Incidence of Ankylosing Spondylitis in Females
The incidence of ankylosing spondylitis (AS) in females is approximately 3 cases per 100,000 person-years across all age groups, which is significantly lower than the 12 cases per 100,000 person-years observed in males. 1
Epidemiological Data
The female-to-male ratio for AS incidence is approximately 1:4, with AS being traditionally considered more common in males 1
Age-specific incidence rates in females vary by age group:
- 0-15 years: 1 per 100,000
- 16-24 years: 4 per 100,000
- 25-44 years: 5 per 100,000
- 45-64 years: 3 per 100,000
- 65-74 years: 1 per 100,000
75 years: 0 per 100,000 1
In contrast, males show higher incidence rates across all age groups, with peak incidence of 23 per 100,000 in the 25-44 age group 1
Recent Research Challenging Traditional Ratios
Recent evidence suggests the gender disparity may be less pronounced than previously thought:
- A 2023 study of US Army personnel found similar incidence rates between males and females (incidence rate ratio 1.16, p=0.23) 2
- This study suggests that previous observations of male predominance may have been influenced by:
- Clinic populations less representative of US race/ethnicity distribution
- Disease ascertainment tools that identified subjects later in disease course
- Lack of organized screenings for musculoskeletal symptoms 2
Historical data from 1975 revealed that when screening HL-A W27 (HLA-B27) positive individuals, the prevalence of AS was similar in both sexes, suggesting underdiagnosis in females 3
Clinical Presentation Differences by Gender
Female AS patients often present with:
- Higher disease activity scores (BASDAI, ASDAS)
- More tender entheses
- Worse physical function and quality of life 4
Male AS patients typically present with:
- More kyphosis
- Greater spinal radiographic damage
- Higher rates of radiographic progression during treatment 4
These differences may contribute to delayed or missed diagnosis in females, as the classic radiographic findings may develop more slowly or be less pronounced 5
Diagnostic Considerations
Early detection of AS in females requires attention to characteristic clinical features:
- Onset before age 30
- Night pain and morning stiffness
- Intermittent, migratory joint pain
- Decreased spine mobility
- Sacroiliac tenderness 5
HLA-B27 testing is useful for detecting early and mild forms of AS, with over 90% of patients testing positive regardless of gender 5
Radioisotope scanning can detect early inflammation of sacroiliac joints before changes are evident on standard radiographs, which may be particularly important for female patients 5
Clinical Implications
- The lower reported incidence in females may lead to underdiagnosis and delayed treatment 5
- Early identification is crucial as proper management may prevent severe fixed deformities of the spine and root joints 6
- Female patients may require more frequent monitoring as they switch TNF-α inhibitors more often than males (32% vs. 14%) despite showing comparable clinical improvement during treatment 4