Phenylephrine

Mechanism

💊 Commercial Names

Colircusi Phenylephrine, Neosynephrine, Phenylephrine Kern, Mydriatic Eye Drops Formulas.

🔬 Pharmacological Group

Selective agonist of α₁ adrenergic receptors. Vasoconstrictor and sympathomimetic mydriatic.

Mechanism of Action

Topical phenylephrine exerts its action by stimulating the adrenergic receptors of the anterior pole of the eye:

• Activation of α₁-adrenergic Receptors: It selectively couples to the α₁ receptors of the dilator muscle of the pupil (radial of the iris) and the conjunctival and episcleral arterioles.
• Active Mydriasis without Cycloplegia: When the dilator muscle contracts, the pupil actively expands. By not binding to muscarinic receptors, phenylephrine preserves accommodation and the pupillary accommodation reflex, allowing a comfortable examination for the patient that does not interfere with long-term close reading.
• Local Vasoconstriction of Superficial Vessels: Contracts the conjunctival microvasculature immediately, clearing the red eye. It is useful in the diagnostic test for scleritis: the vessels of the episclera contract after the instillation of phenylephrine, clearing the conjunctiva, while the inflamed deep scleral vessels remain congested and injected, confirming scleritis.

Pharmacokinetics

Quantitative Ocular and Systemic Pharmacokinetics

Security

Cardiovascular Risk in the Use of 10% Concentration

Instillation of 10% ophthalmic phenylephrine can cause massive systemic absorption capable of inducing an acute hypertensive crisis, intracranial hemorrhage and serious ventricular arrhythmias with a sudden clinical course. By binding to systemic vascular α₁ receptors, it induces profound arterial vasoconstriction that drastically increases peripheral resistance, causing severe hypertension followed by reflex cardiac bradycardia. The 10% concentration is contraindicated in elderly patients, neonates, decompensated hypertensive patients, patients with aneurysms or previous infarctions. It is advisable to use exclusively the 2.5% concentration for routine exploratory use.

Contraindications

• Absolute: Primary angle-closure glaucoma untreated or with active anatomical predisposition; severe uncontrolled or unstable arterial hypertension; severe coronary ischemic heart disease; concomitant use of Monoamine Oxidase Inhibitors (MAOIs) or tricyclic antidepressants; neonates and children with low body weight due to the risk of vasomotor collapse.
• Relative: Patients with advanced diabetes mellitus or autonomic neuropathy (they may present a supersensitive response to ophthalmic phenylephrine due to physiological sympathetic denervation).

Adverse Effects (ADR)

• Eye Locals: Conjunctival stinging and irritation of immediate onset; superficial punctate keratitis; transient tearing; Sun-refractory mydriasis with mild photophobia; increased IOP at closed angles.
• Systemic: Palpitations, sinus tachycardia, transient arterial hypertension, throbbing headache, cardiac arrhythmias and generalized skin pallor.