Friday, October 13, 2006

Section 7-C-3: Alpha-Adrenergic Agonists

Stimulation of the alpha1-adrenergic receptor produces dilation of the pupil, and the alpha2 receptor decreases aqueous production at the ciliary body. They inhibit the production of aqueous, but they can also facilitate the outflow of aqueous from the eye. Both mechanisms act to lower the IOP. Alpha2-specific adrenergic agents include brimonidine (Alphagan, Alphagan-P) and apraclonidine (Iopidine). Older, non-specific adrenergic agents include epinephrine and dipivefrin (Propine). Dipivefrin is a “pro-drug” which is converted in the eye into epinephrine, but the non-specific adrenergic agonists such as dipivefrin are rarely used in 2006.

Topical use of epinephrine and dipivefrin was also associated with significant side effects. These include high blood pressure, rapid heart rate (tachycardia), irregular heart beat (arrhythmias), burning, redness, conjunctival pigmentation, pupillary dilation, and swelling in the macula (cystoid macular edema) in patients who had previous cataract surgery without implantation of an intraocular lens implant. The more specific alpha2-adrenergic agonists, such as brimonidine or apraclonidine, are better tolerated compared to the older nonspecific adrenergic agents. Apraclonidine (Iopidine) was initially reported to be effective for the treatment of short-term IOP elevations after eye laser procedures. Brimonidine is the second generation of alpha2-agonists and has largely replaced apraclonidine. A common side effect of Alphagan is redness and ocular allergy. Alphagan-P contains a different preservative (Purite) which helps reduce the preservative-related allergy. Brimonidine is usually well tolerated and effective in lowering IOP. Common ocular side effects include allergic symptoms of redness, itching, and irritation. Pupillary dilation and upper lid retraction are also seen. Systemic side effects are uncommon but can include sedation, headache, and fatigue. Brimonidine should not be used in children under 2 and should be used with caution under the age of 5 due to respiratory and central nervous system depression (Table 7-5). It is contraindicated in patients taking monoamine oxidase inhibitors due to drug interactions.

Table 7-5: Side effects of Alpha2-agonists:

Ocular

Systemic

Redness

Hypotension (low blood pressure)

Itching

Respiratory depression (especially in infants)

Irritation

Central nervous system depression (especially in infants)

Pupil dilation

Sedation

Lid retraction

Fatigue

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