SYSTEMIC EFFECTSOF OPHTHALMIC DRUGS
Topically applied eye drops are systemically absorbed by vessels in the conjunctival sac and the nasolacri-mal duct mucosa (see Case Discussion). One drop (typically, approximately 1/20 mL) of 10% phenylephrine contains approximately 5 mg of drug. Compare this dose with the intravenous dose of phenylephrine (0.05–0.1 mg) used to treat an adultpatient with acute hypotension. Medications applied topically to mucosa are absorbed systemically at a rate intermediate between absorption following intravenous and subcutaneous injection (the toxic subcutaneous dose of phenylephrine is 10 mg). Children and the elderly are at particular risk of the toxic effects of topically applied medica-tions and should receive at most a 2.5% phenyleph-rine solution (Table 36–4). Coincidentally, these patients are most apt to require eye surgery.Echothiophate is an irreversible cholinesterase inhibitor used in the treatment of glaucoma.
Topical application leads to systemic absorption and a reduction in plasma cholinesterase activity.Because succinylcholine is metabolized by this enzyme, echothiophate will prolong its dura-tion of action. Paralysis usually does not exceed 20–30 min, however, and postoperative apnea is
unlikely. The inhibition of cholinesterase activity lasts for 3–7 weeks after discontinuation of echo-thiophate drops. Muscarinic side effects of echothio-phate, such as bradycardia during induction, can be prevented with intravenous anticholinergic drugs (eg, atropine, glycopyrrolate).
Epinephrine eye drops can cause hypertension, tachycardia, and ventricular dysrhythmias; the dys-rhythmogenic effects are potentiated by halothane. Direct instillation of epinephrine into the anterior chamber of the eye has not been associated with car-diovascular toxicity.
Timolol, a nonselective β-adrenergic antagonist, reduces intraocular pressure by decreasing produc-tion of aqueous humor. Topically-applied timolol eye drops, commonly used to treat glaucoma, will often result in reduced heart rate. In rare cases, it has been associated with atropine-resistant bradycardia, hypo-tension, and bronchospasm during general anesthesia.