Differentials
Ankylosing spondylitis (AS)
SIGNS / SYMPTOMS
Similar spinal involvement to ReA, but in a more symmetrical fashion, particularly in the sacroiliac joints.
INVESTIGATIONS
Axial x-rays reveal syndesmophytes (ossification of spinal ligaments forming bony bridges between vertebrae) that tend to be marginal rather than non-marginal.
Evidence of 'bamboo' spine is indicative of AS.
AS has less prominent peripheral joint involvement than ReA.
Psoriatic arthritis
SIGNS / SYMPTOMS
Skin lesions can appear grossly similar and are histologically identical to those of ReA. In psoriatic arthritis, distal interphalangeal (DIP) joints are more commonly affected.
Psoriatic arthritis and ReA can be difficult to distinguish in: patients with psoriatic arthritis who do not have psoriasis; and, in both groups of patients with skin involvement.
INVESTIGATIONS
Hand x-rays reveal erosion of one end of DIP joint with expansion of the base of the adjacent metacarpal; resorption of terminal phalanges.
Rheumatoid arthritis (RA)
SIGNS / SYMPTOMS
A symmetrical polyarthritis commonly affecting the small joints of the hands and feet. RA does not affect the lumbar spine or sacroiliac joints.
INVESTIGATIONS
Rheumatoid factor and anti-CCP (cyclic citrullinated peptide) antibodies may be positive in RA. Hand x-rays reveal typical erosive changes.
Rheumatic fever
SIGNS / SYMPTOMS
Rheumatic fever is associated with the upper respiratory system before the onset of arthritis. Fevers are common in both entities.
Rheumatic fever starts as an acute migratory arthritis involving both upper and lower extremities. There is no involvement of the axial spine, including the sacroiliac joints. Post-streptococcal ReA is an acute arthritis associated with antecedent streptococcal infection in patients not fulfilling the Jones criteria for acute rheumatic fever.
Post-streptococcal ReA probably has lower-extremity predominance and, therefore, should be included in the differential diagnosis of patients with lower-extremity arthritis. It is not clear if this is a separate disease or a forme fruste of rheumatic fever.
INVESTIGATIONS
Antistreptolysin-O antibodies, antiDNAse B antibodies, and throat culture/rapid antigen test for group A streptococcus may be positive.
Adult-onset Still's disease
SIGNS / SYMPTOMS
Fever can occur in both disease entities. However, the fever in Still's disease is described as double quotidian (2 spikes of fever a day). A characteristic evanescent, salmon-coloured rash occurs at the same time as the fevers in Still's disease.
INVESTIGATIONS
May see leukocytosis and thrombocytosis on FBC count, elevated LDH, abnormal LFTs, and elevated serum ferritin. Hand x-rays may show intercarpal and carpometacarpal joint space narrowing without erosions.
Disseminated gonococcal disease
SIGNS / SYMPTOMS
The arthritis of gonococcal disease occurs as an acute migratory polyarthritis with tenosynovitis, often involving the hands. Another feature of gonococcal infection includes small, painless, maculopapular, pustular, or vesicular lesions on an erythematous base involving the extremities.
INVESTIGATIONS
Genitourinary culture for Neisseria gonorrhoeae.
Arthritis associated with inflammatory bowel disease (IBD)
SIGNS / SYMPTOMS
The presence of documented IBD distinguishes the arthritis associated with IBD from ReA.
INVESTIGATIONS
There are no differentiating tests. However, patients with IBD arthritis tend to have bilateral and symmetrical radiographic sacroiliitis.
IBD arthritis patients might show marginal syndesmophytes on plain radiographs, whereas ReA patients typically display non-marginal syndesmophytes.
Gout
SIGNS / SYMPTOMS
Patients have more distal interphalangeal joint involvement in gout. Patients may also have uric acid tophi, for example in the pinna of the ear.
INVESTIGATIONS
Crystal in the synovial fluid of affected joints.
Septic arthritis
SIGNS / SYMPTOMS
Affects single joints in most cases. The affected joint is often swollen, erythematous, and warm, and patients may have a fever.
INVESTIGATIONS
Culture of the organism from the synovial fluid of an affected joint. Often, the cell count of synovial fluid exceeds 50,000 per mm³ in septic arthritis, with more than 75% polymorphonuclear leukocytes. May also have positive blood cultures.
Post-viral arthritis
SIGNS / SYMPTOMS
Arthralgias are preceded by an influenza-like illness in parvovirus infection. A rheumatoid-like distribution (symmetrical small joint polyarthritis) of arthralgias is typical. Occasionally, patients may have a maculopapular rash.
INVESTIGATIONS
In parvovirus B19 infection, may see positive serology; positive IgM within the first 6 weeks of exposure is diagnostic.
Lyme arthritis
SIGNS / SYMPTOMS
Lyme disease produces a characteristic rash called erythema migrans at the site of the tick bite. Non-specific constitutional symptoms such as headache, fever, and fatigue are common in Lyme disease. The arthritis can be migratory and often affects only 1 joint at a time, primarily large joints such as the knee.
INVESTIGATIONS
Positive Lyme titre with Western blot.
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