A 64-year-old African American woman is brought to the clinic by her husband due to a fall. She was walking in her living room when she tripped over an electrical cord. The patient has had several other ground-level falls in the last 6 months but has had no significant injuries. She also gave up driving a year ago following a near-miss motor vehicle accident. Past medical history is notable for hypertension and type 2 diabetes mellitus, for which she takes appropriate medications. Mental status and neurologic examinations are normal. Ophthalmic examination reveals reduced vision in her peripheral visual fields bilaterally and an elevated intraocular pressure. After discussion of treatment options, the patient is initiated on timolol ophthalmic drops. Which of the following structures is the most likely target of this medication?

Question

A 64-year-old African American woman is brought to the clinic by her husband due to a fall.  She was walking in her living room when she tripped over an electrical cord.  The patient has had several other ground-level falls in the last 6 months but has had no significant injuries.  She also gave up driving a year ago following a near-miss motor vehicle accident.  Past medical history is notable for hypertension and type 2 diabetes mellitus, for which she takes appropriate medications.  Mental status and neurologic examinations are normal.  Ophthalmic examination reveals reduced vision in her peripheral visual fields bilaterally and an elevated intraocular pressure.  After discussion of treatment options, the patient is initiated on timolol ophthalmic drops.  Which of the following structures is the most likely target of this medication?

Quizplus · Accepted Answer

11ec160b_3c22_8839_9249_ffbaad60758c_MD0001_00 This patient with chronic, progressive loss of peripheral vision has typical features of open-angle glaucoma. Glaucoma is a type of optic neuropathy characterized by atrophy of the optic nerve head. It is more common with advancing age and is especially common in certain ethnicities (eg, African Americans). Glaucoma is usually associated with elevated intraocular pressure (IOP) due to increased production or decreased outflow of aqueous humor. Aqueous humor is produced by the epithelial cells of the ciliary body. It is secreted into the posterior eye chamber and transferred through the pupil into the anterior eye chamber. The anterior chamber angle (iridocorneal angle) contains a trabecular meshwork through which the aqueous humor diffuses into Schlemm's canal (scleral venous sinus) and subsequently into episcleral and conjunctival veins. Diagnostic features of glaucoma include elevated IOP and abnormal visual field testing with decreased peripheral vision. Funduscopic examination will show an increased cup-to-disc ratio due to loss of ganglion cell axons. The goal of treatment in glaucoma is to decrease IOP with drugs that either decrease the production of aqueous humor or increase its outflow. Timolol and other nonselective beta blockers work by diminishing the secretion of aqueous humor by the ciliary epithelium. (Choices B, C, D, and E) The intraocular muscles of the eye, including the ciliary muscle and pupillary sphincter of the iris, receive parasympathetic innervation from cholinergic fibers of the oculomotor nerve (cranial nerve III). Cholinergic agonists (eg, pilocarpine) cause miosis by promoting contraction of the sphincter of the iris. This causes the anterior chamber angle to become wider and makes the trabecular meshwork more accessible to outflow of aqueous humor. Cholinergic agonists also promote ciliary muscle contraction, causing the lens to become more convex. Educational objective: Open-angle glaucoma is characterized by progressive loss of peripheral vision from elevated intraocular pressure. Timolol and other nonselective beta blockers work by diminishing the secretion of aqueous humor by the ciliary epithelium. Acetazolamide, a carbonic anhydrase inhibitor, also decreases aqueous humor secretion by the ciliary epithelium. Prostaglandin F2α (eg, latanoprost, travoprost) and cholinomimetics (eg, pilocarpine, carbachol) decrease intraocular pressure by increasing the outflow of aqueous humor.

A 64-Year-Old African American Woman Is Brought to the Clinic