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expert reaction to study reporting an association between cumulative pesticide exposure and risk of developing ALS (a form of motor neuron disease)

Amyotrophic lateral sclerosis (ALS) is a form of motor neuron disease and is the subject of a paper in JAMA Neurology which reports that environmental pollutants measured in the blood were associated with the disease.

 

Dr Belinda Cupid, Research Manager, MND Association, said:

“We know that people get motor neuron disease (MND) due to a combination of many environmental, lifestyle and genetic factors. It’s unlikely that any single factor on its own will cause MND and the results in this research paper aren’t an ‘eureka’ moment. But the evidence that exposure to pesticides is a contributory risk factor towards getting MND is stacking up and I’m sure will be the focus of future research.

“This is good quality research, the conclusions are backed up by solid data, and it agrees with and adds weight to existing evidence on environmental risk factors. There are always limitations to any research study, but they are identified and discussed within the paper and much more within the accompanying commentary. The commentary specifically talks about the (mis)match of the controls to the patients – in terms of differences geographical location and education, and the blood levels of the pesticides.

“For patients – this is too late; the horse has bolted for them, they already have MND.  The paper is suggesting a risk factor for developing MND, not a factor that affects the speed of progression etc. For the general public – more research is needed to understand the detail of how, when and why pesticide exposure may contribute to why people develop MND. Pesticides exposure alone won’t cause MND, and while it increases the risk of developing MND by five times (odds ratio of ~5), MND remains a rare disease.”

 

Prof. Michael Swash, Emeritus Professor of Neurology & Hon Consultant Neurologist, The Royal London Hospital, said:

“This is a well-conducted study but, as noted in the accompanying editorial caution must be applied when interpreting the results. The study suggests that environmental chemical pollutants, insecticides etc., may be risk factors when coupled with genetic predisposition. This is a reasonable conclusion to draw, but one that is very difficult to test. One should remember that ALS was recognised in the nineteenth century, with the advent of neurology as a formal medical discipline, long before any of these compounds were synthesised or available.

“One limitation with the methodology is that all blood measurements were made only once yet the conclusion relates to life-time exposure – surely a major problem, although difficult to avoid. Another major problem, as with all retrospective epidemiological studies of ‘risk factors’, is recall bias, which may affect how participants remember exposure to the substances investigated. The authors of the report acknowledge this problem, which is itself non-addressable

“There is a long history of wondering if environmental toxins might cause ALS or induce it or modify its natural history beginning in the 19th century, when bodily trauma recalled in retrospective enquiries, lead poisoning, previous poliomyelitis, traumatic brain injury, electrical injury (electricity was new at that time) and dietary imbalances were suggested as causative. These suggestions were often accompanied by large case series, all uncontrolled. Although the authors have included a matched group of non-ALS subjects as controls one would have liked to see included a group of ‘disease controls’, e.g. Parkinson disease, as an additional control group. Overall, nonetheless, the possibility of a modest effect of environmental toxin exposure in the pathogenesis of ALS in (presumably) genetically susceptible people remains an intriguing possibility.”

 

Association of Environmental Toxins With Amyotrophic Lateral Sclerosis’ by Su et al. published in JAMA Neurology on Monday 9th May. 

 

Declared interests

Dr Belinda Cupid: “I work full time for the MND Association.”

Prof. Michael Swash: No conflicts of interest.

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