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Cycloplegia is paralysis of the lens of the eye, resulting in a loss of accommodation. The iris is the heavily pigmented colored part of the eye. It has a contractile diaphragm in front of the lens with a central opening called the pupil. It is located between the lens and the cornea, and is attached radially to the ciliary body and the cornea via ligaments called pectinate ligaments. The iris contains two sets of muscles: a radial group for enlargement of the pupil (dilator pupillae), and a circular group set to decrease pupil size on contraction (sphincter pupillae). The muscles regulate the amount of light entering the eye. The sphincter pupillae is innervated by muscarinic receptors. The dilator pupillae are innervated by norepinephrine.

Cycloplegic drugs are generally muscarinic receptor blockers. These include atropine, cyclopentolate, homatropine, scopolamine and tropicamide. They are indicated for use in cycloplegic refractions and the treatment of uveitis. Many cycloplegics are also mydriatic (pupil dilating) agents and are used as such during ophthalmoscopic examinations to better visualize the retina.

Destruction of the sphincter pupillae from any cause can result in a permanent condition, called "photophobia." Light entering an eye with a paralyzed or destroyed sphincter will result in severe pain, because the pupil can not be made smaller.

When cycloplegic drugs are used to dilate the pupil, the pupil in the normal eye regains its function when the drugs are metabolized or carried away. Some cycloplegic drugs can cause dilation of the pupil for several days. Usually the ones used by ophthalmologists wear off in hours, but when the patient leaves the office strong sunglasses are provided for comfort. Anyone who has had their retina examined with the prior use of cycloplegic drugs is aware of this phenomenon.

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