Ankylosing spondylitis

Ankylosing spondylitis (also known as Bechterew's disease and Marie Strümpell disease) is a seronegative spondyloarhropathy which characteristically (as the name suggests) results in fusion (ankylosis) of the spine and sacroiliac joints, although involvement is also seen in large and small joints. 

Epidemiology

There is male predilection of 3:1 or more. It usually manifests in adults, with the first symptoms becoming evident in the third decade, although up to 18% of cases manifest in the second decade. 

Pathology

Genetics 

Patients are rheumatoid factor (Rh) negative (hence seronegative). Approximately 90% of Caucasian individuals have the HLA-B27 gene ⁵. 

Associations

• anterior uveitis
• psoriasis
• ulcerative colitis/ Crohns disease
• upper lobe predominant interstitial lung disease with small cystic spaces (occurs in
approximately 1% of patients) ⁴
• aortic valve disease / aortitis
• amyloidosis (rare)

Radiographic features

Features predominantly affect the axial skeleton although can involve the peripheral joints in ≈ 20% of cases

Plain film

Sacroiliac joints

• a sacroiliitis is usually the first manifestation ⁵ and is symmetrical and bilateral
• joints widen before they narrow
• subchondral erosions, sclerosis and proliferation on the iliac side of SI joints
• at endstage, the sacroiliac joint may be a thin line or not visible

Spine

• early spondylitis is characterized by small erosions at the corners of vertebral bodies with reactive sclerosis
• squaring of the vertebral body
• diffuse syndesmophyitic ankylosis can give a "bamboo spine" appearance
• interspinous ligament calcification can give a "dagger spine" appearance
• ossification of spinal ligaments, joints and discs.
• pseudoarthroses may form at fracture sites.
• enthesophyte formation from enthesopathy.
• Romanus lesions of the spine - shiny corner sign.

Hip(s)

Hip involvement is generally bilateral and symmetric, with uniform joint space narrowing, axial migration of the femoral head, and a collar of osteophytes at the femoral head-neck junction.

Knee(s)

Knees demonstrate uniform joint space narrowing with bony proliferation

Hand(s)

Hands are generally involved asymmetrically, with smaller, shallower erosions and marginal periostitis.

Chest

Radiographs of the lungs may demonstrate progressive fibrosis and bullous changes at the apices. These lesions may resemble TB infection and bullae may become infected.

CT

• CT may be useful in selected patients with normal or equivocal findings on sacroiliac joint radiographs. 
• joint erosions, subchondral sclerosis, and bony ankylosis are better visualized on CT.
• some normal variants of the SI joints may mimic features of sacroiliitis
• CT supplements scintigraphy in evaluating areas of increased uptake
• multi-detector CT (MDCT) is superior to radiographs and MRI in demonstrating injuries
• MDCT is imaging modality of choice in patients with advanced ankylosing spondylitis for whom there is suspicion of cervical spine fracture.
• sagittal reformats should be obtained as axial images poorly assess the transverse fracture plane

MRI

• may have a role in early diagnosis of sacroiliitis
• synovial enhancement on MR correlates with disease activity measured by inflammatory mediators
• enhancement of the interspinuous ligamants is indicative of an enthesitis
• increased T2 signal correlates with edema or vascularized fibrous tissue
• superior to CT in detection of cartilage, bone erosions, and subchondral bone changes
• useful in following treatment results in patients with active ankylosing spondylitis

Bone scintigraphy

• may be helpful in selected patients with normal or equivocal findings on sacroiliac joint radiographs
• qualitative assessment of accumulation of radionuclides in the SI joints may be difficult due to normal uptake in this location. Thus quantitative analysis may be more useful.
• ratios of SI joint to sacral uptake of 1.3:1 or higher is abnormal

Complications

Fracture(s) : Diffuse paraspinal ossification and inflammatory osteitis creates a fused, brittle spine, susceptible to fracture, even with minor trauma. Fractures are more common at the thoracolumbar and cervicothoracic junctions. Recognition of minimally displaced fractures is difficult due to osteopenia and deformity, and it is important to specifically search for disk space widening and discontinuity of the ossified paraspinal ligaments

Andersson lesion(s) : an Andersson lesion is an inflammatory spondylodiscitis which occur in association with ankylosing spondylitis and results in a disc pseudoarthrosis 

Treatment and prognosis

Treatment includes NSAIDs, physiotherapy and, for more severe cases, anti-TNF-alpha therapy.

Differential diagnosis

• general spine : enteropathic arthritis
• cervical spine : juvenile rhematoid arthritis (JRA)
• diffuse idiopathic skeletal hyperostosis