Mydriatics

Mydriatics are drugs which dilates the pupil to facilitate a more thorough examination of the fundus, lens periphery and vitreous in which case it is used as a diagnostic agent. They can also be used as a therapeutic agent in some ocular inflammatory conditions such as anterior uveitis. There are two types of mydriatic agents, both of which act on the two autonomic systems.
1. Parasympatholytics agents eg atropine, homatropine, cyclopentolate, tropicamide. These agents break down the cholinergic system.
2. Sympathomimetics agents mimics the action of the adrenaline on the dilator pupillae. Examples are, phenylephrine (adrenergic agonist), ephedrine, hydroxyl-amphetamine, cocaine.
Properties of an ideal mydriatics as a diagnostic agent
• Quick onset of action.
• Adequate duration.
• Fast recovery after examination.
• Should be able to abolish light reflex.
• There should be no cycloplegia.
• Should be capable of quick reversal in emergency.
• Should not precipitate a rise in intraocular pressure.
• There should be no other pharmacological effect.
• There should be no local toxic reactions.
• No systemic effect.
Indications for the use of mydriatic agent as a diagnostic agent

• Incidents of recent onset of floating of opacities in the vitreous.
• Sudden decrease in visual acuity necessitating an examination of the fundus and muscular area.
• Unexplained loss of visual field.
• Unexplained ocular pain not accompanied by raised intraocular pressure.
• Redness of the eye of no known cause after contusion injury to rule out eye damage.
• Cloudiness of the vitreous or lens when good mydriatics would review fundus details.
Contraindications of the use of mydriatics
• Non-presence of glaucoma, ascertain this by;
1. Questioning the patient
2. Inspection of the eye for signs of therapeutic miosis or previous glaucoma surgery
• Evidence of elevated intraocular pressure derived from tonometric investigation.
• The presence of an abnormally shallow anterior chamber which is almost invariably associated with a narrow anterior angle such an eye is being predisposed to an acute angle glaucoma
• Conjunctival disruption and bleeding
• Patient with hypertension, heart disease
• Thyrotoxicosis
• Prior use of local anesthetics
Mechanism of action of mydriatics
There are two opposing iris smooth muscles that control the pupillary size, the sphincter and dilator pupillae. There are two different modes of action of mydriatics;
1. By mimicking the action of the sympathetic system in the iris by sympathomimetic drugs which will cause the contraction of the dilator pupillae leading to mydriatics. Here, the parasympathetic system is not affected by such drugs. Thus, pupillary light reflex remains active. Smpathomimetic drugs has little or no effect on accommodation.
2. By paralyzing the parasympathetic innervation of the sphincter pupillae muscle using parasympatholytic agents or anti-muscarinic drugs. With this type of drug, the pupillary light reflex is reduced or abolished in addition, this agent also paralyses the accommodation mechanism because they also paralyses the parasympathetic innervation to the ciliary muscles and ciliary body. Thus, this second group of mydriatic agents are said to have cycloplegic effects.
Adverse effects of mydriatics
• Angle closure glaucoma: the use of mydriatics causes pupil dilation which introduces the possibility of the anterior chamber angle blockage, the damage will depend on the degree of dilation and not on the mydriatic employed. In such condition the patient experiences severe pain which may induce vomiting. The conjunctival blood vessels are dilated; the cornea loses transparency and is edematous because of high intraocular pressure cause it to embed water and swell. The pupil will not constrict to light nor to accommodation or to the action of miotics. In such instances the intraocular pressure is highly leading to angle closure glaucoma.
Other local toxic effects from topical sympathomimetic drugs includes.
• Toxic epithelial desquamation of the cornea.
• Liberation of the iris pigment into the anterior chamber has also been reported with the use of phenylephrine 10%
• vasoconstriction
synkinetic reflex in pupillary action
1. constriction to convergence.
2. constriction to light.
3. constriction on accommodation at near.