Lyme arthritis is a form of joint inflammation that occurs as a late manifestation of Lyme disease, which is an infection caused by the bacterium *Borrelia burgdorferi* transmitted through the bite of infected black-legged ticks. When Lyme disease progresses without adequate treatment, the bacteria can invade the joints, leading to Lyme arthritis, most commonly affecting the knees.
The knee is the joint most frequently involved in Lyme arthritis because it is a large, weight-bearing joint with a complex structure that can be susceptible to inflammation. Lyme arthritis typically presents as swelling, pain, and stiffness in one or both knees. The swelling can be quite pronounced, often making the knee visibly enlarged and warm to the touch. Patients may experience difficulty bending or straightening the knee fully, and the joint may feel unstable or weak.
The underlying cause of Lyme arthritis in the knee is the immune system’s response to the bacterial infection. After the initial tick bite and early symptoms such as rash and flu-like illness, the bacteria can persist in the joint tissues. This persistence triggers an inflammatory reaction where immune cells flood the joint space, releasing chemicals that cause swelling and damage to the joint lining (synovium). This inflammation leads to the characteristic symptoms of pain and swelling.
Unlike typical arthritis caused by wear and tear or autoimmune diseases, Lyme arthritis often comes in episodes or flares. A person might have periods where the knee is very swollen and painful, followed by times when symptoms improve or disappear. If untreated, Lyme arthritis can cause lasting joint damage, including cartilage loss and reduced joint function.
Diagnosis of Lyme arthritis involves a combination of clinical history, physical examination, and laboratory tests. A history of possible tick exposure or previous Lyme disease symptoms is important. Blood tests can detect antibodies against *Borrelia burgdorferi*, supporting the diagnosis. Sometimes, fluid is drawn from the swollen knee (joint aspiration) to look for signs of infection and inflammation and to rule out other causes like bacterial septic arthritis.
Treatment for Lyme arthritis usually involves a course of antibiotics, often oral doxycycline or amoxicillin, for several weeks. Most patients respond well to antibiotics, with reduction in swelling and pain. In some cases, if symptoms persist despite antibiotics, additional treatment such as intravenous antibiotics or anti-inflammatory medications may be necessary. Physical therapy can help restore knee function and strength after inflammation subsides.
In summary, Lyme arthritis is a joint inflammation caused by the Lyme disease bacterium, primarily affecting the knees. It causes swelling, pain, and stiffness due to the immune response to bacterial infection in the joint. Early diagnosis and antibiotic treatment are crucial to prevent long-term joint damage and restore normal knee function.