic joint, the prosthetic joint may need to be replaced.

Other complications include:

• Spread of the infection to the adjacent bone or to other sites in the body

Septic arthritis causes

Septic arthritis may develop when an infection elsewhere in your body, like a throat or urinary infection, spreads through your bloodstream to a joint. Less commonly, a puncture wound, drug injection or surgery near a joint allows bacteria to penetrate beneath the skin.

The lining of your joints (synovium) has little to protect itself from infection. Once bacteria reach the synovium, they enter easily and can begin destroying cartilage, the smooth lining that protects and lubricates the bones. Your body’s aggressive reaction to clear the infection contributes to the damage around the joint.

In most cases, septic arthritis is caused by a combination of a weakened immune system plus an entry point for the infection (this may be surgery or injury).

Types of bacteria

A number of strains of bacteria can cause septic arthritis. The most common type involved in septic arthritis is Staphylococcus Aureus (also known simply as ‘staph’ or ‘golden staph’) — a type of bacteria commonly found on your skin, in your nose and in the environment.

In the past, septic arthritis was more frequently caused by the bacterium that causes gonorrhea. Use of safer gender practices has led to a decline in gonorrhea and its complications, including septic arthritis. Still, in sexually active people gonorrhea is a potential cause of septic arthritis.

Some other organisms that cause septic arthritis:

• Streptococcus

Other infectious causes of arthritis

Bacteria are just one cause of joint infections. Viruses also can attack joints (viral arthritis), though this condition usually resolves on its own and causes little joint damage. In rare cases, joint infections can be caused by a fungus (fungal arthritis).

Another infectious type of arthritis is reactive arthritis, which causes joint pain in response to an infection in another part of the body, though the joint itself isn’t infected.

Risk factors for septic arthritis include:

• Existing joint problems. Diseases and conditions that affect the joints — including other types of arthritis, gout & pseudogout, lupus — increase the risk of septic arthritis. An artificial (prosthetic) joint, previous joint surgery and joint injury also increase the risk.
  • Septic arthritis is one of the complications of orthopaedic joint surgery. Doctors will prescribe antibiotics at the time of the surgery to reduce the risk, however in some cases the infection will still occur.
• Taking medications for rheumatoid arthritis. Sufferers of rheumatoid arthritis have a further increase in risk of infection. The medications used to treat rheumatoid, including steroids and methotrexate, suppress the immune system, reducing the body’s capacity to contain infection. In addition, diagnosing septic arthritis in people with rheumatoid arthritis is difficult because many of the signs and symptoms are masked by the medications and their underlying pain.
• Skin fragility. Diseases and conditions that make skin wounds common give bacteria easy access to your body. This includes:
  • Ulcers
  • Psoriasis
  • Eczema and other causes of dry, itchy skin
  • Injectable drugs puncture the skin and consequently increase the risk of infection.
• Weak immune system. People with weakened immune systems, including those with diabetes, kidney and liver problems, as well as the elderly and very young, are at higher risk of septic arthritis. Drugs that suppress the immune system, like steriods and chemotherapy, also carry this risk.

Having a combination of risk factors usually puts you at a greater risk than having just one risk factor.

Toxic synovitis causes

Toxic synovitis (transient synovitis) is a common cause of hip pain and limping in children. Doctors don’t know its exact cause, but some kids develop it after having a viral infection such as a cold or diarrhea. Because of this, some doctors think that toxic synovitis is caused by substances made by the body’s immune system to fight the infection.

Medical literature demonstrates multiple proposed etiologic theories but none of these postulated hypotheses have been conclusively substantiated. Proposed risk factors include but are not limited to ⁵:

• preceding upper respiratory infection
• preceding bacterial infection e.g.,  poststreptococcal toxic synovitis
• preceding trauma

Toxic synovitis can happen at any age, but is most common in kids between 3 and 8 years old. It’s also more common in boys.

Many pediatric patients will present with a history of preceding upper respiratory infection symptoms, or in the setting of recent trauma. According to Kastrissianakis and Beattie ⁶, patients diagnosed with toxic synovitis are more likely to have experienced preceding viral symptoms including vomiting, diarrhea, or common cold symptoms. An earlier study reported that patients with toxic synovitis demonstrated higher serum interferon concentration values ⁷. Seasonal variation in association with toxic synovitis diagnoses remains controversial. One study reported a seasonal variation in the incidence of toxic synovitis, with more cases presenting in October and fewer cases in February ⁸. Studies investigating possible viral pathogen candidates, including parvovirus B-19 and human herpes simplex virus-6, have not been conclusive ⁹.

Other hypothesized risk factors include postvaccine or drug-mediated hypersensitivity reactions or certain allergic predispositions. Another potential clinical association has been proposed for Legg-Calvé-Perthes disease and toxic synovitis. While this relationship remains controversial, some studies have reported increased incidence rates of Legg-Calvé-Perthes disease following toxic synovitis (up to 3%) compared to the relative Legg-Calvé-Perthes disease incidence rate reporting in the general population (0.9 per 100,000 patients) ¹⁰.

Sometimes toxic synovitis can be confused with septic arthritis, or infectious arthritis, a more serious condition caused by a bacterial infection that invades the joints and can cause long-term joint damage. Doctors can rule out septic arthritis or other conditions through a physical exam and diagnostic tests.

Toxic synovitis symptoms

When children have toxic synovitis, the pain starts suddenly (usually following a recent viral infection, such as a cold or stomach virus) and usually is only on one side of the body.

Toxic synovitis symptoms may include:

• Hip pain (on one side only)
• Limp, limping with toes turned outward, the knee bent or straightened
• Thigh pain, in front and toward the middle of the thigh
• Knee pain
• Low-grade fever, less than 101 °F (38 °C)
• Walking on tiptoes
• Hip discomfort after a long period of resting the joint (such as sitting in a car or at a desk, or watching TV)
• Knee or thigh pain with no hip pain
• In younger children, crying
• In some cases, a refusal to walk

In babies, the most common signs of toxic synovitis are abnormal crawling and crying, and they may cry when their hip joints are being moved, such as during diaper changes.

Aside from the hip discomfort, the child does not usually appear ill.

Toxic synovitis diagnosis

Toxic synovitis is diagnosed when other more serious conditions have been ruled out, such as:

• Septic hip (infection of the hip)
• Slipped capital femoral epiphysis (separation of the ball of the hip joint from the thigh bone, or femur)
• Legg-Calve-Perthes disease (disorder that occurs when the ball of the thigh bone in the hip does not get enough blood, causing the bone to die)

The first thing a doctor will do is examine your child, checking to see what kind of movement is painful by moving the knee, the hip, and other joints. This is to confirm that the limping is caused by joint pain.

Next, the doctor may order an ultrasound of the hip. This imaging test will show whether there is fluid in the hip joint. Fluid means there is swelling and inflammation of the tissues. The doctor also may do blood tests to see how much swelling there is.

Sometimes, the blood test results make it hard for the doctor to rule out septic arthritis, so fluid might be taken from the joint for a culture (a lab test to detect bacteria). The doctor also may do other tests and X-rays to look for or rule out other potential causes of your child’s limping and pain.

Tests used to diagnose toxic synovitis include:

• Ultrasound of the hip
• X-ray of the hip
• ESR
• C-reactive protein (CRP)
• Complete blood count (CBC)

Other tests that may be done to rule out other causes of hip pain:

• Aspiration of fluid from the hip joint
• Bone scan
• MRI

A 2017 systematic review and meta-analysis ¹¹ highlighted demographic, clinical, and laboratory variables seen in pediatric patients presenting with toxic synovitis, septic arthritis or lyme arthritis of the hip. The authors noted several key findings that can aid in the clinical differentiation:

Febrile at presentation

• Over 50% of patients with septic arthritis
• 30% of patients with toxic synovitis
• 23% of patients with Lyme arthritis

Refusal to bear weight

• Over 60% in patients diagnosed with either toxic synovitis or septic arthritis
• Only 33% of patients diagnosed with Lyme arthritis

Inflammatory markers

• erythrocyte sedimentation rate (ESR) range for septic arthritis patients was 44 – 64 mm/hr
• erythrocyte sedimentation rate (ESR) range for toxic synovitis patients was 21 – 33 mm/hr
• erythrocyte sedimentation rate (ESR) range for Lyme arthritis patients was 37 – 46 mm/hr

Synovial fluid aspiration results

Synovial white blood cell count counts (cells/mm3) demonstrated a similar trend as noted with measured erythrocyte sedimentation rate (ESR) levels at presentation

• Toxic synovitis (5,644 – 15,388)
• Lyme arthritis (47,533 – 64,242)
• Septic arthritis (105,432 – 260,214)

Peripheral white blood cell count count was similar between each of the diagnostic groups

Toxic synovitis remains a diagnosis of exclusion, although these studies have highlighted the diagnostic utility of a synovial fluid aspiration and analysis.

Additional laboratory workup includes a C-reactive protein (CRP) greater than 2 mg/dl, which has been shown to be an independent risk factor for septic arthritis. A urinalysis and culture are typically normal. Because procalcitonin levels remain low during bouts of inflammatory disease, an increase should raise suspicion of septic arthritis. Depending on the history, consider antinuclear antibody, rheumatoid factor, HLA-B27, and tuberculosis skin testing ¹².

In a Lyme endemic area, only 5% of children with acute, nontraumatic hip pain had a Lyme infection, so routine serology is not necessary. It should be performed if an alternative diagnosis such as septic/pyogenic arthritis is being considered and in those with an atypical clinical course ¹³.

Although plain films may be normal for months after onset of symptoms, the medial joint space is typically slightly wider in the affected hip indicating the presence of fluid. One-half to two-thirds of patients with transient synovitis may have an accentuated pericapsular shadow ¹⁴.

Ultrasound is extremely accurate for detecting an intracapsular effusion. Ultrasound-guided hip aspiration not only relieves pain and limitation of movement but it often provides a rapid distinction from septic arthritis. Ultrasound-guided hip aspiration should be done in all individuals in whom ultrasonography has exhibited evidence of an effusion, and any of the following predictive criteria are present:

• Temperature greater than 99.5 °F (37.5 °C)
• Erythrocyte sedimentation rate (ESR) greater than or equal to 20 mm/hr
• Severe hip pain and spasm with movement

If the aspirate has a positive gram stain, more than 90% polymorphonuclear cells, or a glucose less than 40 mg/dL or markedly different from the serum glucose, the patient is more likely to have septic arthritis and not transient synovitis.

In settings in which routine aspirations of effusions is not performed, a dynamic contrast-enhanced MRI may help differentiate transient synovitis from septic arthritis.

Bone scintigraphy demonstrates mildly elevated uptake; however, it does not help differentiate etiologies.

Multiple algorithms and previously reported step-by-step guidelines are available in the literature ¹⁴.

The Kocher criteria remain a helpful set of clinical risk factors differentiating septic arthritis and toxic synovitis in pediatric patients presenting with hip pain. The criteria include the increasing diagnostic probability in favor of the former, yielding a 99.6% probability favoring septic arthritis as a diagnosis when all four criteria are met:

• White blood cell count > 12,000 cells per microliter of serum
• Inability or refusal to bear weight
• Febrile (> 101.3 degrees fahrenheit or 38.5 degrees celsius)
• Erythrocyte sedimentation rate (ESR) > 40 mm/hr

When none of the above risk factors are present upon presentation, the probability of the patient having septic arthritis of the hip drops below 0.2%. A subsequent study incorporated C-reactive protein (CRP) measurements into the clinical workup. Caird et al. performed a Level I study that concluded that a temperature above 38.5 was the best predictor of septic arthritis followed in decreasing order by C-reactive protein (CRP) (>1mg/dL), erythrocyte sedimentation rate (ESR), refusal to bear weight, and serum white blood cell count count ¹⁵.

Toxic synovitis treatment

Following the appropriate diagnosis of toxic synovitis made following a thorough, comprehensive diagnostic workup. Treatment for toxic synovitis usually includes anti-inflammatory drugs such as ibuprofen or naproxyn. Your child may take these for up to 4 weeks until the inflammation goes away. The doctor also may prescribe medicine for pain, such as acetaminophen.

Other modalities include the application of heat and/or massage modalities. In the setting of clinical concern or when the diagnosis is unclear, admitting the patient for observation can allow for serial observation following an initial period of supportive management.

Resting the hip joint is important, and it’s best if your child avoids putting weight on the hip while recovering. Kids usually can walk again comfortably within a day or two of taking the anti-inflammatory medicine. However, participation in activities like gym class or sports will have to wait until your child fully recovers.

Symptoms generally improve after 24 to 48 hours. Complete resolution of symptoms often takes up to 1 to 2 weeks in up to 75% of patients ¹⁶. The remainder may have less severe symptoms for several weeks. If significant symptoms persist for seven to 10 days after the initial presentation, consider other diagnoses. Patients with symptoms for more than a month have been found to have a different pathology.

Toxic synovitis prognosis

Toxic synovitis usually goes away within a week or two, but sometimes can last for 4-5 weeks.

While most kids have no long-term effects from it, some can develop toxic synovitis multiple times during childhood. If your child has a history of toxic synovitis, let your doctor know.

In total, toxic synovitis of the hip recurs in up to 20% to 25% of patients. Patient should be educated regarding the increased risk of recurrence in the setting of a previously documented diagnosis of toxic synovitis. One study reported the subsequent recurrence rates in patients with a previously documented diagnosis of toxic synovitis were 69%, 13%, and 18% at one-, two-year, and long-term follow-up, respectively ¹⁷.

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