Atropine, Cyclopentolate, Tropicamide

Mechanism

💊 Commercial Names

Isopto Atropina, Colircusi Cicloplejico, Mydriacyl, Tropicacyl, Colircusi Tropicamida.

🔬 Pharmacological Group

Competitive acetylcholine receptor antagonists (parasympatholytics). Synthetic tertiary amines.

🧪 Standard Formulations

Atropine sulfate 0.5% and 1.0%, cyclopentolate hydrochloride 1.0%, tropicamide 1.0%.

Mechanism of Action

These agents act by competitively blocking cholinergic neurotransmission at the autonomic effector junctions of the eye:

• Competitive Blockade of Muscarinic M Receptors₃: They bind selectively and with high affinity to receptor M₃ in the iris sphincter and ciliary muscle, preventing the coupling of endogenous acetylcholine.
• Passive mydriasis: When the pupillary sphincter is inhibited, the sympathetic tone of the iris dilator muscle prevails (controlled by α receptors).₁), causing massive and sustained pupillary dilation (mydriasis).
• Cycloplegia Due to Ciliary Muscle Paralysis: Inhibition of the ciliary muscle prevents ocular accommodation for near vision, fixing the lens at its resting focal length for infinity (useful for accurately determining objective refraction, especially in pediatrics).
• Anti-inflammatory and Analgesic Effect in Uveitis: Relaxes the inflamed spasmodic ciliary body, relieving deep eye pain, and prevents the formation of posterior synechiae (adhesions of the iris to the anterior capsule of the lens) by keeping the pupil dilated and moving.

Pharmacokinetics

Quantitative Ocular and Systemic Pharmacokinetics

Security

Anticholinergic Systemic Toxicity: Atropine Intoxication Syndrome

Systemic absorption of ophthalmic atropine massively drains into the bloodstream, blocking muscarinic receptors of target organs systemically. In children and the elderly, this triggers the classic anticholinergic toxicity syndrome characterized by: 1) Mad as a hatter (delirium, psychomotor agitation, visual hallucinations and disorientation due to muscarinic blockade in the CNS); 2) Red as a beet (thermal compensation peripheral cutaneous vasodilation); 3) Dry as a bone (extreme xerostomia, anhidrosis of the mucous membranes and skin); 4) Hot as a hare (deep central hyperthermia due to inability to dissipate heat through sweat); 5) Blind as a bat (complete cycloplegia with severe photophobia) and severe reflex cardiac tachycardia.

Contraindications

• Absolute: Primary narrow-angle glaucoma or anatomically confirmed closed angle (mydriasis causes folding of the peripheral iris towards the iridocorneal angle, completely occluding the trabecular meshwork and triggering an acute bilateral obstructive glaucoma crisis with an emergent course); hypersensitivity to the chemical group of atropine.
• Relative: Fever of unknown origin or sepsis in children; pre-existing basal tachycardia; obstructive prostatic hypertrophy (risk of acute urinary retention).

Adverse Effects (ADR)

• Ocular Sites: Severe and persistent photophobia (impossibility of adaptive pupillary constriction); inability to read for near vision; allergic follicular conjunctivitis; transient elevation of IOP at predisposed angles.
• Systemic: Tachycardia, dryness of the skin and mucous membranes, urinary retention, constipation of mechanical dural origin, transient delirium and central hyperthermia.