Mydriatics and Cycloplegics

 

Mydriatics and Cycloplegics

Mydriasis means pupillary dilatation

Cycloplegia means paralysis of the ciliary muscles

Iris has two types of muscles- The dilator pupillae (radial muscles) and the constrictor pupillae (circular muscles)

Muscle	Agonistic action	Antagonistic action	Receptor
Radial Muscles	Dilatation	Constriction	Alpha 1 adrenergic
Circular Muscles	Constriction	Dilatation	M3 muscarinic
Ciliary Muscles	Contraction	Relaxation (Cycloplegia)	M3 muscarinic

 

Therefore, topical application of alpha 1 adrenergic drugs like phenylephrine causes pupillary dilatation but no cycloplegia.

And anti-muscarinic drugs cause both pupillary dilatation and cycloplegia.

These drugs are used in two important procedures.

1.      Fundoscopy

2.     Refraction testing

In fundoscopy, the retina of the patient is viewed under magnification by observing through the pupil. Therefore, pupillary dilatation is required. Both adrenergic and anti-muscarinics can be used. But since cycloplegia is not required in this procedure, anti-muscarinics with strong cycloplegia effect are not preferred.

In objective determination of refractive error, the power of the eye is determined with instruments, in contrast to subjective refraction where the patient actively reads the Snellen chart with lens of different powers. Therefore, in objective refraction both pupillary dilatation and cycloplegia is required. So anti-muscarinics are used, especially in children where the sympathetic tone of the ciliary muscles is very high.

Drug	Class	Mydriasis	Cycloplegia
Homatropine	Anti-muscarinic	Yes	Yes
Cyclopentolate	Anti-muscarinic	Yes	Yes
Tropicamide	Anti-muscarinic	Yes	Weak
Atropine	Anti-muscarinic	Yes	Yes, strong
Phenylephrine	Adrenergic	Yes	No