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What I Learned About CAIs/Alpha-2 Agonists That Will Help Any Optometry Student

Posted by Talin Amadian on September 22, 2020 at 12:30 PM
Talin Amadian
Dr. Talin Amadian is a practicing optometrist, writer and content contributor for Optoprep. She graduated from Western University of Health Sciences College of Optometry and continues to practice in Southern California. Her clinical training includes Neuro-Ophthalmology, Cornea and Refractive Surgery, Glaucoma and Ocular Disease. Dr. Amadian takes pride in educating patients and providing specialized care and education based on each patient’s needs. She is passionate about dry eye treatment and management. During her spare time, she enjoys mentoring and helping prospective optometry students succeed.


In the last blog post, we covered first-line glaucoma medications, Prostaglandin analogs, and Beta-blockers. Besides these key players, there are a few more categories of glaucoma medications that surely deserve recognition. Let’s review alpha-2 agonists and carbonic anhydrase inhibitors!

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Alpha-2 Agonists

Mechanism of action:

 Alpha-2 agonists reduce the amount of aqueous secretion and enhance aqueous outflow through the uveoscleral route, thus decreasing intraocular pressure.


The recommended dosage for Iopidine® and Alphagan® is three times per day (TID).

Side effects:

  • Ocular
    • Conjunctival hyperemia
    • Burning/stinging/foreign body sensation
    • Blurred vision
    • Conjunctival vessel blanching
    • Mild miotic effect
  • Systemic
    • Decrease in blood pressure and heart rate
    • Contraindicated in patients taking monoamine oxidase inhibitors (MAOIs)
    • Caution in those with severe cardiovascular disease
    • Avoid in young children due to crossing the blood-brain barrier


  • Iopidine® (Apraclonidine): In addition to decreasing aqueous secretion and increasing uveoscleral outflow, Iopidine® may have additional IOP-lowering effects due to its influence on ocular blood flow. Iopidine® shows a significant drop in IOP that lasts about 12 hours, with a peak effect at about 3-5 hours, which can be up to a 30-40% drop in IOP. However, the benefits of Iopidine® in most patients are short-lived due to the development of tachyphylaxis or ocular allergies that may warrant discontinuation; therefore, Iopidine® is most commonly used after laser surgery of the anterior segment to offset potential acute IOP spikes.
  • Alphagan® (Brimonidine): Alphagan® is a potent and highly selective alpha-2 adrenoreceptor agonist (about 30x more selective than apraclonidine). There is also evidence that shows that Alphagan® may provide neuroprotective properties, though controversial. Peak IOP-lowering with Alphagan® occurs about 2 hours after the instillation of the medication and lasts about 10-12 hours; therefore as a monotherapy, Alphagan® is typically dosed three times daily (TID). Alphagan is usually not the initial drug used to treat glaucoma but is often rather used as a secondary medication for further IOP reduction.

Carbonic Anhydrase Inhibitors (CAIs)

Mechanism of action:

Inhibit aqueous secretion.


TID as monotherapy, BID as an adjunctive treatment.

Side effects:

  • Ocular irritation, foreign body sensation, stinging
  • Blurred vision
  • SPK, conjunctivitis
  • Headaches
  • Bitter taste


  • Caution in patients with allergies to sulfonamides
  • Caution in patients with severe renal and/or hepatic impairment 
  • Avoid in patients who are taking oral carbonic anhydrase inhibitors


  • Trusopt® (dorzolamide): Trusopt® is typically used TID as monotherapy, or BID as an adjunctive treatment. Its IOP-lowering effect has been shown to be similar to betaxolol but inferior to timolol (average decrease in IOP is approximately 23-24%, with the peak effect occurring 2 hours after administration). Trusopt® should be used with caution in patients exhibiting corneal endothelial dysfunction, as some studies have shown that it may cause further decompensation.
  • Azopt® (brinzolamide): The IOP-lowering effect of Azopt® has been proven to be equivalent whether dosed at BID or TID. Its hypotensive effect is similar to that of Trusopt®.

Now that we reviewed the categories of drugs used for Glaucoma treatment, stay tuned for the next blog post for all you need to know about combination drugs!

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Topics: Agonists, CAI's, Alpha-2

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