Overview
Reactive arthritis is an inflammatory joint condition that usually affects the lower extremities, including the knee, after certain infections in the gut or urogenital tract. The inflammation is driven by the immune system's response to the prior infection, not by ongoing damage from the infection itself. When the joint lining and nearby tendon/ligament attachment sites become inflamed, it can cause pain, swelling, stiffness, and reduced motion. Symptoms can also involve the eyes, urinary tract, skin, or the spine in some people.
Symptoms
Many people notice sudden or gradually worsening knee pain and swelling, often along with stiffness that is worse after rest and may limit walking or climbing stairs. Pain may also occur where tendons attach around the ankle/foot, and some patients develop lower back or buttock discomfort. Some people have non-joint symptoms such as burning with urination, changes in urinary frequency, diarrhea, eye redness or irritation, or mild skin changes. Symptoms may improve over weeks but can linger or recur, especially if the initial trigger infection was not fully treated.
Causes
Reactive arthritis typically follows an infection of the bowel (such as certain bacterial gastrointestinal infections) or the urogenital tract (commonly after sexually transmitted infections like chlamydia). The immune system becomes activated and inflammatory chemicals are released, leading to inflammation in the joints and entheses (tendon/ligament attachment sites). In many cases, the original infection is already resolving or resolved, which is why symptoms can appear days to weeks afterward.
Risk Factors
A major risk factor is having a recent bowel or urogenital infection, particularly chlamydia or certain gastrointestinal illnesses. People with a genetic tendency, especially HLA-B27, are more likely to develop reactive arthritis and have more persistent symptoms. Being sexually active without consistent barrier protection can increase risk for triggering infections. Delayed or incomplete treatment of the triggering infection may also contribute to longer or more complicated symptoms.
Prevention
Reducing the risk of trigger infections is the most effective prevention strategy, including practicing safer sex and using barrier protection. Prompt testing and treatment of suspected sexually transmitted infections can lower the chance of reactive arthritis developing afterward. For gastrointestinal triggers, careful food hygiene and avoiding undercooked foods can reduce exposure to causative bacteria. If you have had reactive arthritis before, working with your clinician to prevent recurrences by addressing future infections quickly is important.
How the Diagnosis Is Evaluated
Clinicians usually start with a detailed history of recent diarrhea, urinary symptoms, or sexual exposure, followed by a focused exam of the knee for swelling, warmth, range-of-motion limits, and signs of enthesitis. They may check for associated findings such as red eyes, skin lesions, or mouth sores, and assess for back or heel pain. Blood tests can show inflammatory markers and may include testing for HLA-B27, while testing is often done to look for the triggering infection when indicated. Imaging such as ultrasound or X-ray is sometimes used to rule out other causes and assess effusion, while MRI is reserved for unclear or persistent cases.
Nonsurgical Treatment Options
Treatment focuses on reducing inflammation, relieving pain, maintaining movement, and eliminating any active infection. Anti-inflammatory medicines such as NSAIDs are commonly used and can improve pain and swelling. If there is persistent knee inflammation, a clinician may consider a corticosteroid injection into the knee joint to calm symptoms more quickly. Physical therapy and gentle range-of-motion and strengthening exercises help preserve mobility and support return to normal activity. If a triggering infection is still present, antibiotics may be recommended for that specific infection, and a rheumatology specialist may use disease-modifying medications or biologics for persistent or recurrent reactive arthritis. Short courses of oral corticosteroids may be used in select situations when symptoms are significant and NSAIDs are not enough.
When to Seek Medical Attention
Seek medical attention promptly if you have a hot, swollen knee with difficulty bearing weight, rapidly worsening pain, or symptoms that do not improve within a few days. Get urgent care or emergency evaluation if you have fever, feel very ill, or the knee becomes severely swollen with intense pain (to help rule out infection of the joint). Contact a clinician urgently if you develop new eye pain or light sensitivity, severe urinary symptoms, neurologic symptoms, or other rapidly progressive problems. If symptoms follow a recent infection and you develop joint swelling along with red eyes or urinary symptoms, prompt evaluation is especially important.
Frequently Asked Questions
Reactive arthritis is caused by an abnormal immune inflammatory response that occurs after certain bowel or urogenital infections, leading to inflammation in the knee joint and sometimes tendon attachment sites.
Symptoms often appear days to weeks after the triggering infection, but timing can vary from person to person.
Next Steps
If you suspect reactive arthritis, schedule a prompt evaluation so clinicians can identify the likely trigger infection and safely rule out emergencies such as septic arthritis. Until you are seen, you can use rest, activity modification, and NSAIDs if you can take them safely to help control pain and swelling. If symptoms include fever, severe joint swelling, or red/painful eyes, seek urgent care right away.