Delayed expression of neurotoxicity: the problem of silent damage.

A prolonged period of latency between toxic exposure and disease is a recognized phenomenon in oncology, but is a relatively new concept in neurotoxicology. Suggestions that exposure to neurotoxic compounds may be followed years or decades later by clinically evident neurological disease have received considerable anecdotal support but little empirical evidence. In recent years, evidence has emerged linking environmental factors to several neurodegenerative diseases, including amyotrophic lateral sclerosis and the Parkinsonism-dementia complex. While these diseases have not been definitively demonstrated to arise from exposure to toxins or toxicants, nevertheless the possibility exists that xenobiotic exposure could lead to neurological conditions possessing a period of clinical "silence" prior to expression. Detection of such damage using standard toxicology testing protocols poses a major challenge. Lessons learned from neurodegenerative diseases can be profitably applied to the study of the long-term effects of xenobiotics on the brain and elucidation of the role of aging on the expression of neurotoxicity.