Serum Levels of Vitamin C & Uric Acid in Normal Tension Glaucoma

Oxidative Stress & Normal Tension Glaucoma

The occurrence of normal-tension glaucoma (NTG) varies worldwide. In the US, up to 15-25% of people with primary open-angle glaucoma (POAG) experience NTG. It is considerably more prevalent in Japan, with NTG comprising more than 70% of glaucoma cases.

In NTG, progressive damage can occur even with intraocular pressure (IOP)-lowering intervention. The cause of optic nerve damage in NTG is not fully understood, but oxidative stress is believed to be one contributing factor. Current evidence, for example, points to the association between oxidative stress and ganglion cell death without elevated IOP. In addition, there are some reports of lower levels of circulating antioxidants such as the major cellular antioxidant glutathione, in POAG patients.

These and other findings led Japanese researchers to examine serum levels of antioxidant vitamins A (beta-carotene), C and E, as well as folic acid and endogenous uric acid in NTG patients.

Beta-carotene and vitamin A are fat soluble antioxidants, while water soluble ascorbic acid and uric acid are strong reducing agents and potent antioxidants. In humans, over half the antioxidant capacity of plasma comes from uric acid, and vitamin C is highly concentrated in the aqueous humor. While not an antioxidant, folic acid insufficiency results in microvascular-damaging levels of homocysteine.

Study Design

Sixty patients with newly diagnosed POAG were consecutively screened for inclusion in this study. After diagnosis of POAG, patients underwent 24-hr IOP measurements, and 47 newly diagnosed consecutive NTG patients (18 males, 29 females; mean age 59.5 years) were enrolled. The control subjects were recruited from subjects who came to the clinic for annual refractive check-up. The 44 consecutive control subjects (16 males, 28 females; mean age 62.7 years) had no ocular diseases.

Serum levels of vitamins A, folic acid, C, E, and uric

acid were measured, and the values compared between the NTG and control groups by the Mann-Whitney U test.

Results

Serum levels of vitamin C were significantly lower in NTG patients than in normal healthy controls (P=0.04; NTG 4.6±4.0 μg/ml, and control 6.3±3.9 μg/ml).

Uric acid concentrations were significantly higher in NTG patients compared to controls (P=0.01; NTG 5.8±1.5 mg/dl, and control 4.9±1.4 mg/dl).

No statistically significant differences were observed in the other vitamins measured.

Comments

The results of this study indicate that uric acid levels were increased and vitamin C levels were decreased in NTG patients. The authors speculate that higher serum uric acid and lower serum vitamin C may change the trabecular meshwork physiology. In turn, this may decrease the outflow facility of aqueous humor, causing eventual IOP elevation and glaucomatous optic neuropathy.

The diurnal IOP of the NTG patients in this study was under 21 mmHg, yet significantly higher than that of the normal controls (P=0.001). According to the authors, low vitamin C levels may cause modest IOP elevation and optic neuropathy even in NTG patients.

Vitamin C is thought to play an important role in reducing reactive oxygen species in the trabecular meshwork. Recent studies suggest that oxidative stress accelerates trabecular meshwork cell death. Further, in vitro studies indicate that interactions between uric acid and vitamin C can change the viscosity of some glycosaminoglycans, which are found in the trabecular meshwork.

The authors call for placebo-controlled trials to investigate the effects of vitamin C on IOP and visual field change in patients with NTG.

Reference