alcohol, age, education, motivation and culture, and the presence of comorbid conditions (such as diabetes, depression, and cardiovascular disease).
The committee reviewed the epidemiologic literature on neurologic effects of insecticide and solvent exposure, focusing on studies that examined long-term effects. Four general types of neurologic effects are examined in this chapter: peripheral neuropathy, neurobehavioral effects (assessed by symptom reporting or performance on validated neurobehavioral tests or batteries), neurologic diseases, and sensory effects. For each, the chapter covers studies of Gulf War veterans, when available, and studies of occupational exposure to insecticides and to solvents.
Almost all the available studies of exposure to insecticides focus on exposures to insecticides as a broad group, to insecticide mixtures, or to organophosphorous (OP) insecticides in particular. Similarly, studies of exposure to solvents often examined solvents as a broad group, solvent mixtures, or workers in occupations that were exposed to the solvents of interest (Chapter 2).
The committee was unable to identify epidemiologic studies of sufficient quality to permit a separate evaluation of the long-term neurologic effects of pyrethrins, carbamates, organochlorines, or the insect repellent N,N-diethyl-3-methylbenzamide (DEET). The evidence base for many of those pesticides and neurologic effects generally consisted of case reports and case series—study designs that do not carry the methodologic rigor of cross-sectional, cohort, or case-control studies. Several of the OP-insecticide studies evaluated in this chapter, where noted, did include mixed exposures to OPs and carbamates but not of carbamates alone. The committee was not able to draw conclusions about long-term effects of pesticides other than OP insecticides, because of the lack of methodologically rigorous studies. The effects of pesticides are covered in greater detail in Chapter 3.
The committee reviewed hundreds of peer-reviewed and published studies of neurologic effects of insecticides and solvents, and it selected for detailed evaluation only the studies that met its inclusion criteria, which are listed below.1
The study had to be a published in a peer-reviewed journal and had to have methodologic rigor, including a control or reference group, and reasonable control for confounders. Case studies and case series were generally excluded from the committee’s consideration.
The study had to identify insecticides and solvents relevant to the committee’s charge (Chapters 1 and 2). If solvents were not identified, for example, the study may have been included if it examined occupations with presumed exposure to many of the solvents sent to the Gulf War. For example, studies of painters, workers in paint manufacturing, printers, dry cleaners, or workers in boot or shoe manufacturing and repair were included in the committee’s assessment.
For some neurologic effects, the committee only considered studies that examined long-term rather than short-term effects. That was accomplished typically by examining studies that analyzed only past exposure—by requiring an exposure-free interval of