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INTRODUCTION TO OCULAR AND VISUAL SYSTEM TOXICOLOGY

Environmental and occupational exposure to chemicals, toxicants, gases, and vapors as well as the off target effects of systemic and ocular therapeutic drugs may result in structural and functional alterations to the eye and central visual system (Anger and Johnson, 1985; Grant and Schuman, 1993; Otto and Fox, 1993; Fox, 1998; Santaella and Fraunfelder, 2007; Bartlett and Jaanus, 2008; Fox, 2015; Simunovic, 2016). Almost half of all neurotoxic chemicals affect some aspect of sensory function (Crofton and Sheets, 1989). The most frequently reported sensory system alterations occur in the retina and visual system (Anger and Johnson, 1985; Crofton and Sheets, 1989; Grant and Schuman, 1993; Fox, 1998, 2015). Approximately 3000 substances are toxic to the eye and visual system (Grant and Schuman, 1993). In many cases, alterations in retinal and visual function are the initial symptoms following chemical or toxicant exposure (Damstra, 1978; Hanninen et al., 1978; Baker et al., 1984; Anger and Johnson, 1985; Mergler et al., 1987; Iregren et al., 2002). This suggests that sensory systems, and in particular the retina and central visual system, may be especially vulnerable to toxic insult. Alterations in the structure and/or function of the eye or central visual system are among the criteria utilized for setting permissible occupational or environmental exposure levels for many different chemicals in the United States (http://www.cdc.gov/niosh/npg/; http://www.epa.gov/iris/index.html). New or existing drugs may have visual side effects (Grant and Schuman, 1993; Novack, 2003; Brigell et al., 2005; Santaella and Fraunfelder, 2007). Subtle alterations in visual processing of information (e.g., visual perception, visual motor) can have profound immediate, long-term, and delayed effects on the mental, social, and physical health and performance of an individual. Among the elderly, reduced visual function is a major factor contributing to decreased ability to conduct routine activities of daily living, decreased ability to live independently, and increased risk of falls, car crashes, and other hazards. Ocular and visual system impairments can lead to increased occupational injuries, loss of productive work time, costs for providing medical and social services, lost productivity, and a distinct decrease in the overall quality of life (Gupta et al., 2007).

The overall goals of this chapter are to provide essential information on ocular pharmacodynamics, pharmacokinetics, and drug metabolism; review the procedures for testing visual function; and evaluate and review the structural and functional alterations in the mammalian eye and central visual system produced by environmental and workplace chemicals, toxicants, gases and vapors, and off-target effects of major therapeutic drugs. The adverse effects of these chemicals on the different compartments of the eye (i.e., cornea, lens, retina, and retinal pigment epithelium [RPE]), central visual pathway (i.e., optic nerve and optic tract), and central processing areas (i.e., lateral geniculate nucleus [LGN] and visual cortex) are addressed ...

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