Spatial Clustering of ALS in Ireland: Response to Bradley et al.
James P.Rooney, PhD Student, Trinity College Dublinjrooney@rcsi.ie
Anthony Staines, Dublin, Ireland; Orla Hardiman, Dublin, Ireland
Submitted May 29, 2015
We thank Bradley et al. for their interest and comments on our recent paper. [1] We agree with their point that "If cases are aggregated over large areas, the effect of a localized toxic exposure may be lost." Therefore, we have replicated our analysis at a higher spatial resolution of over 18,000 small areas consisting of no more than 200 residences each. [2] We also agree that "sporadic ALS is a multi-factorial disease, resulting from the interaction of multiple environmental risk factors with multiple genetic predisposing factors." There is recent epidemiological evidence for this in the form of multi-step models for ALS. [3] Of course, we are also cognizant of the ecological fallacy (i.e. inappropriately ascribing group level correlations to individual level variables), and agree that individual risk factors are important to include in models wherever possible.
However, it should be noted that our study did report positive results. Our methods were sensitive enough to find two statistically significant low risk areas, as well as detect 5 cases that presented late and 2 that presented cross border. The algorithms used are agnostic to 'hot' or 'cold' spots; therefore, if 'hot-spots' did exist, we are confident we would have detected them.
1. Rooney J, Vajda A, Heverin M, et al. Spatial cluster analysis of population amyotrophic lateral sclerosis risk in Ireland. Neurology 2015;84:1537-1544.
2. Rooney JP, Tobin K, Crampsie A, et al. Social deprivation and population density are not associated with small area risk of amyotrophic lateral sclerosis. Environ Res 2015;142:141-147.
3. Al-Chalabi A, Calvo A, Chio A, et al. Analysis of amyotrophic lateral sclerosis as a multistep process: a population-based modelling study. Lancet Neurol 2014;13:1108-1113.
For disclosures, please contact the editorial office at journal@neurology.org.
We thank Bradley et al. for their interest and comments on our recent paper. [1] We agree with their point that "If cases are aggregated over large areas, the effect of a localized toxic exposure may be lost." Therefore, we have replicated our analysis at a higher spatial resolution of over 18,000 small areas consisting of no more than 200 residences each. [2] We also agree that "sporadic ALS is a multi-factorial disease, resulting from the interaction of multiple environmental risk factors with multiple genetic predisposing factors." There is recent epidemiological evidence for this in the form of multi-step models for ALS. [3] Of course, we are also cognizant of the ecological fallacy (i.e. inappropriately ascribing group level correlations to individual level variables), and agree that individual risk factors are important to include in models wherever possible.
However, it should be noted that our study did report positive results. Our methods were sensitive enough to find two statistically significant low risk areas, as well as detect 5 cases that presented late and 2 that presented cross border. The algorithms used are agnostic to 'hot' or 'cold' spots; therefore, if 'hot-spots' did exist, we are confident we would have detected them.
1. Rooney J, Vajda A, Heverin M, et al. Spatial cluster analysis of population amyotrophic lateral sclerosis risk in Ireland. Neurology 2015;84:1537-1544.
2. Rooney JP, Tobin K, Crampsie A, et al. Social deprivation and population density are not associated with small area risk of amyotrophic lateral sclerosis. Environ Res 2015;142:141-147.
3. Al-Chalabi A, Calvo A, Chio A, et al. Analysis of amyotrophic lateral sclerosis as a multistep process: a population-based modelling study. Lancet Neurol 2014;13:1108-1113.
For disclosures, please contact the editorial office at journal@neurology.org.