Friday, October 13, 2006

Section 7-C-2: Beta-blockers

Beta-adrenergic blockers (beta-blockers) and alpha-adrenergic agonists (see Section 7-C-3) work to lower IOP by affecting the autonomic (or sympathetic) nervous system. This system exists throughout the body interacting with alpha or beta receptors on cell membranes. Blockers inhibit the action at the receptor while agonists stimulate it. The effects are numerous but in the eye, the main effects are on aqueous production and pupillary constriction/dilation. The two main adrenergic receptors are the alpha and beta receptors. Within each class of receptor, there are subclasses having specific effects. Some medications are selective, meaning that they affect a specific subclass only, while others are nonselective, and they affect the entire class as a whole.

Beta blockers lower IOP by decreasing the production of aqueous. The most commonly used beta-blocker eye drop is timolol (brand name: Timoptic, Timoptic XE, Betimol, Istalol). The usual concentrations are 0.25% and 0.5%. It is very well tolerated and used either once or twice daily. It comes in both liquid and gel forms. The gel form typically is dosed once daily but has the disadvantage of causing transient blurring of vision. Timolol is easily identified by its cap color of yellow.

Beta-blocker eye drops (brand names in parenthesis) approved for glaucoma treatment include timolol (Timoptic, Betimol, Istalol), levobunolol (Betagan), carteolol (Ocupress), metopranolol (OptiPranolol) and betaxolol (Betoptic).

Timolol is the most commonly used nonselective beta-blocker and blocks both the beta1 and beta2 receptors. Stimulation of the beta1 receptor increases cardiac contractility, and the beta2 receptor affects bronchodilation. A nonselective blocker inhibits cardiac contractility and bronchodilation. These are, therefore, contraindicated in patients with asthma, emphysema, chronic obstructive pulmonary disease (COPD), bradycardia (low pulse rate), and congestive heart failure. Topical beta blockers have been widely used for glaucoma treatment since early 1980’s due to their efficacy and cost effectiveness. Studies have noted systemic side effects from absorption into the bloodstream from topical application of timolol. Topical beta-blockers may lose their effectiveness over time (tachyphylaxis) in some patients. If this occurs, another medication may be used to control the IOP. Other nonselective beta-blockers include levobunolol (Betagan), metipranolol (OptiPranolol), and carteolol (Ocupress). Other beta-blockers are selective for a specific beta receptor. Betaxolol (Betoptic) is a selective beta1 receptor antagonist. Its mechanism of action is similar to timolol, but since it is a selective beta1 blocker, it is better tolerated in patients with pulmonary disease than timolol. It could still have a slight beta2 blocking effect and thus, patients who develop shortness of breath should discontinue this medication.

Oral beta-blockers are often used for cardiovascular reasons. Concurrent use of oral beta-blockers may reduce the efficacy of the topical beta blocker. The main ocular side effects include redness, burning, scarring, decreased corneal sensation, decreased ocular blood flow, and inflammation. Although there are numerous reported ocular side effects, beta-blockers are usually very well tolerated. The main systemic side effects include decreased heart rate (bradycardia) and cardiac contractility, irregular heart rhythms (arrhythmias), worsening of congestive heart failure, bronchospasm, difficulty breathing, masking of low blood sugar in diabetics, and depression (Table 7-4). If any of the side effects occur with administration of beta-blocker eye drops, the medication should be discontinued by the prescribing doctor.

Table 7-4: Side Effects of Beta-Blockers:

Ocular

Systemic

Redness

Bradycardia (low pulse rate)

Burning

Arrhythmias (Irregular pulse)

Scarring

Worsening of congestive heart failure

Decreased corneal sensation

Bronchospasm

Decrease ocular blood flow

Masking of hypoglycemic (low glucose level) symptoms

Depression

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