Ankylosing spondylitis

Last updated: July 3, 2023

Summarytoggle arrow icon

Ankylosing spondylitis (AS) is a seronegative spondyloarthropathy and a chronic inflammatory disease of the axial skeleton that leads to partial or complete fusion and rigidity of the spine. It is a part of a spectrum of inflammatory spondyloarthropathies known as axial spondyloarthritis and can also be referred to as radiographic axial spondyloarthritis. Males are disproportionately affected and > 90% of patients are positive for HLA-B27, which is a predisposing factor for the disease. Pain and stiffness in the lower back and neck, caused by inflammation of the vertebral column and the sacroiliac joints, are characteristic early features of AS. Pain typically improves with activity and is particularly prominent at night. Other articular findings include restricted spine mobility and tenderness of the sacroiliac joints on provocation tests, such as the FABER test. The most common extraarticular manifestation is acute, unilateral anterior uveitis. Kyphosis and fusion of the costovertebral joints can cause restrictive lung disease. Diagnosis is primarily based on typical clinical features, elevated inflammatory markers, HLA-B27 positivity, and evidence of inflammatory changes and ankylosis of the sacroiliac (sacroiliitis) and/or intervertebral joints (bamboo spine) on imaging. Typical features may be absent on imaging in early AS, but the presence of several classical clinical and laboratory features is sufficient to confirm the diagnosis and initiate treatment. There is no curative treatment for AS, but regular physiotherapy can slow disease progression. Pharmacotherapy for AS includes NSAIDs and/or TNF-α inhibitors (e.g., infliximab, adalimumab, etanercept). In severe cases, surgery (e.g., arthroplasty) may be considered to improve the patient's quality of life. Patients with AS are at an increased risk of osteoporosis and pathological fractures (chalk stick fractures). Axial spondyloarthritis also includes nonradiographic axial spondyloarthritis, a condition that causes similar symptoms to AS but without inflammatory changes on plain radiographs. Management of nonradiographic axial spondyloarthritis is the same as for AS.

Definitiontoggle arrow icon

Epidemiologytoggle arrow icon

Epidemiological data refers to the US, unless otherwise specified.

Etiologytoggle arrow icon

  • Genetic predisposition: 90–95% of patients are HLA-B27 positive.

Pathophysiologytoggle arrow icon

Clinical featurestoggle arrow icon

Articular manifestations

Extraarticular manifestations

Positive SIJ pain provocation tests

Tenderness over the sacroiliac joints can be elicited on pain provocation tests, such as:

FABER test [9]

Pain in the ipsilateral SIJ on Flexion, ABduction, and External Rotation (FABER) indicates a positive test.

  • Assessed with the patient in the supine position
  • The leg to be tested is placed in a figure-4 position, with the patient's ankle placed on the contralateral knee.
  • Apply pressure on the patient's flexed knee.
  • Pain will be felt in the ipsilateral SIJ if inflammation is present.

Mennell sign [10]

Tenderness to percussion and pain on displacement (distraction) of the SIJ indicates sacroiliitis or degeneration. The SIJ can be distracted using the following maneuvers.

  • Prone position: Lift the leg passively (i.e., extend the hip) with one hand while applying pressure on the ipsilateral SIJ with the other hand.
  • Lateral position
    • Ask the patient to flex their lower hip and knee to ∼ 90°.
    • Lift the upper leg and extend the upper hip with one hand while fixing the pelvis with the other hand.
  • Supine position: Apply posteriorly directed pressure to the anterior superior iliac spines.

Reduced spinal mobility [11]

Spinal mobility is typically significantly decreased in advanced or severe AS.

Schober test

A bedside examination to assess lumbar forward flexion

  • With the patient standing, the examiner marks two points on the patient's back: one at L5 and one 10 cm above. [12]
  • The patient is asked to touch their toes without bending their knees.
  • An increase of < 4 cm between the two points suggests impaired spine flexion.

Lateral lumbar flexion test

A bedside examination to assess lumbar lateral flexion

  • The patient is asked to stand with their back against the wall with hands by the side and flex as far as possible to one side without bending their knees.
  • The distance between the tip of the patient's middle finger and the floor is measured on the ipsilateral side.
  • A larger fingertip-to-floor distance indicates limited spinal mobility.

Chest expansion on deep breathing

A bedside examination to assess thoracic spine mobility [12]

  • Measure the chest circumference after maximal expiration and again after maximal inspiration.
  • A smaller difference in measurements (< 2.5 cm) indicates limited thoracic spine and costovertebral joint mobility. [4][12]
  • Reduced chest expansion can also be due to concomitant pulmonary disease, chest wall deformities, or poor respiratory effort.

Kyphosis [11][13][14]

Kyphosis due to loss of anterior vertebral height and wedging of the thoracic vertebrae is present in up to 50% of patients, especially those with advanced AS. The extent of the deformity may be subtle and is assessed using the occiput-to-wall distance.

  • Ask the patient to stand against a wall with their heels, buttocks, and shoulders pressed against the wall.
  • Measure the distance between the occiput and the wall.
  • In a healthy individual, the occiput should be touching the wall; the inability to do so is considered abnormal.

Diagnosticstoggle arrow icon

Approach [5][15]

  • Suspect AS in patients < 45 years of age who present with features of inflammatory back pain lasting > 3 months.
  • Initial test in all individuals with suspected AS: x-ray of the sacroiliac joints
    • X-ray findings consistent with AS in a patient with typical features of AS: diagnosis confirmed
    • X-ray findings equivocal: Obtain CRP (or ESR) and HLA-B27.
  • HLA-B27 positive
    • High likelihood of AS (compelling clinical picture): diagnosis confirmed; no further imaging required
    • Only a few classical clinical features of AS: MRI of the sacroiliac joints
  • HLA-B27 negative
  • Consider imaging of the spine in patients with pain in the cervical, thoracic, or lumbar spine.
  • Further workup for associated conditions may be appropriate depending on extraarticular manifestations.

Diagnostic criteria [4]

There are several diagnostic criteria for AS (e.g., New York criteria, Rome criteria). The Assessment of Spondyloarthritis International Society (ASAS) classification criteria for axial spondyloarthritis, which is based on a combination of clinical features and imaging or laboratory findings, is detailed here.

Laboratory findings

Interpret a positive HLA-B27 test in conjunction with clinical features as only ∼ 1% of individuals who are positive for HLA-B27 have AS.

Imaging [15]


Sacroiliac joints (PA view) [4]

  • Indication: best initial test to confirm the diagnosis of AS and evaluate disease severity
  • Characteristic findings (usually symmetrical)

Spine (AP and lateral views)

Early stages of AS may be unidentifiable on x-ray (low sensitivity for early-stage). Hence, a patient with several classical clinical and laboratory findings fulfills the diagnostic criteria for AS even if x-ray findings are inconclusive. [5][15]


MRI is the most sensitive method for early detection of inflammatory changes of AS. However, MRI is not routinely required to confirm a diagnosis of AS as specialized MRI techniques and radiology expertise is required to accurately identify AS findings. [19]

Sacroiliac joints with gadolinium contrast

  • Indication: negative or equivocal radiographs of sacroiliac joints in a patient with some classical features of AS [5]
  • Characteristic findings

Spine without contrast

  • Indications: suspected AS in a patient with any of the following
  • Characteristic findings [20]
    • Romanus lesions: inflammatory lesions (high signal intensity) seen anterior or posterior vertebral corners
    • Reparative changes: secondary bone formation, e.g., on the periphery of or within intervertebral disks

CT sacroiliac joints and/or spine

Further evaluation for extraarticular disease

Symptom-based workup for extraarticular manifestations
Extraarticular complication Characteristic clinical features Recommended diagnostic studies and characteristic findings
Restrictive lung disease
Anterior uveitis
Inflammatory bowel disease
Aortic root and valve disease [6]
Atrioventricular blocks [6]
IgA nephropathy

Differential diagnosestoggle arrow icon

Syndesmophytes vs. osteophytes



Radiographic features

  • Horizontal growth
  • Inflammatory spine disease (e.g., AS)

Syndesmophytes grow vertically, as opposed to osteophytes, which grow horizontally!

The differential diagnoses listed here are not exhaustive.

Treatmenttoggle arrow icon

Approach [2][5]

  • Early referral to rheumatology is recommended in all patients with suspected or confirmed AS.
  • Ensure all patients receive supportive therapy.
  • Assess symptom severity.
  • Manage extraarticular complications and associated diseases accordingly.
  • The management of Nr-axSpA is similar to AS.

Supportive therapy

All patients with AS, regardless of the disease stage, should receive appropriate supportive therapy to improve their mobility and prevent complications.

Physical therapy is vital in the management of patients with AS.

Management of active disease

Pharmacological therapy

Pharmacological therapy in ankylosing spondylitis [2]
Medication class Indications Dosage and administration Cautions
  • First-line treatment for patients without concurrent IBD or renal impairment [27]
  • Should be taken continuously (i.e., not on-demand)
  • See “Oral analgesics” for agents and dosages.
TNF-α inhibitors
  • First-line treatment in patients with concurrent IBD or recurrent anterior uveitis
  • Second-line in patients with insufficient response to two different NSAIDs
Antiinterleukin-17 antibodies
Local parenteral glucocorticoids
  • Persistent symptoms despite NSAID treatment and any of the following:
  • Risk of infection (localized or joint)
  • Can cause elevated blood glucose and blood pressure
  • Risk of hepatic and renal toxicity, bone marrow suppression, and development of fibrosing alveolitis; monitor regularly.

Advanced therapeutic options [2]

Spinal osteotomy is a high-risk procedure with unclear benefits; for most patients, the risks outweigh the potential benefits!

Management of stable disease [2]

Patients with AS may have stable disease at diagnosis or following management of active disease.

Monitoring treatment response [2]

  • Treatment efficacy should be assessed by a physician.
  • Treatment response can be assessed based on the impact of symptoms on quality of life and the trend of inflammatory markers and imaging findings.
  • In patients with stable AS, regular follow-up imaging is not recommended.

Management of acute pain [25][29]

Spinal fractures can occur following minor trauma, e.g., a fall from standing, or even spontaneously.

Immobilizing the patient using standard techniques can cause iatrogenic spinal cord injury!

Complicationstoggle arrow icon

We list the most important complications. The selection is not exhaustive.

Referencestoggle arrow icon

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  2. Ward MM, Deodhar A, Gensler LS, et al. 2019 Update of the American College of Rheumatology/Spondylitis Association of America/Spondyloarthritis Research and Treatment Network Recommendations for the Treatment of Ankylosing Spondylitis and Nonradiographic Axial Spondyloarthritis. Arthritis Rheumatol. 2019; 71 (10): p.1599-1613.doi: 10.1002/art.41042 . | Open in Read by QxMD
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