Ankylosing spondylitis (axial spondyloarthritis), a type of seronegative spondyloarthropathy, is a chronic inflammatory disease of the axial skeleton that leads to partial or even complete fusion and rigidity of the spine. Males are disproportionately affected and upwards of 90% of patients are positive for the HLA-B27 genotype, which predisposes to the disease. The most characteristic early finding is pain and stiffness in the neck and lower back, caused by inflammation of the vertebral column and the sacroiliac joints. The pain typically improves with activity and is especially prominent at night. Other articular findings include tenderness to percussion and displacement of the sacroiliac joints (Mennell's sign), as well as limited spine mobility, which can progress to restrictive pulmonary disease. The most common extra-articular manifestation is acute, unilateral anterior uveitis. Diagnosis is primarily based on symptoms and x-ray of the sacroiliac joints, with HLA-B27 testing and MRI reserved for inconclusive cases. There is no curative treatment, but regular physiotherapy can slow progression of the disease. Additionally, NSAIDs and/or tumor necrosis factor-α inhibitors may improve symptoms. In severe cases, surgery may be considered to improve quality of life.
Epidemiological data refers to the US, unless otherwise specified.
- Genetic predisposition: 90–95% of patients are HLA-B27 positive.
- Most common presenting symptoms: back and neck pain
- Limited mobility of the spine (especially reduced forward lumbar flexion)
- Inflammatory enthesitis (e.g., of the Achilles tendon, iliac crests, tibial tuberosities): painful on palpation
- Arthritis outside the spine (hip, shoulder, knee joint)
- Most common: acute, unilateral (∼ 25% of cases)
- Fatigue , weakness, fever, weight loss
- Restrictive pulmonary disease due to decreased mobility of the spine and thorax
- Gastrointestinal symptoms: associated with chronic inflammatory bowel disease (∼ 5–10% of cases, see also: or )
- Physical examination, patient history, and pelvic x-ray: If results are conclusive, no additional testing is required!
- If inconclusive → HLA-B27 testing
- If still inconclusive → pelvic MRI
Chest expansion measurement: to monitor disease severity
Method: measure chest circumference in full expiration and inspiration
- Pathological difference: < 2 cm
- Physiological difference: > 5 cm
- Method: measure chest circumference in full expiration and inspiration
Spine mobility tests
- Schober test : Mark two points, S1 and another point 10 cm above → patient touches toes (without bending the knees) → distance between the two points increases by ≥ 4 cm → physiological test result; a smaller increase in distance between these two points is pathological
- Examination of the hip 
The degree of decrease in chest expansion is an important determinant of disease severity.
- ↑ CRP and ESR
- Auto-antibodies (e.g., rheumatoid factor, antinuclear antibodies) are negative
- HLA-B27 positive in 90–95% of cases
- Helps confirm a diagnosis and evaluate the severity of disease
- Changes are generally more evident in later disease.
- The changes usually occur symmetrically.
Pelvis (best initial test): to examine the sacroiliac joints
- Signs of sacroiliitis, including ankylosis (fusion of the articular surfaces)
- Loss of lordosis with increasing abnormal straightening of the spine
- Sclerosis of the vertebral ligamentous apparatus
- Syndesmophytes resulting in a so-called 'bamboo spine' in anteroposterior radiograph in the later stages (see the table in “Differential diagnosis” below)
- Signs of spondyloarthritis, including ankylosis of intervertebral joints 
- Thorax: ankylosis of costosternal and costovertebral joints
- Mechanical low back pain
- Disc prolapse
- Vertebral osteomyelitis
- Other spondyloarthritides (e.g., reactive arthritis, psoriasis arthritis, arthritis associated with inflammatory bowel disease)
Diffuse idiopathic skeletal hyperostosis (DISH; also called Forestier's disease or hyperostotic spondylosis)
- Definition: degenerative disease of the vertebral column (especially the thoracic and lumbar spine), which is characterized by calcification and ossification of spinal ligaments and entheses
- Limited mobility
- Mild or even no pain at all
- Treatment: symptomatic
- Osteophytes of the spine 
The differential diagnoses listed here are not exhaustive.
- Consistent and rigorous physical therapy
- Independent exercises
- Medical therapy
- Surgery: in severe cases to improve quality of life
Physical therapy is the most important treatment modality!References:
- Complete fusion of the spine → severely limited mobility
- Increased risk of osteoporosis → pathological fractures and possibly spinal cord injury
- Restricted chest expansion and spine mobility → breathing difficulties
- Lungs: fibrosis of upper lobes (apical fibrosis) 
We list the most important complications. The selection is not exhaustive.