MEDizzy - To facilitate a certain eye exam we want to cause mydriasis,

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Pharmacology of peripheral nervous system 2

To facilitate a certain eye exam we want to cause mydriasis, but not alter normal control of accommodation. All of the following drugs are available as topical ophthalmic formulations. Which of the following would achieve our goals the best?

ExplanationStart off with a fundamental review of autonomic innervation of the eye insofar as control of pupil size, and effects on the ciliary muscle (which affects accommodation . . . the ability of the lens of the eye to thicken so we can see close-up). The sympathetic nervous system, via α-adrenergic receptor activation, has the ability to cause mydriasis. Activate those receptors and the pupil dilates; or block the opposing parasympathetic influences (via muscarinic receptor blockade) and the same thing happens, since there’s a relatively comparable influence of both branches of the autonomic nervous system on pupil size. However, there is little resting influence of the sympathetic nervous system (or adrenergic receptors) on the tone of the ciliary muscle. That is mainly a parasympathetic-dependent phenomenon that indirectly affects the shape of the lens of their eyes and, so, the point of sharp focus. And, for all practical purposes, β receptor activation has no influence on pupil size or accommodation. Now, classify the drugs by their sites and mechanisms of action. Eliminate atropine (a) as a possible answer choice. By blocking all ocular effects of normal parasympathetic innervation (muscarinic blockade), it will both dilate the pupil and impair accommodation. The same applies to homatropine (c), a drug that is, for all practical purposes, atropine-like. Pilocarpine (e), a muscarinic agonist, will constrict the pupil of the eye and impair cholinergic mechanisms involved in focusing of the lens. Isoproterenol (d) activates all β-adrenergic receptors, timolol (f) blocks them all, and as we said, β receptors are not relevant in terms of pupil size or accommodation. This leaves us with epinephrine: it has both α-adrenergic and β-adrenergic activating activity and no cholinergic/muscarinic activity. It will dilate the pupil and not significantly affect accommodation.