The pathogenesis of ALS is not fully understood but, as an overwhelmingly sporadic disorder, it is likely to result from a complex mixture of polygenic and environmental risk factors operating in the context of an ageing nervous system. Physical trauma, in particular head injury, has been variably associated with both Alzheimer's and Parkinson's disease, and largely discounted in relation to multiple sclerosis. Several case-control studies in ALS have reported an association with physical tra...