Reactive arthritis is a type of joint inflammation that develops as a reaction to an infection elsewhere in the body, often in the intestines, urinary tract, or genitals. It is not caused by the infection directly invading the joints but rather by the immune system responding abnormally after the infection has occurred. This immune response leads to inflammation, pain, and swelling in one or more joints, typically affecting areas like the knees, ankles, and feet.
The condition usually appears within a few weeks after an infection, most commonly gastrointestinal infections caused by bacteria such as Salmonella, Shigella, Yersinia, or Campylobacter, or genitourinary infections caused by Chlamydia trachomatis. These infections trigger the immune system to mistakenly attack the joints, causing symptoms of arthritis even though the joint itself is not infected.
Reactive arthritis is part of a group of diseases called spondyloarthropathies, which share certain genetic and clinical features. One important genetic factor associated with reactive arthritis is the presence of the HLA-B27 gene, which increases susceptibility to developing the condition after infection.
The main symptoms of reactive arthritis include joint pain and swelling, often in the lower limbs, along with stiffness and reduced mobility. Besides joint symptoms, people may experience inflammation of other tissues such as the eyes (conjunctivitis or uveitis), urinary tract (urethritis), and skin. This combination of symptoms is sometimes referred to as Reiter’s syndrome.
The exact mechanism behind reactive arthritis involves an abnormal immune response where the body’s defense system continues to react even after the initial infection has resolved. This immune reaction causes inflammation in the joints and other tissues, leading to the characteristic symptoms. The bacteria that trigger reactive arthritis are often no longer present in the joints, but fragments of bacterial proteins or DNA may persist and stimulate the immune system.
Reactive arthritis can affect people of all ages but is more common in young adults, particularly men. The severity and duration of symptoms vary widely; some individuals experience mild symptoms that resolve within months, while others may develop chronic arthritis lasting years.
Treatment focuses on managing symptoms and controlling inflammation. Nonsteroidal anti-inflammatory drugs (NSAIDs) are commonly used to reduce joint pain and swelling. In more severe cases, corticosteroids or disease-modifying antirheumatic drugs (DMARDs) may be prescribed. Addressing the initial infection with appropriate antibiotics is also important, especially if the triggering infection is still active.
Because reactive arthritis is triggered by infection and involves the immune system, maintaining good hygiene and prompt treatment of infections can help reduce the risk. However, not everyone who has an infection will develop reactive arthritis, indicating that genetic and environmental factors play a role in susceptibility.
In summary, reactive arthritis is an inflammatory joint condition caused by an abnormal immune response following certain bacterial infections, leading to joint pain, swelling, and other systemic symptoms. It is a complex interplay between infection, immune system, and genetics that results in this unique form of arthritis.