EYE DROPS
In humans, the adult eyeball measures approximately one inch (2.5 centimeters) in diameter, and of its total surface area, only the anterior one- sixth is exposed to the external environment. This exposed area is protected by accessory structures of the eye, including the eyelids, eyelashes, eyebrows, lacrimal (tearing) apparatus, and extrinsic eye muscles. Each eyelid possesses a conjunctiva, which is a thin, transparent, and protective mucus membrane. The conjunctiva lines the inner aspect of the eyelids as the palpebral conjunctiva and folds back (reflects) over the eyelids onto the anterior exposed surface of the eyeball as the bulbar (ocular) conjunctiva. The bulbar conjunctiva covers only the “white” of the eye (not the cornea that covers over the iris and pupil). Because the bulbar conjunctiva is quite thin, blood vessels are clearly visible beneath it. When these blood vessels are dilated and congested from local irritation or infection (conjunctivitis, or inflammation of the conjunctiva), the result is the appearance of reddened and irritated “bloodshot” eyes.
Many consumers purchase over-the-counter eye drop medications for eye symptoms such as redness and/or itchiness or dry eyes. The “redness- reliever” products are generally appropriate for clearing up red or blood- shot eyes that have been exposed to general minor irritants such as smoke, chlorine, and wind. These products usually contain an active redness- reliever ingredient such as ephedrine hydrochloride (a-[1-(methylamino) ethyl]benzyl alcohol hydrochloride; C10H15NO·HCl) that occurs naturally in Ma-huang and several other Ephedra plant species, naphazoline hydrochloride [2-(1-naphthylmethyl)imidazoline; C14H14N2·HCl], phe- nylephrine hydrochloride ((-)-m-hydroxy-a-[(methylamino)methyl]benzyl alcohol hydrochloride; C9H13NO2·HCl), or tetrahydrozoline hydrochlo- ride [2-(1,2,3,4-tetrahydro-1-naphthyl)-2-imidazoline; C13H16N2· HCl] ranging in concentrations from 0.25 to 0.012 percent. Their primary mechanism of action is vasoconstriction (decrease in blood vessel diameter), which is accomplished by direct stimulation of specific receptors (a- receptors) on blood vessels of the eye. These drugs act as adrenergic “sympathomimetic” agents, targeting adrenergic receptors within the vasculature of the eye and mimicking the effects of the sympathetic division of the autonomic nervous system. These agents may also decrease aqueous humor flow rate within the eye. Aqueous humor, a liquid found in the anterior cavity of the eye, helps nourish the lens and the cornea. In addition, lubricants (e.g., polyethylene glycol, polyvinyl alcohol, purified water, glycerin) and preservatives (e.g., benzalkonium chloride, edetate disodium, disodium EDTA, boric acid, sodium borate, sodium chloride, sodium hydroxide, hydrochloric acid) can be added to moisturize eye dry- ness, ensure product purity, and adjust pH. Specialized antihistamine eye drops usually contain an added ingredient such as pheniramine maleate (2-[cx-(2-dimethylaminoethyl)benzyl]pyridine maleate; C16H20N2·C4H4O4), a propylamine-derivative first-generation antihistamine that antagonizes the action of the chemical mediator histamine, and are generally recom- mended for eyes that are itchy and inflamed as a result of dust, pet dander, pollen, or other allergens.
Natural human tears (or lacrimal secretions) are solutions produced by the lacrimal gland of each eye. Each lacrimal gland lies within the or- bit above the lateral end of the eye and releases tears through the excre- tory lacrimal ducts that empty the tears onto the surface of the conjunctiva of the upper eyelid. From there, the tears pass medially over the anterior surface of the eyeball through the action of blinking. Tears eventually drain away from the eye through a series of cavities and ducts into the nasal cavity. Tears consist of a watery solution containing salts, some mucus, antibodies, and a bactericidal enzyme called lysozyme. Thus, tears cleanse and protect the eye surface as they moisten and lubricate, allowing for clarity of vision. The lacrimal glands become less active with increasing age, so the elderly are more susceptible to eye dryness, infection, and/or irritation. Eye lubricants, or artificial tears, are often used to treat dry eyes by adding moisture and lubrication. Dry eye symptoms include burning, stinging, or a sensation of grittiness caused by a variety of factors, including smoke, fumes, dust, airborne particles, and changes in external tem- perature and humidity. Over-the-counter eye medications indicated for dry eyes can contain active eye-lubricating ingredients within a saline solution base. These ingredients may include polyethylene glycol, glycerin, hydroxypropylmethylcellulose, carboxymethylcellulose sodium, dextran, polyvinyl alcohol, and povidone. In addition, preservatives that are chemically similar to those found in eye redness-reliever treatments are often included in these formulations. Ophthalmic ointments obtained without prescription may also be used to treat dry eyes. These medications are applied to the inside of the eyelid and tend to treat symptoms for a longer duration than traditional eye drop treatments. However, they tend to blur vision and are mostly used as an overnight treatment. Such eye- lubricating ointments include active ingredients such as white petroleum and mineral oil in a ratio of approximately 80 to 20 percent.
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