What is ankylosing spondylitis?
Ankylosing spondylitis (often referred to as Bechterew's disease) belongs to the group of spondyloarthropathies, alongside psoriatic, reactive, and enteropathic arthritides.
The disease debuts with inflammatory back pain and is most frequently diagnosed in adolescents and young adults carrying the HLA-B27 antigen. The primary target of the pathological process is the axial skeleton (sacroiliac joints and spine); however, in one-third of patients, peripheral joints (specifically, hip and shoulder joints) are also affected.
Pathogenesis and spinal changes
The underlying cause of spinal involvement in AS is enthesitis—inflammation of the sites where ligaments, tendons, and joint capsules attach to bone.
In the discovertebral segment, inflammation initially involves the attachment sites of the peripheral fibers of the annulus fibrosus (Sharpey's fibers). As the disease progresses, typical lesions develop:
- Romanus lesions — sclerotic changes in the corners of the vertebral bodies, which appear as "shiny" corners on lateral radiographs. Subsequently, thin, vertically oriented bone bridges—syndesmophytes—form at these sites.
- Combined with ossification of the anterior longitudinal ligament, the vertebral bodies acquire a characteristic "squared" shape.
Diagnostic imaging: from early signs to ankylosis
The choice of imaging modality depends on the stage of the disease:
- MRI (magnetic resonance imaging): Considered the gold standard for early diagnosis. MRI can detect the very first sign of the disease—subchondral bone marrow edema in the sacroiliac joints and vertebral corners even before radiographic changes appear.
- Conventional radiography and CT: Demonstrate later structural changes. These scans visualize erosions, joint space narrowing, sclerosis, and later, osteoproliferation (syndesmophytes).
- “Bamboo spine”: The classic appearance in late stages, reflecting complete fusion of the vertebral bodies and sacroiliac joints.
Spinal complications
An ankylosized ("fused") spine paradoxically becomes extremely fragile. Osteopenia frequently develops against the background of the disease.
- Fractures: The rigid spine is highly susceptible to trauma. Transverse fractures through ankylosed segments (especially in the cervical spine, e.g., C6/C7) are extremely unstable.
- Andersson lesion: When the majority of the spine has fused, motion concentrates in the remaining mobile segments. This leads to localized inflammatory damage that can be mistaken on radiographs for infectious spondylodiscitis (due to contour irregularity and sclerosis).
Briefly: answers to topical questions
What are the radiographic stages of sacroiliitis described in Bechterew's disease?
In Bechterew's disease, early, late, and end-stage changes of sacroiliitis are described:
- Early sign — subchondral bone marrow edema is detected on MRI.
- Late changes — erosive changes and subchondral sclerosis are detected on conventional radiography and CT.
- End stage — in the absence of treatment, changes progress to complete bony ankylosis of the sacroiliac joints.
For a definitive diagnosis, the criterion is the presence of bilateral stage II–IV sacroiliitis combined with any of the three clinical signs.
How do syndesmophytes in ankylosing spondylitis differ from osteophytes in deforming spondylosis?
Syndesmophytes in ankylosing spondylitis differ from osteophytes primarily in shape and direction of growth:
| Sign | Syndesmophytes | Osteophytes |
|---|---|---|
| Orientation | Vertical | Not specified in sources |
| Thickness | Thin | Not specified in sources |
Initially, syndesmophytes appear as thin bony outgrowths developing at the sites of Romanus lesions—sclerosis of the vertebral body corners. Their vertical orientation and thinness help distinguish them from osteophytes. Later, they form bridging structures, leading to the "bamboo spine" sign.
What extra-articular manifestations are characteristic of ankylosing spondylitis?
Ankylosing spondylitis is characterized by multiorgan pathology and the development of the following extra-articular manifestations:
- Ocular involvement — occurs in approximately one-third of patients (iritis, uveitis, episcleritis).
- Renal involvement — development of renal amyloidosis, which is complicated by renal failure and uremia.
- Cardiovascular involvement — most commonly presents as aortitis, which can lead to aortic and mitral valve disease.
- Respiratory system involvement — indirect impairment due to restricted chest wall excursion, predisposing to respiratory diseases and tuberculosis.
- Enthesopathies — pain in the region of the Achilles tendon insertion or plantar aponeurosis.
Class and exam questions
What are Romanus lesions?
These are sclerotic changes in the vertebral body corners at the attachment sites of Sharpey's fibers, resulting from enthesitis. On radiographs, they appear as "shiny" corners.
Why is the risk of spinal fractures high in ankylosing spondylitis?
Due to the development of osteopenia and total ankylosis, the spine becomes rigid (losing its shock-absorbing properties), making it extremely vulnerable even to minor trauma.
What else to study on this topic
- Radiographic signs of peripheral joint involvement in AS
- Comparison of syndesmophytes in AS and psoriatic arthritis
- Differential diagnosis of AS and degenerative spinal diseases on MRI
- Features of shoulder and hip joint involvement in AS
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