---
title: Ankylosing Spondylitis
type: condition
specialty: Rheumatology
aliases:
cssclasses:
  - img-grid
updated: 2026-04-06T07:53
created: 2026-04-04T14:34
---

> [!etymology] Etymology
> **Ankylos** (bent/stiff) + **spondylos** (vertebra) + **-itis** (inflammation)

# Theory 
## Definition
- Ankylosing spondylitis (AS) is a chronic inflammatory disorder that primarily affects the axial skeleton (in particular the sacroiliac joints and spine) [^2].
- It is a seronegative condition – it has no specific antibodies or markers like other rheumatological conditions.

## Epidemiology and Risk Factors
- AS is most commonly diagnosed before the age of 40. It is more prevalent in men than in women (4:1). There is a strong association with HLA-B27 gene.

## Pathophysiology
- The cause of AS is unclear, however it results in chronic inflammation that affects the axial skeleton, in particular the lumbar spine and sacroiliac joints. It may be the result of genetic predisposition and exposure to environmental triggers. 
- HLA-B27 aggregates in joints and entheses and leads to a degenerative cascade, with a cytotoxic T-cell autoimmune reaction against HLA-B27 aggregates.
- Enthesitis (inflammation of tendon attachments) is the particular driving force, as it leads to bony erosion, surrounding soft-tissue ossification, and eventually joint ankylosis (fusion. 
- Inflammation of the annulus fibrosis (tough outer layer) of the interverterbal discs lead to bridging osteophyte formation (syndesmophytes).
- AS can affect other organ systems including the skin (psoriasis), bowels (inflammatory bowel disease), respiratory (pulmonary fibrosis), eyes (uveitis), cardiovascular (conduction issues, aortic root dilatation).

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# Diagnosis 

## Presentation
- Symptoms:
	- Lumbosacral pain – insidious onset and worse in morning (improves with activity)
	- Neck and thoracic pain – later sign, typically later in life with untreated disease
	- Shortness of breath – may occur if costovertebral joints are involved
- Signs:
	- Reduced [[Schrober Test|Schrober test]] (e.g. <15cm in lumbar flexion).
	- Positive [[Faber Test|Faber test]] indicating sacroiliac damage
	- Reduced chest expansion
	- Spinal kyphosis

![[Ankylosing Spondylitis - Schrober Test.png]] ![[Ankylosing Spondylitis - Faber Test.png]]

## Differentials
1. Diffuse Idiopathic Skeletal Hyperostosis
2. Psoriatic Arthritis
3. Lumbar spinal stenosis
4. Mechanical lower back pain

## Investigations
- Bloods:
	- Acute phase reactants – ↑ CRP and ESR
- Imaging 
	- May have no imaging changes in early disease
	- Recommended approach is full-length AP and lateral of spine + Ferguson pelvic tilt view of hips (to assess for SI joint erosion or issues) [^1]
	- Lumbar x-ray may demonstrate bamboo spine in advanced disease (lumbar fusion)
	- MRI is the best imaging modality for changes in early disease in young patients as can 
- Genetics 
	- Typically HLA-B27 positive (90% of patients)


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# Management

## Medications
- First-line:
	- NSAIDs (e.g. diclofenac) and COX-2 inhibitors (e.g. celecoxib)
	- Used to manage pain and inflammation
- Second-line:
	- TNF-α blockers (e.g. infliximab, adalimumab, etanercept)
	- Block Tumour Necrosis Factor α, a pro-inflammatory cytokine released by macrophages. 
	- IL-17 inhibitors are being introduced as novel treatments (Secukinumab)

## Physiotherapy
- Focus on maintaining mobility and flexibility

## Operative Approaches
- Spinal decompression with instrumented fusion, typically used for spine trauma 
- Lumbar osteotomy for patients with spinal deformity 

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# Outcomes

## Complications
- Chronic pain and disability
- Aortic regurgitation or root dilatation
- Pulmonary fibrosis
- Cauda equina syndrome
- Mood disorders

## Prognosis
- Earlier onset is associated with poorer functional outcomes, however as treatments improve this may improve functional outcomes in the future for this patient group.
- Severe physical disability is relatively uncommon in AS. Most patients have good levels of functioning and live active and fulfilling lives.
- Mortality is increased due to cardiovascular complications.

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# Study
## Key Points
- Classical exam presentation is a man <40 with insidious-onset inflammatory back pain
- Physical examination may show ↓ chest expansion, ↓ lumbar mobility and sacroiliac joint pain
- HLA-B27 testing is supportive but not truly diagnostic 
- Many patients with early disease with no imaging treatments
- First-line treatment is anti-inflammatories and physiotherapy

## Flashcards

T | Ankylosing Spondylitis – Pathophysiology |
CQ | The pathophysiology of AS involves HLA-B27 aggregation triggering a cytotoxic T-cell reaction, with {{c1::enthesitis}} driving bony erosion and subsequent {{c1::syndesmophyte}} formation that leads to joint {{c1::ankylosis}} |
I | Enthesitis is the primary driver of structural damage in AS, distinguishing it from other arthritides. |
G | Rheumatology |

T | Ankylosing Spondylitis – Genetic Association |
Q | Which HLA allele is strongly associated with Ankylosing Spondylitis, and what is its approximate positivity rate in patients? |
A | HLA-B27 (positive in approximately 90% of patients) |
G | Rheumatology |

T | Ankylosing Spondylitis – Inflammatory Back Pain |
Q | What are the three classic clinical features of inflammatory back pain in Ankylosing Spondylitis? |
A | Insidious onset, worse in the morning, and improves with activity |
I | These features help differentiate inflammatory back pain from mechanical causes. |
G | Rheumatology |

T | Ankylosing Spondylitis – Pharmacological Management |
CQ | First-line pharmacological management for AS consists of {{c1::NSAIDs}} and {{c1::COX-2 inhibitors}}, while second-line therapy includes {{c2::TNF-alpha blockers}} and {{c2::IL-17 inhibitors}} |
I | Biologics are typically introduced when NSAIDs fail to control symptoms or disease progression. |
G | Rheumatology |

T | Clinical Scenario – Back Pain |
MCQ | A 26-year-old man presents with 10 months of low back pain and morning stiffness. Pain improves with exercise but not rest. He reports alternating buttock pain and reduced spinal flexibility. He recently started a desk job involving prolonged sitting. What is the best next step in management? |
A | Pelvic and lumbar spine x-rays |
O | CT scan of lower back | 
O | Analgesia |
O | Serology |
G | Rheumatology |

T | Ankylosing Spondylitis – Pathophysiology |
OQ | Put the pathophysiological sequence of ankylosing spondylitis in order |
1 | Enthesitis initiation |
2 | Cytokine-driven inflammation | 
3 | Erosion and repair | 
4 | Ankylosis | 
G | Rheumatology |

[^1]: Orthobullets – Ankylosing Spondylitis (https://www.orthobullets.com/spine/2044/ankylosing-spondylitis)
[^2]: NIH StatPearls – Ankylosing Spondylitis (https://www.ncbi.nlm.nih.gov/books/NBK470173/)

