There is an association between increasing prevalence and increasing latitude for some autoimmune diseases, including rheumatoid arthritis (RA). Furthermore, in RA patients, a geographical variation in methotrexate pneumonitis has been suggested at a regional level in New Zealand.
The objective of the study was to determine if there is an increased incidence of methotrexate pneumonitis with increasing latitude in New Zealand.
A search was conducted using the NZ Ministry of Health's National Minimum Data Set for patients with discharge codes for RA (M05, M06) or history of RA and drug-induced lung disease (J702, J703, J704) or other (J189, J680, J90, J984) and methotrexate (Y431), for the period July 1, 1999, to June 30, 2008. Anonymous data were provided by the Ministry of Health for the 43 patients fulfilling these coding criteria and also the latitude and population of each domicile code. A Poisson regression analysis was undertaken with latitude as a continuous variable, adjusting for the total population at different latitudes.
The incidence rate ratio for methotrexate pneumonitis shows a 16% increase per 1 degree of latitude (95% confidence interval, 7%-27%; P = 0.02).
There was a latitudinal gradient seen in the rate of patient discharges for methotrexate pneumonitis, in the defined period. This supports the hypothesis that there is a latitude-dependent risk factor for this disorder and raises questions regarding possible environmental cofactors. It also supports the growing pool of evidence that certain immune-mediated conditions are more common at higher latitudes.