Associations between occupational exposures and neurodegenerative disorders (including amyotrophic lateral sclerosis (ALS), Parkinson’s and Alzheimer’s diseases) have not been studied as extensively as cancer and respiratory diseases and their causes remain largely unknown.
Due to complicated clinical diagnosis and lack of registries, case ascertainment is a limiting factor when studying neurodegenerative disorders. Case-control studies are best suited for inclusion of incident cases, but prone to recall bias. Studies on neurodegenerative disorders may additionally suffer from lack of recall because of disease characteristics. Cohort studies are free from recall bias and offer opportunities to study associations with occupational exposures. Moreover, baseline blood may be informative about exposures (e.g. lead) well before disease onset. Although occupational information collected in most cohorts is limited, successful studies in cancer and respiratory epidemiology have shown that occupational studies in general populations can be informative.
For ALS, several efforts are being conducted to tackle the aforementioned challenges. Examples are: an ongoing nation-wide case-control study in the Netherlands (PAN: currently ~2300 cases and ~4200 controls), to study risk factors and possible gene-environment interactions; international pooling of case-control studies; using existing cohorts (e.g. EPIC) ; and aiming to register all Dutch ALS cases. Consistent positive associations between smoking and ALS were observed within PAN and EPIC, indicating that a similar approach for occupational risk factors would be informative.
Because each study design has its advantages and disadvantages, neurodegenerative disorders should be looked at more in a range of (occupational) studies to gain better understanding of the aetiology.