Friday, October 13, 2006

Section 7-C-4: Carbonic Anhydrase Inhibitors (CAIs)

The topical CAIs include dorzolamide (Trusopt) and brinzolamide (Azopt). Both are much better tolerated than the systemic CAIs due to reduced systemic side effects. Both are dosed either two or three times daily. In addition, dorzolamide also comes in a combined form with timolol (Cosopt, Figure 7-3) for better compliance in administration. The CAIs inhibit the enzyme carbonic anhydrase, which in turn, reduces aqueous humor formation. Patients are typically placed on topical glaucoma medications prior to resorting to oral CAIs due to their systemic side effects which are often poorly tolerated. Topical CAIs are effective at lowering the IOP. They are well-tolerated and have limited systemic side effects. Ocular side effects include irritation, redness, bitter taste after administration, and corneal toxicity. Although much less frequent compared with systemic CAIs, they may produce a bitter taste and low blood cell counts (Table 7-5).


Figure 7-3. Combination beta-blocker and carbonic anhydrase inhibitor: timolol maleate/dorzolamide hydrochloride (Cosopt, Merck Co., Whitehouse Station, NJ)

Table 7-5: Side effects of topical Carbonic Anhydrase Inhibitors




Allergic symptoms


Bitter taste with administration


Low blood counts

Corneal toxicity

The CAIs are the only class of chronic glaucoma therapy which is administered systemically (orally). Acetazolamide (Diamox) and methazolamide (Neptazane, GlaucTabs) are the two oral CAIs which may be used for the treatment of glaucoma. Methazolamide is usually better tolerated since it has fewer side effects than acetazolamide. However, it may not be as effective as acetazolamide. Reported side effects include tingling of fingers/toes (paresthesias), urinary frequency, electrolyte imbalances, fatigue, anorexia, depression, low potassium levels (hypokalemia), poor taste, nausea, diarrhea, kidney stone formation (nephrolithiasis), and low blood cell counts. Systemic CAIs are contraindicated in those with an allergy to sulfa, since it is a sulfonamide drug, and in those with a history of renal disease, kidney stones, low potassium (especially in those taking thiazide diuretics), or abnormal blood counts. Oral CAIs may exacerbate these conditions, and should be avoided or used with caution (Table 7-6).

Table 7-6:
Systemic side effects of systemic Carbonic Anhydrase Inhibitors

Tingling of fingers/toes

Urinary frequency

Electrolyte imbalances



Low potassium levels

Metallic taste



Kidney stone formation

Abnormal blood counts

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