Association Between Ankylosing Spondylitis and Endometriosis
Yes, there is a significant epidemiological association between endometriosis and ankylosing spondylitis, with endometriosis patients showing a 61% increased risk of developing AS, though the clinical implications remain primarily centered on heightened awareness rather than routine screening.
Epidemiological Evidence
The association between these conditions is supported by robust population-based data:
A large Taiwanese retrospective cohort study of 92,015 patients demonstrated that endometriosis patients have a significantly elevated risk of developing AS (adjusted HR = 1.61,95% CI = 1.11-2.35) 1. This represents a 61% increased risk compared to women without endometriosis.
The cumulative incidence of AS was significantly higher in the endometriosis cohort over the 12-year follow-up period 1.
Among endometriosis subtypes, infertile endometriosis specifically showed a protective association with AS (OR = 0.83, P = 0.02), though this finding requires cautious interpretation given the complexity of the relationship 2.
The most recent comprehensive analysis (2025) confirmed increased risk across multiple autoimmune conditions, with endometriosis patients showing 30-80% increased risk for classical autoimmune diseases including rheumatoid arthritis, multiple sclerosis, and coeliac disease 3.
Genetic and Biological Basis
The association appears to have underlying genetic and immunological mechanisms:
Genetic correlation analyses revealed significant overlap between endometriosis and osteoarthritis (rg = 0.28), rheumatoid arthritis (rg = 0.27), and multiple sclerosis (rg = 0.09) 3.
Mendelian randomization analysis suggested a potential causal relationship between endometriosis and rheumatoid arthritis (OR = 1.16,95% CI = 1.02-1.33) 3.
The KIR2DS5 gene showed protective effects for both conditions, with its presence associated with reduced risk of AS (OR = 0.47, P = 0.003) and endometriosis (OR = 0.25, P = 0.03) 4. This shared genetic susceptibility pattern suggests common immunological pathways.
Shared genetic loci have been identified, with enrichment in seven biological pathways common to both conditions 3.
Clinical Risk Stratification
Certain patient characteristics modify the association:
Patients with major depressive disorder and endometriosis show markedly elevated AS risk (adjusted HR = 5.05,95% CI = 1.85-13.78) 1.
NSAID users with endometriosis demonstrate lower AS risk compared to non-users (adjusted HR 1.35 vs 4.57, P for interaction = 0.031), suggesting that anti-inflammatory treatment may modify disease progression 1.
The association remains consistent across age subgroups 1.
Clinical Implications
While the association is statistically significant, practical management considerations include:
Clinicians managing endometriosis patients should maintain awareness of potential AS development, particularly monitoring for inflammatory back pain characteristics: morning stiffness, pain in the second half of the night, and improvement with exercise 5.
The absolute risk remains relatively low despite the elevated relative risk, so routine screening for AS in all endometriosis patients is not warranted based on current evidence.
Heightened clinical suspicion is appropriate in endometriosis patients presenting with chronic back pain, particularly those with comorbid depression or those not using NSAIDs 1.
The shared inflammatory pathways suggest that NSAIDs used for endometriosis management may provide dual benefit in reducing AS risk 1.
Important Caveats
The evidence base consists primarily of observational studies from Asian populations, which may limit generalizability to other ethnic groups 1.
Causality remains incompletely established; the association may reflect shared genetic susceptibility, common inflammatory pathways, or surveillance bias 2, 3.
No specific screening protocols or preventive interventions have been validated for this association.