Inflammation in the spine and sacroiliac (SI) joints is caused by a type of chronic, immune-mediated arthritis called ankylosing spondylitis. The condition causes low back pain, spinal rigidity, postural abnormalities, and hip and buttock pain.
Arthritis of peripheral joints, such as hips, shoulders, fingers, and toes, also occurs in approximately one-third of cases.
The global prevalence of ankylosing spondylitis is less than 2 percent.
In This Article:
The Immune System’s Role in Spinal Inflammation
Ankylosing spondylitis mainly affects the entheses, where ligaments attach to bone.
Ankylosing spondylitis primarily affects the entheses – the regions where ligaments, tendons, and joint capsules insert into bone. Over time, chronic inflammation of the entheses (enthesitis) triggers abnormal bone formation, resulting in gradual fusion of the spinal vertebrae, facet joints, and sacroiliac joints. This gradual ossification of the spine leads to loss of flexibility and mobility.,
1. The immune system is triggered
Ankylosing spondylitis is an autoimmune disease, meaning the immune system attacks the body's own tissues.
Individuals with ankylosing spondylitis have alterations in specific immune cells (T-cells), which make these cells produce more proinflammatory proteins, including,,:
- Tumor necrosis factor- α (TNF-α)
- Transforming growth factor-β (TGF-β)
- Interleukin (IL) 23 and IL-17
These proinflammatory proteins stimulate inflammation, cartilage breakdown, and bone erosion in the joints affected by ankylosing spondylitis.
2. Enthesitis occurs at different sites in the spine
The inflammatory processes cause repetitive microtraumas at the enthesis, which triggers an inflammatory response in the adjoining tissues, such as fibrocartilage, bursae, synovium, and trabecular bone (the spongy inner bone layer).,
In ankylosing spondylitis, inflammation typically begins in the SI joints, but as the disease progresses, the entheses of the facet joints, outer ring of the intervertebral discs, and peripheral joints are affected.
3. Spinal and sacroiliac joint fusion take place
Over time, the inflamed entheses around the SI joints induce changes such as swelling, tissue damage, and bone erosion. The body responds to this inflammation by growing new bone tissue, called enthesophytes, through a process known as calcification.
As the cycle of inflammation, healing, and enthesophyte growth continues, the SI joints can eventually fuse together leading to limited mobility of the lower back or chronic pain.,
Progression of ankylosing spondylitis leads to the involvement of the facet joints and intervertebral discs, and the same cycle of inflammation and new bone growth occurs and leads to the fusion of the spinal segments. This process results in reduced function and loss of normal structure in the spine.,
See Development and Progression of Ankylosing Spondylitis
Additionally, the vertebral bodies undergo erosion and inflammation, leading to a transformation in their shape, making them more square-like. New bone tissue, called syndesmophytes, grows over these square-shaped vertebrae, resulting in complete fusion of the vertebrae (ankylosis) and spinal immobility.,
Hallmark Symptoms and Signs of Ankylosing Spondylitis
Ankylosing spondylitis causes pain in the lower back, hip, and buttocks.
Several areas of the low back, pelvis, and thigh are affected in ankylosing spondylitis. The symptoms typically appear before the age of 45 and include some combination of,:
- Chronic low back pain that typically improves with movement and exercise
- Spinal stiffness that is generally worse in the mornings
- Hip pain that typically goes into the thigh
- Buttock pain that typically alternates but is sometimes one-sided
- SI joint pain that usually occurs when sitting or standing
Ankylosing spondylitis can also involve the peripheral joints and cause pain in the shoulders, ankles, and knees; neck pain and stiffness; and inflammation and swelling of the fingers and toes (dactylitis), leading to the appearance of sausage digits.
Co-Occurring Conditions with Ankylosing Spondylitis
Certain inflammatory conditions with similar characteristics co-occur with ankylosing spondylitis, including,:
- Inflammatory bowel disease: inflammation of the gastrointestinal tract, which affects up to 50% of people with ankylosing spondylitis
- Acute anterior uveitis: inflammation of the uveal tract in the eyes, which affects 25% to 35% of people with ankylosing spondylitis
- Psoriasis: an inflammatory condition affecting the skin, which affects 10% of people with ankylosing spondylitis
These conditions share genetic susceptibility factors and alterations in immune functions with ankylosing spondylitis.,
The Course of Ankylosing Spondylitis
Ankylosing spondylitis cannot be cured, but managing inflammation is a critical factor in limiting the course and severity of the disease.
As the condition advances, chronic inflammatory processes in the upper back (thoracic spine) affect the costotransverse and costovertebral joints where the ribs attach to the spine, affecting posture and restricting lung capacity., Neck pain develops in the later stages as the cervical spine becomes involved.
Severe disability is uncommon, and most people with ankylosing spondylitis remain able to work and participate in daily activities. However, the presence of one or more co-occurring inflammatory conditions (eg, psoriasis) is associated with poor prognosis and negatively influences quality of life.
See Progression of Ankylosing Spondylitis in the Spine
Impact of Ankylosing Spondylitis on Daily Life and Activities
Quality of life doesn't necessarily decline significantly for every individual with ankylosing spondylitis.
The most common limitations include a reduced ability to keep physically active and increased discomfort when sitting for long periods of time during travel or work.
When Ankylosing Spondylitis Is Serious
In the advanced stages, ankylosing spondylitis is characterized by postural abnormalities and severe spinal damage, including:
- Cauda equina syndrome: a medical emergency involving damage to the cauda equina nerve roots that causes problems with bladder and bowel control and impotence.
- Spondylodiscitis: an infection that affects the vertebrae, spinal discs, and adjacent structures.,
- Hunched back: also referred to as hyperkyphosis, hunched back is characterized by an excessive forward curvature of the upper spine, leading to an exaggerated rounding of the upper back caused by abnormal bone growth in the advanced stages of ankylosing spondylitis.
- Fractures: both vertebral and non-vertebral fractures occur due to the fragility of the bones caused by low bone mineral density and spinal rigidity.
- Bamboo spine: a hallmark late-stage finding in ankylosing spondylitis caused by fusion of the vertebrae and is associated with progressive back stiffness.
Cauda equina syndrome requires immediate medical treatment to prevent permanent nerve damage, such as bladder dysfunction and paralysis. Spondylodiscitis and spinal fractures also require medical intervention to prevent complications, but they are not usually considered emergencies.,,
Complications in the Peripheral Joints and Organs
Non-spinal complications of ankylosing spondylitis occur in the peripheral joints and organs that are affected by chronic inflammation, including,,:
- Heart disease, such as high blood pressure, heart failure, aortic regurgitation, stroke, and acute coronary syndrome
- Respiratory conditions, such as pulmonary fibrosis and restrictive pulmonary disease, which is also caused and/or or worsened by reduced chest wall expansion
Additionally, chronic pain and disability lead to fatigue, sleep disturbances, and mood disorders such as depression in some cases.
Specialists Who Treat Ankylosing Spondylitis
A rheumatologist or spine specialist usually diagnoses, treats, and monitors patients with ankylosing spondylitis.
Consultation with a physical therapist is also beneficial for individualized exercise plans to improve posture, spine mobility, and strength.