Geographical clustering of amyotrophic lateral sclerosis in South-East England: a population study.

BACKGROUND: Amyotrophic lateral sclerosis (ALS) is a degenerative disease of motor neurons that causes progressive paralysis and eventually results in death from respiratory failure. Environmental factors that trigger ALS might result in a pattern of geographical clustering of cases. We tested this hypothesis using the South-East England ALS population register, which covers south-east London, Kent and parts of neighbouring counties. METHODS: The register's catchment area was divided into postcode districts and sectors. The expected rates of ALS (adjusted for age and sex) were compared with the observed rates using a standardised residuals method and the SaTScan programme. RESULTS: There were 406 cases of ALS identified in the catchment area during the study period. Four of the 126 postcode districts, all in Greater London, had residuals >2.5 SDs from the mean. Similarly, there were 15 postcode sectors (out of 420) that had residuals >1.96 SDs from the mean. Nine of these were in Greater London. SaTScan identified an area that had a 5.61-km radius in which the relative risk of ALS was 1.70 (p = 0.012). This area overlapped with the postcode districts and some of the sectors identified using the residuals method. CONCLUSIONS: These findings suggest an excess of ALS cases in some postcode districts in south-east England.