• Inflammatory arthritis triggered by antecedent gastrointestinal or genitourinary infections.
  
• Asymmetric oligoarthritis most commonly affecting the lower extremities.
  
• Enthesitis and dactylitis.
  
• Association with extra-articular manifestations, such as conjunctivitis, anterior uveitis, urethritis, circinate balanitis, oral ulcers, and keratoderma blennorrhagicum.

Reactive arthritis is an inflammatory arthritis that develops after certain infections of the gastrointestinal or genitourinary tracts. Symptoms can frequently be self-limited with spontaneous remission after several weeks to months, while other times the symptoms can become chronic. At onset, the arthritis is usually an acute, asymmetric oligoarthritis of peripheral joints that occurs within several weeks after the infection. While isolating the causative infectious agent aids in the diagnosis, this is neither required nor always possible. Treatment of an active infection with antibiotics is indicated but is frequently ineffective in preventing the acute arthritis from becoming chronic.

Currently, there is still no consensus on the classification and diagnostic criteria for reactive arthritis. Nonetheless, it is generally classified as one of the spondyloarthropathies, a group of diseases that also includes psoriatic arthritis, ankylosing spondylitis, and inflammatory bowel disease–associated arthritis. Arthritis of the axial skeleton (ie, the sacroiliac joints and spine), enthesitis (inflammation of the insertion sites of tendons to bone), asymmetric oligoarthritis or symmetric polyarthritis of peripheral joints, and the absence of rheumatoid factor are shared characteristics of the spondyloarthropathies. A more restrictive definition of reactive arthritis, formerly known as Reiter syndrome, is the classic triad of peripheral arthritis, conjunctivitis, and urethritis or cervicitis.

Reactive arthritis usually occurs in young adults between 20 and 40 years of age. The post-gastrointestinal tract infection subtype of reactive arthritis equally affects men and women, while the post-genitourinary tract infection subtype overwhelmingly affects men more frequently than women (9:1 ratio). The prevalence of reactive arthritis is estimated to be 30 to 40 per 100,000, and the incidence is estimated to be 5 to 28 per 100,000 per annum.

Reactive arthritis frequently develops 1–4 weeks after a bout of gastroenteritis caused by Shigella, Salmonella, Campylobacter, or Yersinia, or after acquisition of a sexually transmitted infection, most commonly Chlamydia trachomatis and occasionally HIV. In up to 25% of patients, however, there are no symptoms of an antecedent infection. This observation suggests that reactive arthritis ensues either after a subclinical infection or that other environmental triggers are involved.

In addition to environmental triggers, genetic factors may also play a role in disease susceptibility. Approximately 30–50% of patients with reactive arthritis are positive for HLA-B27. As such, there is a greater prevalence of the disease in whites, in contrast to other ethnic groups that have a lower frequency of HLA-B27, such as blacks. A strong family history may be elicited in patients with reactive arthritis. The exact role of HLA-B27 in the disease pathogenesis remains elusive.

The triggering microbial agents and their chromosomal DNA occasionally have been isolated from synovial fluid of affected joints. However, the causative agents and their associated components have been discovered in joints from patients without reactive arthritis. Consequently, the significance of these findings remains to be determined.

Symptoms and Signs

Articular Manifestations

The predominant manifestation of reactive arthritis is an acute, asymmetric oligoarthritis of peripheral joints, frequently 1–4 weeks after the onset of the inciting infection. Joints of the lower extremities (eg, knees, ankles, and feet) are preferentially affected compared with joints of the upper extremities. The sternoclavicular and temporomandibular joints are sometimes involved. Affected joints are usually stiff, swollen, and tender.

Axial skeleton disease most commonly presents as inflammatory low back pain, which occurs in up to half of patients with reactive arthritis. Radiographic evidence of sacroiliitis, which is often unilateral, develops in approximately 20–25% of patients. This is in contrast to the bilateral, symmetric sacroiliitis of ankylosing spondylitis. A minority of patients with sacroiliitis have spondylitis with extensive fusion of the spine resembling severe ankylosing spondylitis. The prevalence of axial skeleton disease is greater among those with chronic disease and those with HLA-B27 (90% of patients with radiographic evidence of sacroiliitis are HLA-B27 positive).

Enthesitis

A prominent feature of reactive arthritis is enthesitis, inflammation of sites where tendons attach to bone. Common sites for enthesitis are the Achilles tendon, plantar fascia, and pelvic bones. Findings of enthesitis include soft tissue swelling with overlying warmth and tenderness to palpation.

Dactylitis

The combination of synovitis and enthesitis in a toe or finger can cause the entire digit to become diffusely swollen, producing dactylitis or “sausage digit.” Dactylitis is a feature of the spondyloarthropathies, frequently observed in reactive arthritis and psoriatic arthritis. Toes are more commonly affected than the fingers.

Mucocutaneous Lesions

Circinate balanitis is an inflammatory lesion on the glans or shaft of the penis, and it is one of the characteristic lesions associated with reactive arthritis. In an uncircumcised man, these lesions are shallow, moist, serpiginous, painless ulcers with raised borders. In a circumcised man, these lesions can appear as dry, hyperkeratotic plaques resembling psoriasis and be painful.

Another cutaneous lesion associated with reactive arthritis is keratoderma blennorrhagicum, a papular, waxy rash that affects primarily the palms and soles. At first, the rash can be vesicular and then evolve into maculopapular nodules before turning into scaly, hyperkeratotic lesions resembling psoriasis. These lesions can coalesce to cover large areas of skin, extending proximally beyond the palms and soles.