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Oral Meds:  Carbonic Anhydrase Inhibitors


This category of medicines is used in three main arenas within the context of eye care:

• Chronic glaucoma forms
• Acute angle-closure attacks
• Pseudotumor cerebri

Cerebrospinal fluid (CSF) and aqueous humor are produced by very similar mechanisms. By reducing secretory tissue activity of carbonic anhydrase, CSF and aqueous production is reduced, usually by 30-50%. Only hyperosmotic agents, such as 50% glycerin, can diminish IOP as markedly as an oral CAI.

With the advent of topical CAIs, oral use has dramatically decreased, although there are still clinical circumstances where an oral CAI is necessary. The two most commonly used oral CAIs are acetazolamide (Diamox) and methazolamide (Neptazane).

Acetazolamide is best suited for acute pressure rises of any etiology, whereas methazolamide is best suited for chronic, protracted care as in POAG.

The typical dosage of acetazolamide for acute pressure spikes is two 250mg tablets, followed by an additional dose in 3 to 4 hours if needed. The 500mg Diamox Sequels are best suited for chronic therapy because of their time-release formulation. This time-release pharmacodynamic is why these sequels are better tolerated. In acute pressure rises, a time-release formulation of acetazolamide is not optimum; rather the tablet formulation provides rapid drug effect, which is much desired.

The typical dosage for methazolamide is 25mg bid, stepping up to 50mg bid as needed. Methazolamide is usually well-tolerated and carries a much reduced risk of renal calculi (kidney stones) compared to acetazolamide.

All CAIs are sulfonamide-type drugs (just as oral sulfonylureas are used in treating diabetes). Therefore, the first question to ask any patient for whom CAIs (either topical or systemic) are contemplated is their history regarding drug allergies.

As with all drugs, there are circumstances where CAIs are contraindicated. They are:

• Allergy to sulfa drugs
• Diabetics susceptible to ketoacidosis
• Advanced hepatic insufficiency
• History of kidney stones
• COPD because of CO2 retention
• History of blood dyscrasias

Suffice it to say, this class of drug presents a clinical challenge in chronic dosing. However, if the patient is not sulfa-allergic, then using 500mg in managing acute pressure spikes is indeed simple and very straightforward.

In the exceedingly rare circumstance in which you have a patient in acute angle closure who is truly allergic to sulfa-based drugs, then substitute an orally administered hyperosmotic agent. The only such product available is 50% glycerin, marketed as Osmoglyn, by Alcon Surgical. While there are exact and precise dosing guidelines, we much prefer a direct, clinically practical approach. Simply have the patient drink about one-half of a typical 8oz. glass of Osmoglyn over cracked ice. Ask the patient to drink this thick, Karo syrup-like liquid slowly over about five to ten minutes, which will minimize the likelihood of vomiting. The glycerin makes the blood slightly hyperosmotic and effectively attracts water from the vitreous, causing it to shrink rapidly, thus dramatically lowering IOP.

This packs a caloric load and must be used cautiously in patients with diabetes. Our endocrine consultants advise: Patients with IDDM should monitor blood glucose every couple of hours and adjust insulin on a sliding scale; for patients with non-IDDM, no therapeutic adjustment is necessary. The blood glucose level will spike, but will also return to baseline in one to two days.