We have always included questions about a patients history of head injury and loss of consciousness in our questionnaires designed to assess for associations between exposure to chemicals and age at onset of neurodegernative diseases such as Parkinson's. We believe that head injuries like chemical exposures can modify the subclinical course of neurodegenerative diseases to thereby alter age at onset of overt symptoms and therefore, must be considered in the differential diagnosis.
Link to story at ABC News