Question 258

Question:

A 55-year-old man develops loss of visual accommodation while taking an antidepressant drug. The most likely cause is blockade of which of the following receptors?

Correct Answer: D.  Muscarinic

Visual accommodation occurs by contraction of the ciliary muscle to increase the curvature of the lens. Parasympathetic nerve fibers are responsible for ciliary muscle contraction and these act on muscarinic receptors. The muscarinic receptors are activated by acetylcholine to stimulate contraction of the ciliary body. Many antidepressant drugs have anti-cholinergic properties, thus blocking the muscarinic receptors and resulting in the loss of visual accommodation.

Antidepressant medication also may have anti-histaminergic and anti-serotoninergic properties. However, these two receptors are not found in the ciliary muscle and are not responsible for stimulating visual accommodation.

There are no beta-adrenergic receptors that stimulate the ciliary muscle of the eye. Therefore, blockade of beta-adrenergic receptors would not cause loss of visual accommodation.

There are alpha-adrenergic receptors located in the radial muscle of the eye, but this muscle is not responsible for visual accommodation.