Although it is arguable that the eyes are the mirror to the soul, it is certain that the eyes reveal a great deal of information with regard to toxicology. In addition to exhibiting findings of systemic toxicity, they are also subject to the direct effects of xenobiotics and serve as a portal of entry for systemic absorption. An understanding of ophthalmic principles will allow the clinician to make timely and more accurate diagnoses that can be sight-saving or lifesaving and is essential to efficient, organized patient care.
As a matter of convention, the routine eye examination is performed in the following sequence: visual acuity, pupillary response, extraocular muscle function, funduscopy, and, when indicated, a slit-lamp examination. Examination of the pupillary size and response to light can help determine the presence of a toxic syndrome. For example, opioids and cholinergics produce miosis, whereas anticholinergics and sympathomimetics produce mydriasis. Assessment of the extraocular muscles often reveals xenobiotic-induced nystagmus or cranial nerve abnormalities. Funduscopy typically reveals pink discs characteristic of poisoning by methanol or carbon monoxide. The slit-lamp examination allows for evaluation of toxic exposure to the lids, lacrimal systems, conjunctiva, sclera, cornea, and anterior chamber. However, before considering specific xenobiotic exposures in detail, it is important to review the anatomy and physiology of the visual pathways and how alteration of the normal physiology and anatomy correlate with clinical signs and symptoms.
OPHTHALMIC ANATOMY AND PHYSIOLOGY
The eye is a spherical structure referred to as a globe. The globe is divided into anterior and posterior structures (Fig. 24–1). The most anterior structures are the cornea, conjunctiva, and sclera. Posterior to the cornea are the iris, the lens, and the ciliary body. The space between the cornea and the iris is the anterior chamber, and the space between the iris and the retina is the posterior chamber. The anterior chamber contains aqueous humor, which is produced by the ciliary processes; this fluid nourishes the cornea, iris, and lens. The iris, the ciliary processes, and the choroid compose the uvea. The posterior chamber is filled with a transparent gelatinous mass termed the vitreous humor. The vitreous humor is an important body fluid in forensic toxicology as it is less susceptible to postmortem redistribution (Chaps. 139 and 140). The fundus is the most posterior structure and includes the retina, retinal vessels, and the head of the optic nerve or disc.
The major xenobiotics that cause ophthalmic toxicity and their areas of ophthalmic injury.
Visual Acuity and Color Perception
Normal vision is dependent on light transmission through the globe reaching intact posterior neural elements. As such, appropriate light transmission requires a clear cornea, clear aqueous humor, proper pupil size, an unclouded ...