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Purpose: To assess novel differences in serum levels of glucose, lactate and amino acids in patients with normal-tension glaucoma (NTG) compared to agematched controls, at baseline and in response to universal hypoxia. Methods: Twelve patients diagnosed with NTG and eleven control subjects underwent normobaric hypoxia for 2 hr. Peripheral venous blood samples were taken at baseline, during hypoxia and in the recovery phase. Serum glucose and lactate levels were measured by a blood gas analyser. Amino acids were analysed by high-performance liquid chromatography. Results: Baseline levels of lactate and total amino acids were significantly lower in patients with NTG compared to healthy controls. No differences were seen in blood glucose levels between the two groups. Lactate levels remained unchanged during hypoxia in the control group, but increased in patients with NTG. In the recovery phase, total amino acid levels were reduced in the control group, whereas no changes were found in patients with NTG. Conclusion: Reduced serum levels of lactate and total amino acids were identified as potential markers for NTG. Moreover, significant differential regulatory patterns of certain amino acids were found in patients with NTG compared to control subjects. Overall, our results suggest a link between systemic energy metabolites and NTG and support a novel understanding of glaucoma as an inner retinal manifestation of a systemic condition.

The main risk factor for primary open-angle glaucoma (POAG) is increased intraocular pressure (IOP). It is of interest that about half of the patients have an IOP within the normal range (normal-tension glaucoma, NTG). Additionally, there is a group of patients with a high IOP but no glaucomatous neurodegeneration (ocular hypertension, OHT). Therefore, risk factors other than IOP are involved in the pathogenesis of glaucoma. Since the retina has a very high oxygen-demand, decreased autoregulation and a fluctuating oxygen supply to the retina have been linked to glaucomatous neurodegeneration. To assess the significance of these mechanisms, we have utilized a human experimental model, in which we stress participants with a fluctuating oxygen supply. Levels of oxidative stress molecules, antioxidants, and lipid mediators were measured in the plasma. Patients with NTG, OHT, and control subjects were found to have similar levels of oxidative stress markers. In contrast, patients with OHT had a higher level of total antioxidant capacity (TAC) and pro-homeostatic lipid mediators. Thus, we suggest that OHT patients manage fluctuating oxygen levels more efficiently and, thus, are less susceptible to glaucomatous neurodegenerations, due to enhanced systemic antioxidant protection.

Purpose Autoregulation is essential for a constant circulation of O2. The apparent dissimilarity of vulnerability towards increased intraocular pressure in patients with normal‐tension glaucoma (NTGs) and individuals with ocular hypertension (OHTs) has been linked to disturbed autoregulation. The retina is particularly O2‐dependent and susceptible towards oxidative stress. The purpose was to evaluate the impact of a fluctuating O2 level on retinal vessel diameters and plasma levels of vasoregulators, nitric oxide (NO) and endothelin‐1 (ET‐1), and total antioxidant capacity (TAC) in strictly characterized NTGs, OHTs and age‐matched controls. Methods NTGs (n = 10–16), OHTs (n = 9–10) and controls (n = 9–14) were exposed to 2 h of hypoxia followed by 30 min of normoxia. Fundus pictures and blood samples were taken before hypoxia (“baseline”), during hypoxia (“hypoxia”) and after hypoxia (“recovery”). NO, ET‐1 and TAC was measured in plasma. Retinal arterial and vein diameters were measured with MATLAB. Results Retinal arteries dilated in NTGs and OHTs during hypoxia, while controls constricted. No significant changes were seen in retinal veins. During hypoxia controls and NTGs increased plasma levels of ET‐1 and NTGs sustained a high level in recovery. OHTs did not regulate ET‐1 but maintained a lower level compared to NTGs during hypoxia and recovery. No significant differences were seen in NO. OHTs had a higher TAC compared to NTGs and controls throughout the experiment. Conclusions The present study suggests a link between NTG and dysfunctional autoregulation. ET‐1 increase in controls and NTGs during hypoxia. As a vasoconstrictor, a vascular constriction is expected as evident in controls. The observed dilation in NTGs therefore might indicate that molecular mechanisms for vasoregulation are abolished. Surprisingly, OHTs present a similar pattern of autoregulation to NTGs but there is a prominent difference in the TAC. The higher level of TAC in OHTs compared to NTGs and controls indicates that the antioxidant defence might be important in providing resistance towards glaucomatous neurodegeneration.