A Multicenter, Double-Masked, 2-Arm Parallel Group Study Comparing the Effect of Brimonidine 0.2% Versus Timolol 0.5% on Visual Field Stability in Patients With Low-Pressure Glaucoma

The term glaucoma describes a specific pattern of optic nerve head and visual field damage caused by a number of different diseases of the eye, most (but not all) of which are associated with an elevated eye pressure. Glaucoma is currently considered to be a progressive neurodegenerative disorder. Low-pressure glaucoma (LPG) is a type of open-angle glaucoma (OAG) with progressive visual field and optic nerve damage despite an untreated eye pressure in the statistically normal (mean 15.9, SD 2.9 mmHg) range, usually less than 21 mmHg. Therefore, in this condition, pressure-independent mechanisms (e.g., vascular or structural defects of the optic nerve) may be the main, if not the sole, cause of the optic neuropathy. LPG represents 6.7% to 68.3% of all OAGs.

Current glaucoma treatment is directed to lowering eye pressure using medical therapy (eye drops), laser treatment, and/or surgery, to a level that stops progressive optic nerve damage. The efficacy of lowering eye pressure in LPG has been reported. Both protocol medical treatments, brimonidine and timolol, show similar efficacy to lower eye pressure.

Laboratory research over the past decade indicates the potential to manage glaucoma not only by lowering eye pressure, but with treatment modalities aimed at the damage occurring at the optic nerve. Possible therapies may include agents effective as neuronal protectants to increase or prolong the survival rate of injured retinal ganglion cells. Treatments could also be directed to the rescue of nerve fibers from secondary degeneration, as stimulants to expand dendritic fields, and to promote nerve regeneration or neural transplantation.

Selective α2-adrenergic agonists have been shown to have a neuroprotective effect in animal models of focal cerebral ischemia. Brimonidine is reported to protect the optic nerve and retinal ganglion cells from secondary degeneration following a partial crush lesion to the adult rat optic nerve. One molecular mechanism for this neuroprotection may relate to up-regulation of neuronal survival factors. In rats, systemic α2-adrenergic agonists induce basic fibroblast growth factor mRNA in the retina. Treatment with α2-agonists before and during constant light exposure reduces retinal photoreceptor degeneration in albino rats. Animal studies demonstrate that topical administration of brimonidine results in pharmacologic concentrations of drug in the vitreous (100-170 nM). Therefore, ocular dosing with brimonidine provides a route for drug delivery to the retina in amounts sufficient to bind and activate the α2-adrenoceptor and provide a neuroprotective effect.

The study hypothesis is to evaluate the ability of topical treatment with 0.2% brimonidine, a highly selective α2-adrenergic agonist, to impart neuroprotection to the damaged optic nerve in patients with LPG. Comparison is made to 0.5% timolol, a nonselective β-adrenergic antagonist, without reported neuroprotective properties. Patients will be randomly assigned to twice daily double-masked treatment with one of these drugs. Neuroprotection will be assessed by evaluation of automated static visual fields performed at 4 month intervals for 4 years of treatment.