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ammation, and has
been used to assess the response of tenosynovitis, synovitis, and enthesitis to
treatment.
Musculoskeletal ultrasound is a sensitive way to detect in
fl
3.3 Reactive Arthritis
Reactive arthritis is a form of seronegative in
ammatory arthritis that develops after
an infection somewhere in the body outside of the joints. Some gastrointestinal and
urinary infections are considered causal, including those due to the bacteria
Chlamydia trachomatis, Yersinia, Salmonella, Shigella, Campylobacter [
60
],
Escherichia coli, Clostridium dif
fl
cile and Chlamydia pneumonia [
61
].
Reactive arthritis is uncommon, and affects men and women equally. Symptoms
of arthritis develop several days or weeks after the initial infection [
60
]. A pre-
ceding infection is not always identi
ed, even in patients with a typical presenta-
tion. Often, pain and swelling develops in a few joints, particularly in the knees or
ankles. Enthesitis is not uncommon. Patients may have signs of eye in
ammation,
urinary symptoms, and skin rashes. In most patients, the arthritis subsides after
6 months, however, in a small proportion, it may become chronic.
Spine involvement usually manifests as inflammatory back pain. Twenty-five
percent of patients develop radiographic changes of the sacroiliac joints, usually
affecting only one side. Extensive spine fusion is very uncommon. Patients with
spine involvement often have HLA-B27 [
62
].
Treatment of reactive arthritis is mainly symptomatic. NSAIDs are used to
control acute in
fl
ammation. Limited evidence supports the use of antibiotics when
Chlamydia is the cause [
63
]. In chronic reactive arthritis, methotrexate, sulfasala-
zine, and biologics have been used with various responses.
In acute reactive arthritis, plain radiographs of the affected joints are mainly used
to exclude other diagnoses. In patients with spinal involvement, radiographic
changes of the sacroiliac joint can be seen after some duration of symptoms, with
more severe changes on one side compared to the other. Syndesmophytes may
develop that are often bulky, asymmetric and extend laterally [
64
] (Fig.
11
). MRI of
the spine and entheses is potentially useful for the assessment of in
fl
ammation and
monitoring responses to treatment, but has not been evaluated extensively.
fl
3.4 Undifferentiated Spondyloarthritis
Undifferentiated spondyloarthritis refers to spondyloarthritis that does not ful
ll
criteria for AS, psoriatic arthritis, reactive arthritis, or enteropathic spondyloarthritis
[
65
]. Most patients are young men with in
ammatory low back pain who may have
HLA-B27. Some may have arthritis in peripheral joints or enthesitis. The major
fl
