Does Lyme arthritis usually cause visible joint swelling or effusion?

Lyme arthritis is a distinct objective inflammatory syndrome, and serologic findings require interpretation in that clinical setting—particularly in an older adult, in whom alternative causes of mono- or oligoarthritis also warrant consideration.

Clinical evidence

• Lyme arthritis is strongly supported as typically presenting with marked, objectively demonstrable swelling of one or a few large joints, especially the knee, often with an effusion. Joint pain may be disproportionately mild relative to visible swelling.
• Arthralgia without swelling, effusion, or synovitis is not equivalent to Lyme arthritis. Objective inflammatory findings are therefore central when considering active joint involvement.
• In a selected pediatric cohort of 87 patients with Lyme knee arthritis, all had an effusion; this illustrates the characteristic phenotype but does not provide a population-wide frequency estimate.

Serology and interpretation

• Persistent Borrelia burgdorferi IgG is well established to occur after prior infection, including following successful treatment. In long-term follow-up 10–20 years after treated Lyme disease, IgG remained detectable in 25% of those with prior early Lyme disease and 62% of those previously treated for Lyme arthritis despite no active signs or symptoms.
• Lyme arthritis itself generally produces a robust, expanded IgG response. Thus, serology is useful as first-line testing when compatible objective arthritis is present.
• The assertion that persistent IgG alone does not establish active joint infection is classified as unsupported in the finalized assessment; clinically, however, the supplied evidence emphasizes that antibody results require correlation with objective arthritis and that serology is not a test of cure.

Mechanistic context

• Joint swelling and effusion reflect synovial inflammation (synovitis) associated with disseminated Borrelia infection. Inflammation may persist after antimicrobial therapy through post-infectious immune mechanisms, while bacterial DNA can also persist without proving viable intra-articular infection.

Bottom line

• Objective large-joint swelling/effusion plus compatible serology is the clinically meaningful pattern for Lyme arthritis; persistent IgG should be interpreted in the full clinical context rather than as an isolated marker.