Ankylosing spondylitis

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 joins 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.

• Axial spondyloarthritis ^[1]
  • Chronic inflammatory arthritis affecting the axial skeleton
  • Affected individuals are commonly positive for HLA-B2.
  • Extraarticular manifestations such as anterior uveitis, inflammatory bowel disease (IBD), and enthesitis are common.
• Ankylosing spondylitis (radiographic axial spondyloarthritis; AS) ^[2]
  • Symptoms of axial spondyloarthritis with inflammatory changes in the sacroiliac joints and spinal ankylosis seen on plain radiographs
• Nonradiographic axial spondyloarthritis (nr-axSpA) ^[1]
  • Symptoms of axial spondyloarthritis without features of sacroiliitis or spinal ankylosis on plain radiographs
  • The majority of patients have sacroiliitis detectable on MRI.
  • Up to 30% of affected individuals will progress to AS within 10 years of diagnosis.

• Sex: ♂ > ♀ (3:1)
• Age: 15–40 years
• Lifetime prevalence: ∼ 0.5%

Epidemiological data refers to the US, unless otherwise specified.

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

Articular manifestations

• Features of inflammatory back pain (most common presenting symptom) ^[3][4]
  • Age at onset < 45 years
  • Insidious onset of dull pain that progresses slowly
  • Morning stiffness > 30 minutes that improves with activity
  • Pain is independent of positioning
  • Pain persists with rest and is also present at night.
  • Alternating gluteal pain
• Tenderness over the sacroiliac joints (SIJ)
• Reduced spinal mobility
• Extraspinal joint pain
  • Inflammatory enthesitis
  • Dactylitis
  • Arthritis outside the spine (hip, shoulder, knee joint)

Extraarticular manifestations

• Acute, unilateral anterior uveitis (most common extraarticular manifestation; ∼ 25% of patients)
• Constitutional symptoms such as fatigue , weakness, fever, and weight loss
• Restrictive pulmonary disease
  • Due to decreased mobility of the thoracic spine and costovertebral joints
  • Secondary to apical pulmonary fibrosis or more widespread interstitial lung disease ^[5]
• Gastrointestinal symptoms: due to associated chronic IBD (∼ 5–10% of patients)
• Prostatitis
• Aortic root inflammation and subsequent aortic valve insufficiency, atrioventricular blocks ^[6][7]
• IgA nephropathy (rare)

Positive SIJ pain provocation tests

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

FABER test ^[8]

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 ^[9]

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 ^[10]

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. ^[11]
• 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 ^[11]

• 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. ^[3][11]
• Reduced chest expansion can also be due to concomitant pulmonary disease, chest wall deformities, or poor respiratory effort.

Kyphosis ^[10][12][13]

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.

Approach ^[4][14]

• 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
  • 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
  • Some (2–3) classical clinical features of AS present: MRI of the sacroiliac joints
  • ≤ 1 classical clinical feature: Consider differential diagnoses of AS.
• 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 ^[3]

• Lower back pain for > 3 months in patients < 45 years of age and one of the following:
  • Sacroiliitis confirmed on x-ray or MRI and ≥ 1 typical clinical or laboratory finding
  • A positive HLA-B27 test and ≥ 2 typical clinical or laboratory findings
• Clinical and laboratory findings used in the diagnostic criteria for AS include:
  • Articular manifestations: inflammatory back pain, dactylitis, arthritis
  • Extraarticular manifestations: enthesitis (may present with heel pain), uveitis, IBD, psoriasis
  • Patient history
    • Positive family history for axial spondyloarthritis
    • Symptoms relieved within 24–48 hours of full-dose NSAIDs
  • Laboratory findings: positive for HLA-B27; ↑ CRP and/or ESR

Laboratory findings

• CRP and ESR are typically elevated.
• HLA-B27: Positive in 90–95% of patients with axial spondyloarthritis ^[15][16]
• Autoantibodies (e.g., rheumatoid factor, antinuclear antibodies) are negative
• CBC: may show anemia ^[17]

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 ^[14]

X-ray

Sacroiliac joints (PA view) ^[3]

• Indication: best initial test to confirm the diagnosis of AS and evaluate disease severity
• Characteristic findings (usually symmetrical)
  • Signs of sacroiliitis: erosion and sclerosis (increased radiodensity) of the sacroiliac joints
  • Ankylosis: fusion of the articular surfaces

Spine (AP and lateral views)

• Indications
  • Pain in the cervical, thoracic, or lumbar spine
  • Suspicion of AS remains after a negative radiograph and MRI of sacroiliac joints
• Characteristic findings
  • Loss of lumbar lordosis: abnormal straightening of the spine
  • Ankylosis of costosternal and costovertebral joints
  • Dagger sign
    • A radiodense line running through the center of vertebral bodies on AP view
    • Caused by ossification of vertebral ligaments
  • Bamboo spine: seen in later stages and is caused by the following
    • Ossification of outer fibers of the annulus fibrosis resulting in ankylosis (fusion) of intervertebral joints
    • Syndesmophytes between adjacent vertebral bodies (see “Syndesmophytes vs. osteophytes”)
  • Shiny corners sign: reactive sclerosis of the superior and inferior margins of vertebrae

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. ^[4][14]

MRI

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. ^[18]

Sacroiliac joints with gadolinium contrast

• Indication: negative or equivocal radiographs of sacroiliac joints in a patient with some classical features of AS ^[4]
• Characteristic findings
  • Active inflammatory lesions: erosion of the sacroiliac joints and bone marrow edema (high signal intensity) ^[4]
  • Reparative changes: secondary bone formation (low signal intensity)

• Other inflammatory spondyloarthritides (e.g., reactive arthritis, psoriasis arthritis, arthritis associated with inflammatory bowel disease)
• Mechanical low back pain
• Fibromyalgia
• Disc prolapse
• Vertebral osteomyelitis
• Osteophytes of the spine ^[20][21]
• See also “Nonurgent spinal causes of acute back pain.”
• Diffuse idiopathic skeletal hyperostosis (DISH; also called Forestier's disease or hyperostotic spondylosis): degenerative disease of the vertebral column (especially the thoracic and lumbar spine), which is characterized by calcification and ossification of spinal ligaments and entheses
  • Not related to HLA-B27 ^[22]
  • May be asymptomatic or present with limited spinal mobility
  • Back pain may be absent or mild
  • Mostly affects men and is more common in patients with diabete
  • X-ray of the spine : osteophytes may be present (see table below); no evidence of sacroiliitis
  • Treatment is primarily symptomatic

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

The differential diagnoses listed here are not exhaustive.

Supportive therapy

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

• Provide regular physical therapy to maintain range of motion and posture ^[4]
• Refer patients for:
  • Fall evaluation and counseling
  • Group or individual patient education on self-management
• Encourage smoking cessation. ^[23][24]
• Screen and treat patients for osteoporosis. ^[25]
• Advise patients with severe osteoporosis or spinal fusion to avoid spinal manipulation.

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

Management of active disease

Pharmacological therapy

• First-line pharmacotherapy in most patients: NSAIDs
• Second-line : TNF-α inhibitors (e.g., etanercept, adalimumab).
• Alternative therapies (e.g., DMARDs such as sulfasalazine, intraarticular glucocorticoids): Consider in patients with significant peripheral disease or insufficient response to first-line therapies.

Advanced therapeutic options ^[2]

• Tofacitinib (a targeted DMARD): Consider in patients who do not respond to NSAIDs, TNF-α inhibitors, or antiinterleukin-17 antibodies.
• Surgery
  • Only indicated in patients with severe functional impairment to improve their quality of life.
  • Options include:
    • Hip arthroplasty for severe hip arthritis
    • Spinal osteotomy in patients with severe kyphosis causing significant pain, physical disability, or psychological distress ^[27]

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

• Complete fusion of the spine → severely limited mobility
• Increased risk of osteoporosis → pathological fractures and possibly spinal cord injury
• Chalk stick fractures: transverse fractures through the fused spinal column caused by reduced flexibility due to a fused spine and osteoporosis
• Restricted chest expansion and spine mobility → breathing difficulties
• Lungs: fibrosis of upper lobes (apical fibrosis) ^[5]

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

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