Design
Controlled clinical trial to evaluate the effect of oral magnesium therapy on ocular blood flow and visual field perimetry indices in patients with normotensive glaucoma (NTG). Blood magnesium level measurement, visual field analysis, and color Doppler imaging of the orbital vessels were done before treatment and at 1 month.
Participants
Fifteen patients with NTG (study group) received 300 mg oral magnesium citrate for 1 month, while 15 patients with NTG (control group) received no treatment.
Key Findings
In the study group, visual field measurements improved significantly: Mean deviation improved from -3.7+/-1.9 (mean +/- standard deviation) at baseline to -2.5+/-1.8 at 1 month (P<0.05) and pattern standard deviation improved from 3.6+/-2.3 at baseline to 2.8+/-2.6 at 1 month (P<0.05). Color Doppler imaging indices did not change after magnesium therapy. Other mechanisms than increased ocular blood flow may be responsible for the improvement in the visual field with oral magnesium therapy.
Practice Implications
Glaucoma reflects damage to the optic nerve. Glaucoma can result in the gradual loss of peripheral vision resulting in tunnel vision and ultimately blindness. Usually, the damage to the optic nerve is caused by increased pressure within the eye, which results from greater production than outflow of the fluid of the eye (the aqueous humor). The normal intraocular pressure (IOP) is about 10 to 21 mm Hg. In chronic glaucoma, the intraocular pressure is usually mildly to moderately elevated (22 to 40 mm Hg). In acute glaucoma, the intraocular pressure is greater than 40 mm Hg.
In some cases glaucoma develops in people with normal IOP. Referred to as low-tension glaucoma or normotensive glaucoma, this form accounts for approximately 25 to 30 percent of all glaucoma cases in the United States.
Since elevated IOP is not a factor in NTG, other factors are responsible for the optic nerve damage. Suggested causes include:
Reduced blood flow
Early nerve cell death
Nerve irritation
Excess glutamate production
Autoimmune disease
The condition is more common in women than in men and affects adults averaging 60 years old. A common risk factor for NGT is low blood pressure.
This study showing benefits in visual field improvements after oral magnesium supplementation is highly significant, as NTG is not effectively treated by conventional medications. It is also extremely thought-provoking, as a mechanism of action must be identified.
Since the benefits of magnesium therapy are not due to changes in blood flow, it is possible that the benefit is related to reducing increased glutamate concentrations in the extracellular spaces around the optic nerve. Glutamate is a major excitatory neurotransmitter and an important neurotoxin that promotes neuronal cell death. It is definitely implicated in NTG. Since magnesium has been shown to stimulate the blockade of neural receptors that result in a decrease in glutamate release during ischemia,1 this mechanism could also explain its benefits in NTG.