Prostaglandin drops work by increasing uveoscleral outflow. Aqueous humor is the clear fluid the eye makes inside its front chamber. Most of that fluid leaves through the drainage angle. A smaller share leaves by a second route, through the ciliary body and the wall of the eye. That second route is the uveoscleral pathway. Prostaglandins open it wider. More fluid drains out, so the pressure falls. The drop does not change how much fluid the eye makes.
This is what sets prostaglandins apart from the other classes. Beta-blockers, carbonic anhydrase inhibitors, and alpha-2 agonists all lower pressure by making less fluid. Miotics work on drainage, but they pull open the trabecular meshwork in the angle instead. No glaucoma drop raises fluid production; that would push the pressure up. So the choices split neatly into two groups: make less fluid, or let more fluid out. Prostaglandins are given as a single evening dose, which is one reason patients stick with them. The drugs in this class are latanoprost (Xalatan), bimatoprost (Lumigan), and travoprost (Travatan).
Warn your patient about the side effects so he is not alarmed. These drops cause longer, thicker eyelashes, red eyes, and darker color in the iris and the eyelid skin. The iris change can be permanent. Ask at every visit whether he is still using the drop each evening. A once-daily drop is easy to forget, and a missed week can let the pressure climb. Remind him to close the lids gently for 2 minutes after each drop. He can also press lightly at the inner corner of the lids for 60 seconds. Knowing how each glaucoma class lowers pressure is a core part of the COA pharmacology content area.
Incorrect Answers
- A. Decreasing fluid production is how beta-blockers and carbonic anhydrase inhibitors work. Prostaglandins act on drainage instead, so they sit in a different group.
- B. No glaucoma drop is designed to make more aqueous humor. Adding fluid would raise the pressure, not lower it.
- C. Opening the trabecular meshwork is the action of miotics such as pilocarpine. Prostaglandins use the second, uveoscleral route.
Testing Pearls
- Prostaglandins are dosed once each evening.
- The class includes Xalatan, Lumigan, and Travatan.
- Side effects are lash growth, red eyes, and darker iris color.
- Beta-blockers, carbonic anhydrase inhibitors, and alpha-2 agonists all cut fluid production.
- Miotics lower pressure by opening the trabecular drainage angle.
References
- American Academy of Ophthalmology. Ophthalmic Medical Assisting: An Independent Study Course. Chapter 6, Basics of Ophthalmic Pharmacology.
