Ankylosing Spondylitis

Ankylosing spondylitis (AS) is a chronic inflammatory arthritis that primarily affects the spine and sacroiliac joints. Over time, persistent inflammation can cause the vertebrae to fuse together — a process called ankylosis — leading to progressive spinal stiffness and, in severe cases, a rigid, fixed spine. It is part of a broader group of diseases called spondyloarthropathies.

Ankylosing spondylitis is one of the most important conditions to distinguish from DISH, which produces similar spinal bridging and stiffness through a metabolic rather than inflammatory mechanism. As AS progresses, the resulting spinal rigidity and altered biomechanics can contribute to Compression Fractures — even with minor trauma — and the prolonged corticosteroid use sometimes employed in its treatment is a recognized cause of Epidural Lipomatosis.

Causes

Genetic and immune factors.

Ankylosing spondylitis has a strong genetic component. The HLA-B27 gene is present in the vast majority of people with AS, though not everyone with this gene develops the condition. The disease is caused by an abnormal immune response in genetically predisposed individuals, leading to chronic inflammation at the sites where tendons and ligaments attach to bone (entheses).

Other contributing factors

AS is more common in men and typically begins in late adolescence or early adulthood. Environmental factors, including certain gastrointestinal infections, may trigger the inflammatory process in susceptible individuals. Family history of AS or related conditions significantly increases risk.

Symptoms

The hallmark symptom is chronic low back pain and stiffness that is characteristically worse in the morning or after periods of inactivity, and improves with exercise and movement — the opposite pattern of mechanical back pain. Pain typically begins in the sacroiliac joints and progresses up the spine. Patients may also experience peripheral joint involvement, enthesitis (tendon and ligament inflammation), uveitis (eye inflammation), fatigue, and reduced chest expansion due to involvement of the costovertebral joints. In advanced disease, progressive spinal fusion can lead to significant postural deformity (kyphosis).

Diagnosis

Diagnosis is based on clinical features, imaging, and laboratory findings.

MRI of the sacroiliac joints and spine is the most sensitive early imaging tool, detecting bone marrow edema and active inflammation before structural changes are visible on X-ray. A lumbar spine MRI evaluates the sacroiliac joints and lumbar vertebrae, while a cervical spine MRI assesses upper spinal involvement and any cord compression from a fused, rigid cervical spine. X-rays show classic findings in established disease including sacroiliac joint erosions and fusion, and spinal changes such as squaring of vertebral bodies and the "bamboo spine" appearance of complete vertebral fusion. Blood tests for HLA-B27 and inflammatory markers (ESR, CRP) support the diagnosis, though neither is diagnostic in isolation.

Disease Severity Classification

AS severity is assessed using validated clinical tools alongside imaging findings.

• Non-radiographic axial spondyloarthritis (nr-axSpA): Active sacroiliac inflammation on MRI without established X-ray changes — early form of the disease.
• Radiographic AS: Definite structural changes on X-ray (sacroiliitis grade 2 bilaterally or grade 3–4 unilaterally) meeting modified New York criteria.
• Advanced/Fused spine: Progressive vertebral bridging and fusion — may require surgical consideration for severe deformity.

Treatments

While there is no cure for ankylosing spondylitis, treatment can effectively control inflammation, relieve symptoms, and slow structural progression. Early diagnosis and consistent treatment are key to preventing long-term disability.

Regular physical activity and dedicated spinal stretching exercises are fundamental to maintaining flexibility and posture. Smoking cessation is strongly encouraged, as smoking accelerates spinal damage in AS.

Medical treatment: NSAIDs are the first-line pharmacological treatment and can effectively control pain and inflammation in many patients. For those who do not respond adequately, biologic medications — particularly TNF inhibitors (such as adalimumab, etanercept) and IL-17 inhibitors (such as secukinumab) — have transformed outcomes by targeting the specific inflammatory pathways driving the disease. JAK inhibitors are a newer class of oral targeted therapies also approved for AS.

Surgical treatment

Surgery is rarely needed for AS itself but may be required for complications. Spinal osteotomy — surgically cutting and realigning the fused spine — can correct severe fixed kyphotic deformity. Total hip replacement is often needed for advanced hip joint involvement. Fractures of the fused spine, which can occur even with minor trauma, may require surgical stabilization.

Related Spine Conditions

• DISH
• Compression Fracture
• Epidural Lipomatosis
• Spinal Stenosis
• Scoliosis
• Back to all Spine Conditions