Ethnicity and disease severity in ankylosing spondylitis

February 2018

A cross-sectional analysis of three ethnic groups

OVERVIEW

A person’s genetic makeup, including HLA-B27, is thought to make up roughly 90% of the disease risk in ankylosing spondylitis (AS.) It’s not surprising then that ethnicity plays a large role in the frequency of AS throughout the world. New research now suggests that ethnicity may also play a role in disease activity and severity. A study published in the Journal of the International League of Associations for Rheumatology investigated AS severity in three different ethnic/racial groups – Blacks, Whites, and Latinos –analyzing also the association of HLA-B27 with AS in these groups.

THE STUDY

925 adult AS patients from the U.S. and Australia were recruited from the study investigator’s clinics, patient organizations – such as the Spondylitis Association of America, and other various rheumatology practices. Genetic and statistical analyses were conducted, and clinical and radiographic measurements and tools were used to analyze disease activity and severity in the three groups.

Though it has previously been established that Blacks experience the lowest rates of both ankylosing spondylitis (AS) and axial spondyloarthritis (axSpA) due to lower frequencies of HLA-B27, the study found that the Black AS patients enrolled had the highest levels of disease activity, functional capacity/impairment, and most radiographic severity of the three groups studied.

Disease Activity

Disease activity is classified and measured using three separate scientific methods.

1. The Bath Ankylosing Spondylitis Disease Activity Index (BASDAI) is a quick and subjective measure in which patients classify their discomfort level on a scale of 1-10 in six different categories, such as fatigue and stiffness, with the numbers then being averaged. The BASDAI found that the Black AS patients had the highest levels of discomfort (5.9), followed by Latino AS patients (4.5), and then White AS patients (3.5).
2. The erythrocyte sedimentation rate (ESR) is a blood test used to detect inflammatory activity in the body. Since inflammation causes cells to lump together, they are denser than normal cells. The ESR measures the distance the cells descend in a test tube in one hour – the higher the number, the more inflammatory activity. The ESR was also highest in the Black patients (27 mm/hr), followed by Latino patients (17 mm/hr), and White patients (10 mm/hr).
3. The C-reactive Protein (CRP) test is a similar blood test that identifies the amount of CRP in a patient’s blood. High levels of CRP indicate high rates of inflammation and often more disease activity. Once again, Black patients had the highest levels of CRP (1.2 mg/dl) followed by Latinos (0.9 mg/dl) and Whites (0.4 mg/dl).

Functional Capacity/Impairment

Unlike disease activity, functional impairment is classified using only one measure: the Bath Ankylosing Spondylitis Functional Index (BASFI). The BASFI uses a similar 1-10 scale, but rates a patient’s confidence in their ability to complete certain activities or tasks, such as putting on socks or standing unsupported for 10 minutes without discomfort. In this category as well, Black patients were more significantly impacted by the disease (6.25) than both Latinos (3.81) and Whites (2.78) enrolled in the study.

Radiographic Severity

Radiographic severity was measured in two ways.

1. The modified Stoke Ankylosing Spondylitis Spine Score (mSaSSS) assesses and analyzes the existence and severity of erosion, deterioration, and squaring at the anterior corners of the cervical and lumbar spine. It is based on a 0-72 scale, with 72 being the most severe. The Black AS patients had a dramatically higher score in this category (38.2) compared to Latinos (8.1) and Whites (6.4).
2. Similar results were found in the Bath Ankylosing Spondylitis Radiographic Index (BASRI), which grades sacroiliac, lumbar, and cervical fusion/damage on scales from 0-4 and adds the three together. The highest number, indicating the most damage, again belonged to the Black patients (9.5) while the Latino and White patients tied at 7.3.

CONCLUSION

Despite the lower frequency of HLA-B27 and the resulting lower prevalence of AS/axSpA in the Black population, the results of this study demonstrate higher levels of disease activity, greater functional impairment, and greater radiographic severity in Black AS patients. The exact causes of this remain unknown however, and will need to be investigated in future studies.

Further Reading and Sources Used:

1. Breakthroughs in genetic studies of ankylosing spondylitis

2. Ethnicity and disease severity in ankylosing spondylitis a cross-sectional analysis of three ethnic groups