Ankylosing spondylitis

Ankylosing spondylitis (AS) is a type of axial spondyloarthropathy that most commonly affects young men (aged 20-30 years old). It is sometimes referred to as Bechterew disease or Marie Strümpell disease.

Pathophysiology

AS results in ankylosis of the joints. The term ankylosis means stiffness/fixation of a joint (either through pathological or surgical methods). The mechanism by which this occurs is immune-mediated.

There are 3 key processes that occur in AS:

1. Inflammation of the joint
2. Cartilage erosion
3. Reparation/ossification

It is this 3rd step that differs AS from rheumatoid arthritis (RA). This osteoproliferative process then ossifies the annulus fibrosus of the intervertebral discs which then results in fusion of the joints due to syndesmophyte formation. Syndesmophytes are bony growths that originate inside a ligament. They differ from osteophytes, which are degenerative osteoproliferative lesions, by being inflammatory osteoproliferative lesions. These syndesmophytes then bridge together and cause ankylosis of the joint.

⚠️ Risk factors

• HLA-B27 - about 90% of patients with AS have the HLA-B27 gene.
• Family history
• Male sex - affected at a ratio of 3:1 when compared to women.
• Klebisella pneumoniae - most patients with AS have antibodies against klebsiella pneumoniae and so this could be a potential trigger but its role is still unclear.

😷 Presentation

⭐️ The predominating symptom is back pain. This pain can be described as inflammatory back pain. It commonly affects the lumbar and cervical spine.

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What characterises inflammatory back pain?

1. Early morning stiffness - lasts >30 minutes.
2. Improves with activity
3. Worse at night - may waken the patient up from sleep but improves when they get up.
4. Chronic pain - it has a gradual (insidious) onset over a period of >3 months.

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On examination:

• Reduced lateral flexion
• Reduced forward flexion - we will discuss this further when looking at physical exam tests in the investigations section of this page.
• Reduced chest expansion - as mentioned previously.
• Dorsal kyphosis
• Loss of lumbar lordosis

Other symptoms include:

• Enthesitis - inflammation at the region where tendons/ligaments insert into the bone. Most commonly in the lower limbs (such as achilles tendonitis and plantar fasciitis).
• Peripheral arthritis - more common in females. Most commonly affecting hips and shoulders. The involvement is typically asymmetrical.
• Dyspnoea - this is due costovertebral joint involvement leading to reduced chest expansion. Lung expansion may also be reduced due to kyphosis deformation of the spine. Spirometry may indicate restrictive lung disease as a result.
• Fatigue
• Weight loss

Let’s take a look at some of the extra-articular manifestations that may arise with AS:

• Psoriasis
• Inflammatory bowel disease - ~60% of AS patients have histological evidence of bowel inflammation (although only approximately 7% have a concomitant diagnosis).
• Anterior uveitis - if anterior uveitis is suspected then refer to ophthalmology urgently (same day).
• Aortitis - which can lead to aortic regurgitation.
• Apical lung fibrosis
• AV node block
• AA (secondary) amyloidosis - this is a rare complication.
• IgA nephropathy
• Cauda equina syndrome - once again, a rare complication in advanced disease and its mechanism is poorly understood.

🔍 Investigations

It is important to note that there is no single test that can diagnose or rule out AS.

🥇 The first thing we can order are simply inflammatory markers such as CRP/ESR. However, this does not rule out diagnosis if they are normal.

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Imaging:

🥇 Plain X-rays are the most useful investigation to assess disease.

• Pelvic X-ray findings:
• Sacroiliitis - indicated by subchondral erosions and sclerosis. It may be unilateral or bilateral.
• Ankylosis - in advanced disease.

• Spinal X-ray
• Squaring of lumbar vertebral bodies
• Syndesmophyte formation
• ⭐️ Syndesmophyte bridging - this leads to the characteristic “bamboo spine” in later stages.

🏆 MRI - if X-ray findings are negative but suspicion remains high. It is the most sensitive test and can show abnormalities despite normal X-ray findings. It is especially useful in early disease.

DEXA scan - should be performed every 2 years to assess for osteoporosis in patients with axial AS. Osteoporosis is the most common complication and predisposes patients to fractures. It is important to note that spinal DEXA scans may indicate raised bone density (due to syndesmophyte formation) and therefore a pelvic measurement is more reliable.

Let’s also quickly look at a physical examination which may prove useful:

Schober’s test

This test is used to assess if there is a limitation of the movement within the lumbar spine. It is worth remembering how to perform it for OSCE purposes:

1. Ask the patient to stand up straight.
2. Identify the L5 vertebrae.
3. Mark a point 10cm above this point, as well as a point 5cm below this point (15cm total distance).
4. Ask the patient to touch their toes/bend forward as far as they can.
5. The distance should increase by a further 5cm at least (20cm total distance), otherwise it indicates limited lumbar flexion and may support a diagnosis of AS.

NICE provides guidelines on who to refer to rheumatology, as their assessment is required for confirmation of diagnosis:

3 criteria need to be met:

1. Low back pain beginning before 45 years old
2. Pain lasts >3 months
3. 4 or more of the following additional criteria:
1. Low back pain beginning before 35 years old
2. Symptoms wake them up in the second half of the night
3. Buttock pain
4. Improves with activity
5. Improves within 48 hours of NSAID use
6. Spondyloarthritis in 1º relative
7. Current/past arthritis, enthesitis, psoriasis.

• If these 3 criteria are met → refer to rheumatology.
• If 3 of the additional criteria are met → do HLA-B27 test.
• If positive → refer to rheumatology.
• If negative or they have less than 3 of the addition criteria met → monitor and refer them for further assessment if they have any new signs, symptoms or risk factors.

🧰 Management