Ocular β-Blockers in Glaucoma Management
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Topical β-blockers reduce the intraocular pressure (IOP) by blockade of sympathetic nerve endings in the ciliary epithelium causing a fall in aqueous humour production. Two types of topical β-blockers are available for use in glaucoma: nonselective, which block both β1- and β2-adrenoceptors; and cardioselective, which block only β1-receptors.
Of the β-Blockers commercially available, timolol, levobunolol, metipranolol and Carteolol are nonselective, and betaxolol is cardioselective. Twice-daily timolol is probably the most effective agent in lowering IOP, although levobunolol is equally effective and can be used once daily with little difference in effect. Carteolol is used twice daily and any theoretical advantage in diminished side effects confered by its partial β-agonist activity compared with timolol has not been fully substantiated. Metipranolol is effective twice daily and does not have partial β-agonist activity. Betaxolol has an effect comparable to timolol in lowering IOP, but is less effective in some patients.
β-Blockers can be used with other antiglaucoma medications, but their combined action with epinephrine (adrenaline) is suspect, particularly in the case of the nonselective β-blockers, and the effect should be assessed in patients on an individual basis. Local stinging can be a problem in some patients with betaxolol. The most serious side effects of β-blockers are the exacerbation of chronic obstructive airways disease with nonselective agents and the precipitation of bron-chospasm in some patients. Betaxolol seems relatively free of adverse respiratory effects, although this may be dose-related and extreme caution should still be exercised in patients with any history of respiratory illness. Because of the lower risk of precipitating side effects, betaxolol is probably the β-blocker of first choice for use in glaucoma; timolol or levobunolol are reserved for patients who do not respond satisfactorily to betaxolol and are quite free of respiratory disease.
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