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Image: “Fracture of the 2nd lumbar vertebra – old patient with Bechterew’s disease. The fracture affects all three pillars and is instable. This type of fracture ain’t untypical for Bechterew’s disease.” by Hellerhoff. License: CC BY-SA 3.0



Spondyloarthropathies are a group of overlapping inflammatory rheumatic disorders that mostly affect the spine. Patients usually present with back pain caused by the inflamed vertebral column.

Common characteristics of spondyloarthropathies are the involvement of the axial skeleton and of the entheses (the sites where tendons or ligaments insert into the bone). They are also associated with the MHC class I antigen, HLA-B27.

The entities grouped under spondyloarthropathies often include ankylosing spondylitis, reactive arthritis, psoriatic arthritis, enteropathic arthritis, and juvenile-onset spondyloarthritis.

Based on clinical and radiological findings, spondyloarthropathies can be grouped in 2 classes:

  • Axial spondyloarthritis (including ankylosing spondylitis)
  • Peripheral spondyloarthritis (affecting the arms and legs)

Image: ‘The ankylosis process, made by Senseiwa, with an image of the spine from Gray’s Anatomy’ by Senseiwa. License: CC BY-SA 3.0

Ankylosing spondylitis (AS)

Ankylosing spondylitis is a type of chronic arthritis of the spine and sacroiliac joints. When the disease progresses, it leads to a stiffening of the spine. Besides the axial skeleton, the peripheral joints may also be affected.


Image: “Ankylosing Spondylitis” by BruceBlaus. License: CC BY 3.0

Terminology and synonyms

Ankylosing spondylitis is also known as Bechterew’s disease, Marie-Strümpell disease, or rheumatoid spondylitis.

In the English-speaking world, ankylosing spondylitis is the most commonly used term. The obsolete term Bekhterev disease refers to the Russian neurologist Vladimir Bekhterev. Marie-Strümpell disease refers to French neurologist Pierre Marie and his German colleague Adolph Strümpell.


The incidence of ankylosing spondylitis is highest in circumpolar arctic and subarctic regions and in Northern European countries with a value of around 7:100,000 of the general population. This incidence declines significantly when approaching the equator. In the Southern hemisphere, ankylosing spondylitis is virtually absent.

The sex ratio of ankylosing spondylitis is approximately 3:1, with 62% of patients being male and 37% female.

The age of onset is usually between late adolescence and early adulthood; 90% of patients experience the first manifestation of the disease between 15 and 40 years of age. The peak incidence is at age 25. Often, 5–10 years may pass between the first onset of symptoms and the final diagnosis. This delay of diagnosis is often because insufficient attention is paid to the patient’s medical history and because of the insidious development of the disease.


The prevalence of ankylosing spondylitis fluctuates between 0.1 and 1.4% and is strongly dependent on the presence of HLA-B27. Ninety percent of people with ankylosing spondylitis are HLA-B27 positive. This gene is located on chromosome 6p21.33.


Causes of ankylosing spondylitis

The etiology of ankylosing spondylitis is unclear. While there is a strong association between ankylosing spondylitis and HLA-B27, with about 95% of patients expressing this gene, only 4–7% of HLA-B27 carriers actually develop ankylosing spondylitis. Other genetic factors and external factors like bacteria may likely play a role here. Infections with Klebsiella, Salmonella, Shigella, Yersinia, and Chlamydia are suspected to contribute to the pathogenesis.

The detection of TNF-α is important with regard to the treatment of the affected joints.


Musculoskeletal Symptoms

Note: The main symptom is chronic back pain (> 12 weeks).

The inflammatory back pain can affect the entire spine. However, the pain mainly manifests in the sacroiliac region and inferior thoracic area. The pain is independent of position and typically occurs at night and in the morning hours, improving with movement over the course of the day.

Increasing stiffness of the spine can be observed (especially morning stiffness), which results in changed body posture including:

  • Vertical tilt of the pelvis
  • Loss of lumbar lordosis
  • Increased thoracic kyphosis and cervical lordosis
  • Tendency to bend the hip and knee joints
  • Lateral rotation of the scapula
  • Hyperextension of the abdominal muscles with predominant abdominal breathing
  • Atrophy and hyperextension of the lumbar muscles
  • Hypotension of the gluteal and neck muscles and lack of movement of the spine while walking.

Bamboo spine: In the course of the disease, a so-called ‘bamboo spine can develop. First, the extension and then the flexion of the spine become impaired.

Other Symptoms of Ankylosing Spondylitis

Image: “Ankylosing spondylitis: Rigid thoracic kyphosis with typical abdomen" by Mehlauge. License: CC BY-SA 3.0

Image: ‘Ankylosing spondylitis: Rigid thoracic kyphosis with typical abdomen’ by Mehlauge. License: CC BY-SA 3.0

Enthesopathies: A typical symptom is a pain stemming from inflammations in the areas where the tendons insert (entheses). It mainly occurs in the area of the Achilles tendon (achillodynia) and of the plantar fascia.

Peripheral involvement: In addition, ankylosing spondylitis can involve the peripheral joints. Fifty percent of patients show symptoms of oligoarthritis (e.g., of the knee, hand, or ankle joints).

Uveitis: Acute anterior uveitis occurs in about 25% of patients, either as a single event or several. Uveitis includes acute inflammation of the uvea (vascular layer of the eyeball) and of the ciliary body. Acute anterior uveitis is therefore also referred to as iritis or iridocyclitis.

Affected patients often experience blurred vision and general visual impairments. The temporal connection between uveitis and arthritis activity is mostly accidental.

Cardiac complications: Cardiac complications are very rare. If they occur, they do so in the form of conduction disorders (AV blocks, left bundle branch block) and aortic insufficiency after aortitis. The frequency of cardiac involvement depends on the duration of the disease. Examples from the literature strongly differ.

Intestinal involvement: About 5–10% of patients experience colitis or terminal ileitis. Fifty percent show clinically silent intestinal inflammation in endoscopic studies.


Classification criteria for ankylosing spondylitis

Currently, the most common classification criteria for ankylosing spondylitis are the modified New York criteria. These criteria require 1 of the 2 following conditions to be present: radiographic evidence of sacroiliitis and > 1 further criterion; or HLA-B27 positive and > 2 further criteria:

As per the 2009 Criteria for the Classification of Axial Spondyloarthritis (which is applicable to patients with back pain ≥ 3 months and age of onset of 45 years) with regard to the diagnosis of ankylosing spondylitis, either of 2 criteria ‘Sacroiliitis on Imaging Plus ≥ 1 Spondyloarthritis Feature (SpA)’ or “HLA-B27 Plus ≥ 2 Other SpA Features’ is sufficient.

SpA features include:

  • Inflammatory back pain
  • Arthritis
  • Enthesitis (heel)
  • Anterior uveitis
  • Dactylitis
  • Psoriasis
  • Crohn disease or ulcerative colitis
  • Good response to NSAIDs
  • Family history of SpA
  • HLA-B27
  • Elevated CRP
Image: “Anterior uveitis” by Jonathan Trobe. License: CC BY 3.0

Image: “Anterior uveitis” by Jonathan Trobe. License: CC BY 3.0

Clinical Presentation of Ankylosing Spondylitis

Different functional tests of the spine and sacroiliac joints are used for diagnosis and follow-up examinations. The Bath ankylosing spondylitis disease activity index (BASDAI) score is generally used clinically to measure the severity and plan the treatment.

Joint Examination
Sacroiliac joints Pressure pain threshold, Mennell sign
Lumbar spine Reduced measure in Schober test
Thoracic spine Reduced measure in Ott test
Cervical spine Distance between chin and sternum with flexed head > 2 cm (normal: 0 cm)
Rib joints Limited chest expansion: < 4 cm with maximum inspiration and expiration (normal: > 4 cm)

Mennell Sign for Ankylosing Spondylitis

Retroflexion of the upper leg provokes pain in the sacroiliac joint.

Laboratory and Imaging Tests

Laboratory results of ankylosing spondylitis

No laboratory test is diagnostic for ankylosing spondylitis. Laboratory tests show slightly increased inflammatory parameters (CRP, BGL?), and the rheumatoid factor is negative.

Imaging of Ankylosing Spondylitis

The most important pathological localizations of ankylosing spondylitis are the sacroiliac joints, spine, and hip joints. The optimal radiological method for imaging the sacroiliac joints is MRI. Typical findings in patients with acute inflammation are irregularities of the joint cavities with erosions and bone edema. Active inflammation of the sacroiliac joints on MRI is considered equivalent to definite radiographic sacroiliitis.


Image: “Bamboo spine in a patient with ankylosing spondylitis” by Stevenfruitsmaak. License: CC BY-SA 3.0

In the early stages, bone scintigraphy shows symmetric pathological accumulations at the sacroiliac joints and parts of the spine.

The plain radiograph in a progressed stage shows ankylosis, sclerosis, and erosions. Important findings at the spine are syndesmophytes. Syndesmophytes are bony protrusions from the edge of one vertebra to the next and they appear primarily at the thoracolumbar junction.

In the final stage, the so-called bamboo spine becomes apparent, which is accompanied by a complete inability to move.

Differential Diagnoses

With regard to differential diagnoses, the most relevant are other spondyloarthropathies  including:

  • Psoriatic arthritis
  • Crohn disease, Ulcerative colitis
  • Post-infectious genesis (Reactive arthritis, Reiter syndrome)

Furthermore, degenerative changes in the joints and spine are possible differential diagnoses that can trigger similar symptoms.


Treatment of ankylosing spondylitis has significantly changed in the past decades. In the 1990s, the focus was mainly on physical therapy and balneotherapy. With the development of TNF-α inhibitors, a new era of treatment for ankylosing spondylitis was introduced.

Note: Treating the cause of ankylosing spondylitis is not currently possible.

Treatment focuses on pain reduction and a decrease of inflammatory activity. The goal is to halt the deterioration of movement impairment and deformation of the spine.

Basic Treatment of Ankylosing Spondylitis

  • NSAIDs: These represent the most important drug group against inflammatory pain in the axial skeleton, and are first-line therapy.
  • Local steroid administration: Temporary, in case of extreme exacerbations
  • Physical therapy for the spine: Crucial for maintaining mobility. The patient must learn to perform exercises that aim at the mobilization of the vertebral joints and muscle stability. Exercise to maintain posture and range of motion is a part of treatment at all stages of the disease.
  • Balneotherapy: Alleviates symptoms and improves mobility
  • Warmth and cold: Warmth is good for combating the spine pain. Cooling is recommended for peripheral arthritis.

Treatment with TNF-α Inhibitors

  • If patients do not respond to the basic treatment, TNF-α inhibitors can be used. Studies have shown the efficacy of infliximab, etanercept, adalimumab, golimumab, and certolizumab pegol. These substances are mainly aimed at reducing pain and fatigue and at improving mobility.
  • Other symptoms of ankylosing spondylitis, such as enthesitis, which are resistant to treatment with NSAIDs, and extra-articular manifestations respond well to TNF-α inhibitors.
  • Other agents being experimented in refractory patients include secukinumab and tofacitinib.

Surgical Interventions

If the disease is in a markedly progressed state, surgical measures can be considered. While fusion of the spine through spondylodesis and wedge osteotomy cannot restore the original mobility, it improves the quality of life of the patient. If the hip joints are affected, partial or complete prostheses may be considered.

Disease Course and Prognosis

The course of ankylosing spondylitis is variable and ranges from mild stiffness and normal radiographs on one hand to a totally fused spine on the other. Some patients develop severe bilateral hip arthritis, severe peripheral arthritis, and extra-articular manifestations.

For about 30% of the patients, the severe functional impairments lead to psychosocial problems at work, in everyday life, and with recreational activities.

The complete clinical manifestation of ankylosing spondylitis comprises chronic pain and complete stiffening of the spine.

Unfavorable factors for disease prognosis are:

  • Male sex
  • Young age of onset (< 25 years)
  • Early peripheral arthritis (especially coxitis)

The use of TNF-α inhibitors has significantly improved these unfavorable prognostic factors.

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