Ankylosing spondylitis (AS) is a chronic inflammatory condition primarily affecting the spine. This progressive form of arthritis can lead to significant pain and discomfort. Understanding its prevalence involves examining how common it is across different populations and the factors influencing its occurrence.
Understanding Ankylosing Spondylitis
Ankylosing spondylitis is a type of arthritis characterized by inflammation that largely targets the spine. This inflammation can affect the vertebrae, the individual bones of the spine, leading to chronic pain and stiffness. Over time, this inflammation can cause new bone formation, potentially fusing sections of the spine. This fusion can result in a rigid, inflexible spine, sometimes described as a “bamboo spine.”
Beyond the spine, AS can also impact other joints, including the hips and shoulders, and areas where tendons and ligaments attach to bones. Non-joint areas can also be affected, such as the eyes, where it can cause uveitis (inflammation of the eye’s middle layer), or the bowels, leading to gastrointestinal symptoms. Symptoms often begin with lower back pain and stiffness, particularly in the morning or after periods of rest.
Global and Regional Prevalence
Ankylosing spondylitis is considered a relatively rare condition, with global prevalence estimates typically ranging from 0.1% to 1.4% of the population. A 2014 study estimated between 1.30 and 1.56 million cases in Europe and 4.63 to 4.98 million cases in Asia.
Prevalence rates for AS show notable variations across different geographic regions. North America has reported a mean prevalence of approximately 31.9 cases per 10,000 people. In contrast, Europe’s mean prevalence is around 23.8 per 10,000, while Asia’s is about 16.7 per 10,000. Latin America and Africa report lower rates, with approximately 10.2 and 7.4 cases per 10,000 people, respectively.
Demographic and Genetic Influences
Ankylosing spondylitis often manifests during late teens to early forties, with approximately 80% of patients experiencing symptoms by 30 years of age. Historically, AS was thought to affect men more frequently than women, with a male-to-female ratio often cited as 2:1. However, more recent research suggests a more even distribution, or even higher rates in women for specific disease presentations.
Ethnic and racial disparities also influence prevalence patterns. While the HLA-B27 gene is strongly linked to AS, its prevalence varies among ethnic groups, affecting 7.5% of non-Hispanic white individuals compared to 4.6% of Mexican Americans. A 2017 study found that Black individuals might experience more severe AS symptoms despite a lower percentage testing positive for HLA-B27 (62.5% versus 85.3% in white individuals).
The human leukocyte antigen (HLA)-B27 gene is significantly linked to AS. This gene is found in over 80% of individuals with AS. While HLA-B27 is a major risk factor, its presence does not guarantee disease development; only about 5–6% of people with the HLA-B27 gene develop AS. Not all individuals with AS carry the HLA-B27 gene, indicating a complex interaction between genetic predisposition and environmental factors.
Challenges in Measuring Prevalence
Accurately measuring the prevalence of Ankylosing Spondylitis presents several complexities. A significant challenge is diagnostic delay, the time between symptom onset and definitive diagnosis. This delay is estimated to be 5 to 7 years in the United States, and typically ranges from 2 to 6 years globally. This prolonged period can lead to underestimation of true prevalence, as many individuals remain undiagnosed.
Misdiagnosis is another factor. Early AS symptoms, such as chronic back pain, can be non-specific and are often mistaken for other common back conditions or different forms of arthritis. Patients may consult multiple healthcare providers before receiving an accurate diagnosis. This delays proper classification and inclusion in prevalence statistics.
Variations in diagnostic criteria over time and regions also complicate accurate prevalence measurement. Historically, diagnosis often relied on structural damage visible on X-rays, which appears later in the disease course. While newer criteria, such as those from the Assessment of Spondyloarthritis International Society (ASAS), allow for earlier classification based on inflammatory markers and MRI findings, inconsistencies in application can affect reported figures. These factors suggest the actual prevalence of AS may be higher than current statistics indicate.