How do dilation drops work?
Eye dilation drops are essential to every eye care professional’s toolbox.
The pupil is the window located in the center of the iris. The iris enlarges or constricts the pupil to control the amount of light entering the eye. This function allows you to focus on objects up close or far away.
Dilation drops temporarily enlarge the pupil to better examine the back of the eye, including the optic nerve and retina. Ophthalmologists also use these drops during eye surgery, such as cataract removal. Another use for dilation drops is the treatment of myopia (nearsightedness) progression in children, amblyopia (lazy eye), and uveitis (inflammation in the eye).
Types of dilation drops
Mydriatics and cycloplegics are the two categories of dilation drops. Pupillary dilation occurs because these medications stimulate the iris dilator (muscle that widens the pupil) and inhibit the iris sphincter (muscle that shrinks the pupil). Most eye care professionals use a mydriatic combined with a cycloplegic because dilation with a mydriatic alone is not sufficient.
Common mydriatics include alpha-adrenergic agonists, such as epinephrine and phenylephrine. Alpha-adrenergic agonists are medications that work by binding and activating alpha-adrenergic receptors. Agonists increase the activity of the sympathetic nervous system, while antagonists decrease activity.
Activation of the sympathetic nervous system causes the adrenal gland to release adrenaline, along with increases in heart rate, blood pressure, breathing rate, and pupillary dilation. These effects are most commonly experienced when feeling scared or excited.
In addition to dilating the pupils, cycloplegics cause temporary paralysis of the eye muscles involved in accommodation (the eye’s ability to adjust its power to focus on an object). Cycloplegic drops include tropicamide, cyclopentolate, scopolamine, homatropine, and atropine.
Cycloplegics are muscarinic antagonists. Muscarinic receptors are part of the parasympathetic nervous system, which controls the body’s functions at rest. These actions include digestion, urination, regulating resting heart rate, pupillary constriction, and production of saliva and tears.
Cycloplegic medications bind to muscarinic receptors on the ciliary body (the muscle responsible for accommodation) and iris sphincter, inhibiting their activity. As a result, these muscles relax, the pupil dilates, and accommodation relaxes.
Side effects of mydriatics and cycloplegics
Dilation drops have several temporary side effects. Most dilations last from 4 to 6 hours, sometimes up to 24 hours. The duration depends on the medication type, concentration, and the individual person. Side effects tend to last longer in people with lighter-colored irises.
Ocular side effects include dry eyes, blurry vision (primarily up close), and light sensitivity. Because the body absorbs a small amount of the eye drops, some people may experience elevated heart rates, higher blood pressure, constipation, drowsiness, and dry mouth. However, systemic side effects are infrequent.
Side effects of dilation drops can be uncomfortable. However, these medications are necessary in helping eye care professionals diagnose and manage various eye conditions.