The eye is a key part of the CNS connecting to the brain via the second cranial or optic nerve. Many other receptors and signal transduction process in the eye recapitulate those present in the CNS proper although the cellular architecture of the eye and its function are unique (see 6.12 Ophthalmic Agents). The anterior pole of the eye, composed of the cornea, iris, and lens, serves to focus light onto the photoreceptors of the retina. The retina is a layered structure composed of retinal ganglion cells, amacrine cells, horizontal cells, and photoreceptors that transduces, processes, and integrates visual stimuli. The ganglion cells are the output neurons of the retina, sending axons to the lateral geniculate nucleus by way of the optic nerve. This highly evolved complex system allows for accurate processing of visual stimuli with an exceptional dynamic range. However, it is susceptible to multiple disorders including glaucomas and macular degeneration that ultimately can produce visual defect and blindness.
The glaucomas are a group of disease associated with elevated intraocular pressure. If left untreated, the increase in pressure produces irreversible vision loss by damaging the optic nerve. Glaucoma is the second most common cause of vision loss, and the leading cause of blindness in the USA, affecting over 3 million individuals. Glaucoma typically produces no overt symptoms until vision is irreparably lost. Fortunately, early detection is enabled by a number of facile and routine diagnostic tests such as air-puff tonometry. First-line treatment is usually with topical application of agents that reduce the secretion of aqueous humor such as ^-adrenoceptor blockers (e.g., timolol), and carbonic anhydrase inhibitors (e.g., dorzolamide), or agents that increase the outflow of aqueous humor such as prostaglandin analogs (e.g., latanoprost) and parasympathomimetic drugs (e.g., pilocarpine).
Macular degeneration is a disorder in which the macula, the cone-rich central part of the retina responsible for fine detail and color vision, degenerates. The degeneration can be either a dry form which appears as an atrophy of the retinal pigmented epithelium, or a wet form in which degeneration is caused by the leaking of choroidal neovascularizations. The disorder develops gradually and with age, and is therefore often termed 'age-related macular degeneration.' It is the leading cause of vision loss affecting more than 1.6 million individuals in the USA. Symptoms include a loss of visual acuity, blurred or distorted vision, or a loss of vision in the center of the visual field. Diagnosis is typically made by visual examination, tests of central visual acuity, and fluorescein angiography to assess neovascularization.
Until recently, treatment options were only available for wet macular degeneration and were limited to surgical approaches such as photocoagulation. Several emerging therapies show promise including, intravitreal injections of the vascular endothelial growth factor (VEGF) antagonist, pegaptanib for wet macular degeneration, or the use of high dose combination antioxidant zinc therapy to slow the progression of dry macular degeneration.
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