ankylosing spondylitis (AS)

introduction

one of the seronegative spondyloarthropathies with a strong genetic predisposition which appears to relate to the ARTS1 and IL23R genes in addition to HLA-B27
men are affected more than women by a ratio of about 3:1, and the disease is generally more severe in males
~90% of AS patients express the HLA-B27 genotype BUT only 5% of individuals with the HLA-B27 genotype contract the disease
it mainly affects joints in the spine and the sacroiliac joint in the pelvis, and can cause eventual fusion of the spine.
onset of symptoms is usually before age 45yrs with average age of onset being 23yrs, but given the non-specific nature of early symptoms diagnosis is usually not made until ~10 years later.
in the 1st 10 yrs of the disease, it is cervical dominant in 5%, lumbar dominant in ~20% and equi-dominant or minimal in each area in 75%, while 80% have symmetric sacro-iliitis ¹⁾

initial symptoms are chronic back pain and stiffness in the thoracic spine, often with pain referred to a buttock or back of thigh from sacro-iliac joint involvement
pain usually worst at rest improving with activity
adolescent onset may cause pain and swelling of large limb joints, particularly the knee
in prepubescent cases, the ankles and feet may be involved, where calcaneal spurs may also develop.
40% develop iritis and uveitis, causing redness, eye pain, vision loss, floaters and photophobia
5-10% also have or develop inflammatory bowel disease (IBD)

there is no specific test
Dx is usually made on clinical features, and MRI and XRay of the spine although plain XRay features are usually not present until 10yrs after onset
Schober's test is a useful clinical measure of flexion of the lumbar spine performed during examination
ESR and CRP may be raised during acute inflammatory periods
a minority are ANCA +ve

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