Cyclopegic Drugs, Cyclopegics

Cyclopegic drugs, also known as mydriatics, paralyze the eye muscle that controls the size of the pupil causing cycloplegia; it is used in the treatment of some of the diseases of the eye. Cyclopegic drugs may be either sympathomimetics or parasympatholytics. The latter cause cycloplegia or paralysis of accommodation at high doses and may precipitate glaucoma. Mydriatics are used in eye diseases and to facilitate eye examination.

Sympathomimetics

Sympathomimetics stimulate the adrnergic receptors of the dilator muscle and may cause mydriasis (dilation of the pupil). Phenylephrine and epinephrine are examples of sympathomimetics. They are available as topical ophthalmic solutions.

Parasympatholytics

Tropicamide is available as topical ophthalmic solution. It has minimal effect on the ciliary muscles, has a rapid onset of action (20 min), is short acting (3 to 4 hours), and is a drug of choice for eye examination. Atropine has a slow onset of action (45 min) and the pupillary dilation is accompanied by cycloplegia. Atropine is available as a topical ophthalmic solution or ointment and is a systemic medication that can be administered under the conjunctiva. Topical or subconjunctival atropine is often used for pupil dilation and eye muscle paralysis in uveitis in dogs and cats.1

Atropine should be used with care if tear production is reduced (since it reduces tear production even further), and it must not be used in glaucoma cases in dogs and cats since it will further increase intraocular pressure. Atropine has a bitter taste and can cause salivation after topical application, as the drug drains down nasolacrimal ducts into the mouth. Ointments causes less of a problem and are the formulation of choice in cats.2

References

  1. Small animal ophthalmology: a problem-oriented approach. Robert L. Peiffer, Simon M. Petersen-Jones
  2. Veterinary ophthalmology: a manual for nurses and technicians. Sally Turner