Does Lyme arthritis typically cause obvious swelling of one or a few large joints, especially the knee?

Lyme arthritis is a late musculoskeletal manifestation of Borrelia burgdorferi infection with a distinctive inflammatory pattern. The claim is strongly supported by guideline and cohort evidence.

Clinical presentation

• Lyme arthritis usually causes marked, objectively visible swelling of one joint or a small number of joints (typically fewer than five), rather than diffuse small-joint or widespread polyarticular disease.
• A substantial joint effusion is common. Swelling, warmth, and stiffness may be striking, sometimes appearing disproportionate to the level of pain.
• The knee is the characteristic site. The 2020 IDSA/AAN/ACR guideline specifically describes marked swelling of one or a few joints—especially the knee—as typical. In a relatively small PCR-confirmed cohort, the knee was involved in 98% of adults and all children; this illustrates the predominance of knee disease but is not a universal frequency.
• Other large joints can be affected, but small-joint involvement and extensive polyarthritis are uncommon in the usual presentation.

Mechanistic and treatment implications

• The effusion and swelling reflect inflammatory synovitis, driven largely by the host immune response to B. burgdorferi.
• In most patients, antimicrobial treatment clears infection and the arthritis resolves. A minority have persistent inflammatory arthritis after treatment, reflecting ongoing inflammation rather than necessarily persistent active infection.

Bottom line

• Lyme arthritis typically presents as obvious swelling/effusion of one or a few large joints, with the knee especially often involved. This is a characteristic clinical pattern, though not an absolute rule in every individual.