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Eye dilating drops are a type of medication administered to the eyes to cause the pupils of the eyes to dilate, or widen. They are also called mydriatic eye drops. They consist of a solution of water and sodium chloride (NaCl), commonly called saline, containing drugs that cause pupil dilation, called mydriatic drugs. Eye dilating drops are used for eye examinations, eye surgery, and the treatment of some vision disorders.
The pupil is the part of the eye that allows light into the retina. Pupils dilate naturally in low-light conditions so that the eye receives more light to see and contract in brighter conditions to prevent too much light from getting in. Some chemicals can cause the pupils to remain dilated even when the amount of light reaching the eyes would normally cause them to contract, a condition called mydriasis.
The mydriatic drugs in eye dilating drops are usually anticholinergic agents, which means that they block the neurotransmitter called acetylcholine. This interferes with the normal transmission of signals from the parasympathetic nervous system, which governs various bodily processes that occur unconsciously. One of its responsibilities is signaling the pupils to dilate or contract according to the amount of light being detected, so applying an anticholinergic drug to the eyes temporarily stops the pupils from receiving the parasympathetic nervous system's instructions to contract.
Commonly used short-acting anticholinergic agents used in eye dilating drops include homatropine (C16H21NO3) and tropicamide (C17H20N2O2). These drugs usually cause mydriasis for only a few hours, and so are frequently used for eye examinations to make the retina, lens, and vitreous humor more visible, or for surgery performed on the eyes. Other anticholinergic drugs used in eye dilating drops have longer-lasting effects. The drug atropine (C17H23NO3), for example, can cause mydriasis for a week or more. Longer-lasting drugs are used in eye dilating drops for longer-term uses, such as treating some types of glaucoma and improving the vision of people with eye problems such as cataracts or a dislocated lens, a condition called ectopia lentis.
Mydriasis can also be induced by sympathomimetic drugs, which have effects similar to those of neurotransmitters associated with the sympathetic nervous system such as epinephrine and norepinephrine. The sympathetic nervous system governs the body's response to stress, which includes dilating the pupils during the fight-or-flight response. Thus, applying sympathomimetic drugs to the eyes can cause them to respond as if they were receiving instructions from the sympathetic nervous system. An example of these commonly used in eye dilating drops is the short-acting mydriatic agent phenylephrine (C9H13NO2), which can bond to adrenergic receptors in cells and mimic the effects of epinephrine.