Establishing that a drug therapy is neuroprotective requires that efficacy with regard to visual field preservation can be proven.¹ Traditional theories on glaucoma have suggested that successful lowering and control of IOP would result in slowing of visual field loss progression. Brimonidine is an ?₂- agonist that is believed to possess neuroprotective properties. Argon laser trabeculoplasty (ALT) is a surgical therapy that focuses solely on lowering IOP. According to Stefano Gandolfi, MD, the comparison of these two treatments in regard to visual field loss progression may yield insight into the neuroprotective qualities of brimonidine.²
Brimonidine is available as an ophthalmic solution and is currently used in the treatment of open-angle glaucoma. Approved by the U.S. Food and Drug Administration (FDA) in 1996, brimonidine is the first drug in its class to be used for the treatment of glaucoma. This drug lowers IOP by reducing the production of aqueous humor and increasing the outflow of aqueous humor.³ ALT is a laser therapy that is used to lower IOP in patients with glaucoma. Comparison of visual field progression in patients treated with brimonidine versus patients treated with ALT may be revealing, since ALT is a treatment that focuses on targeting the trabecular meshwork to increase outflow of aqueous. ALT, unlike brimonidine, is not a drug therapy and specifically works by lowering IOP.²

A prospective, randomized, investigator-masked, clinical trial examined IOP-unrelated effects of brimonidine on visual function in patients with glaucoma. Eligibility requirements were as follows: glaucomatous visual field defect (24/2 Humphrey field test), IOP less than or equal to 19 mm Hg with two drugs, open-angle glaucomatous neuropathy (Heidelberg Retinal Tomograph [Heidelberg Technology, Heidelberg, Germany], Moorfield regression analysis) and clear lens. According to the Lens Opacities Classification System II scores, best-corrected visual acuity scores should be lower than 0.2 LogMAR refraction within 5 D spherical to 2 D spherical, with no comorbidity.²

This study involved an 18-month observational period to find patients with an established progression as determined by visual field testing. This period was followed by interventional therapy (for the patients that showed progression) with brimonidine (0.2%, twice daily) or ALT (360°), in addition to the preexisting dual medical therapy. Visual field was tested every 3 to 4 months, with five to six visual field tests per eye.²

Fifty-two eyes were enrolled in the study, 27 randomized to brimonidine (0.2% twice daily) and 25 to ALT therapy (360°). In cases where IOP was decreased less than 10%, the eyes were crossed over to the alternative treatment. After failing eyes were crossed over to treatment, 50 eyes qualified for follow-up. 29 in the brimonidine group where IOP was decreased less than 10%, and 21 in the ALT group. Forty-one eyes completed follow-up (22 brimonidine and 19 ALT).²

Results indicated that, despite offering less IOP control, brimonidine 0.2% was more effective than 360° ALT in reducing the progression of visual field deterioration in glaucoma. Using a paired Student t-test, the effect of brimonidine on mean slope was found to be significant with a power of 90% and an alpha probability of 5%. Visual field progression after treatment with brimonidine was slower than treatment by ALT with a power of 85% and an alpha probability of 10%.²

Comparison of visual field progression in patients treated with brimonidine versus patients treated with ALT may be revealing, since ALT is a treatment that focuses on targeting the trabecular mesh work to increase outflow of aqueous.

Results indicated that, brimonidine 0.2% was more effective than 360° ALT in reducing the progression of visual field deterioration in glaucoma.

The study concluded that a comparison of brimonidine with ALT treatment in regard to effect on visual field loss progression has shown that IOP lowering alone does not account for the benefits of brimonidine. Brimonidine was shown to be more effective in slowing visual field loss progression than ALT despite less effectiveness in the lowering of IOP This suggests that brimonidine may have neuroprotective properties.²