Latitude gradient influences the age of onset of rheumatoid arthritis: a worldwide survey

Abstract

The age of onset of rheumatoid arthritis (RA) is an important outcome predictor. Northern countries report an age of RA onset of around 50 years, but apparently, variability exists across different geographical regions. The objective of the present study is to assess whether the age of onset of RA varies across latitudes worldwide. In a proof-of-concept cross-sectional worldwide survey, rheumatologists from preselected cities interviewed 20 consecutive RA patients regarding the date of RA onset (RAO, when the patient first noted a swollen joint). Other studied variables included location of each city, rheumatologist settings, latitudes (10° increments, south to north), longitudes (three regions), intracountry consistency, and countries’ Inequality-adjusted Human Development Index (IHDI). Data from 2481 patients (82% females) were obtained from 126 rheumatologists in 77 cities of 41 countries. Worldwide mean age of RAO was 44 ± 14 years (95% CI 44–45). In 28% of patients, RA began before age 36 years and before age 46 years in 50% of patients. RAO was 8 years earlier around the Tropic of Cancer when compared with northern latitudes (p < 0.001, 95% CI 3.5–13). Multivariate analysis showed that females, western cities, and latitudes around the Tropic of Cancer are associated with younger age of RAO (R2 0.045, p < 0.001). A positive correlation was found between the age of RAO and IHDI (r = 0.7, p < 0.01, R2 0.5). RA often begins at an early age and onset varies across latitudes worldwide. We postulate that countries’ developmental status and their geographical and geomagnetic location influence the age of RAO.