Treating glaucoma: what are carbonic anhydrase inhibitors?

Glaucoma can be categorised through high or imbalanced intraocular pressure (IOP) resulting in damage to the optic nerve fibres. It is this high eye pressure that can lead to the progressive loss of vision, and why glaucoma is considered a serious condition.

There are a number of different ways that glaucoma is treated, including eye drops, corrective lenses, and even surgery. The most common treatment for glaucoma is eye drops. Your ophthalmologist may have talked to you about carbonic anhydrase inhibitors: eye drops or medication that work to reduce intraocular pressure.

Here, we’ll take a closer look at what carbonic anhydrase inhibitors actually are, and how they can help to treat glaucoma.

How do carbonic anhydrase inhibitors work in glaucoma?

A part of the eye called the ciliary body produces a fluid called aqueous humour. This thin, transparent fluid nourishes the cornea and the lens, and ultimately works to maintain eye pressure. It does this by being produced and drained at an equal rate. Numerous studies have shown the main component of this fluid is sodium bicarbonate.1

A group of enzymes called carbonic anhydrases are responsible for the secretion of aqueous humour, but sometimes they get carried away and produce too much fluid, causing a rise in intraocular pressure. To tackle problem carbonic anhydrase inhibitors (CAIs are used to suppress the activity of carbonic anhydrase enzymes. This, in turn, reduces the rate of bicarbonate and aqueous humour secretion.2

Are there different types of carbonic anhydrase inhibitors?

CAIs are available as eye drops as well as oral medication. The difference here often comes down to whether your glaucoma is controlled by eye drops at present.

If, for example, you require more than one type of eye drop, then you may need to take each medicine in a certain order as prescribed by your doctor. If you are using more than one type of eye drop, then it’s important to wait five minutes between eye drop medicines.

The oral form, on the other hand, is usually an alternative for people whose glaucoma is not controlled by medication eye drops.

Acetazolamide

CAIs, such as acetazolamide, were one of the first generation of diuretics (to decrease fluid build-up) and have been in use for a long period of time.

Acetazolamide is the best studied version, and is frequently administered long-term in oral form, due to its efficient reduction of IOP and minimal toxicity.

Dorzolamide and brinzolamide

Dorzolamide and brinzolamide are potent water-soluble CAIs that penetrate the cornea. They are usually administered in the form of eye drops, and can acutally show fewer systemic and renal side effects when compared against other treament options.3

As these two CAIs are strong enough to penetrate the cornea and reach the ciliary body, both can be used topically, and are often used in combination with other treatment options when necessary.

What are the side effects of carbonic anhydrase inhibitors?

The side effects of CAIs differ slightly depending on how they are administered.

Systemic medications (medications that can affect the whole body) like acetazolamide can cause side effects like numbness and tingling of extremities, a metallic taste in the mouth, fatigue, and gastrointestinal irritation.4 5

Whereas if applied topically (applied to a particular spot in or on the body), side effects tend to be less severe: including a stinging sensation, burning or reddening of the eye, or blurred vision — which is due to the acidic nature of the eye drop solution.

What next?

In addition to CAIs, there’s a whole host of treatment options that are available. Your optometrist can give you some indications of treatments available and refer you to an ophthalmologist for specialist discussions surrounding specific treatments. Whether you have questions about treatment options, or just want advice on maintaining eye health, you can book an appointment today.

For the time being, if you’re interested in learning more about the glaucoma treatment process, head to our dedicated glaucoma treatment resources page to learn more about the condition.

References

1. Scozzafava, A., Mastrolorenzo, A. & Supuran, C. T. Carbonic anhydrase inhibitors and activators and their use in therapy. Expert Opin. Ther. Pat. 16, 1627–1664 (2006).
2. Mincione, F. et al. The development of topically acting carbonic anhydrase inhibitors as antiglaucoma agents. Curr. Top. Med. Chem. 7, 849–854 (2007).
3. Sam R et al (2015): Diuretic agents. Chapter 15. In: Basic and Clinical Pharmacology. 13th Ed. Katzung BG, Trevor AJ (Editors). McGraw-Hill / Lange.
4. Epstein, D. L. (1977). Carbonic Anhydrase Inhibitor Side Effects. Archives of Ophthalmology, 95(8), 1378.
5. Scozzafava, A. et al. Carbonic anhydrase inhibitors. Part 74. Synthesis of water-soluble, topically effective, intraocular pressure-lowering aromatic/heterocyclic sulfonamides containing cationic or anionic moieties: is the tail more important than the ring? J. Med. Chem. 42, 2641–2650 (1999)

Ross Campbell
BSc (Hons) MCOptom Prof Cert Glauc. Prof Cert Med Ret.

Ross is an experienced optometrist, pre-registration optometrist supervisor, and lead assessor for the Wales Optometry Postgraduate Education Centre (WOPEC)… Read more