Vitreous amino acid concentrations in patients with glaucoma undergoing vitrectomy

Objective: To measure vitreous concentrations of glutamate and other amino acids in patients with glaucoma undergoing vitrectomy. Methods: Undiluted vitreous samples were collected from patients undergoing vitrectomy at the University of Iowa (Iowa City) between 1997 and 1998 (n=69). Vitreous concentrations of 16 amino acids, including glutamate, were determined using high-pressure liquid chromatography. Patients with a history of diabetes mellitus were excluded from the analysis. The study group consisted of those with a history of glaucoma (n=8), and the control group included those with an epiretinal membrane and/or macular hole with no history of glaucoma (n=17). Comparison of amino acid concentrations between the 2 groups was performed using a multifactor main effects model that adjusted for the effect of 10 selected covariates. Power analysis was done to determine the level of significant difference in amino acid concentrations, Results: The glaucoma group comprised vitreal specimens from patients with primary open-angle (n=3) and angle-closure glaucomas that included aqueous misdirection (n=2), uveitis with secondary angle-closure (n=2), and Axenfeld Rieger syndrome (n=1). Indications for vitrectomy in this group included epiretinal membrane, retinal detachment, aqueous misdirection, and uveitis. The control group included specimens from patients with a macular hole (n=11) and epiretinal membrane (n=7), with 1 eye having both. Surgical indications in controls were macular hole, retinal detachment, and epiretinal membrane. The mean±SD levels of vitreous glutamate, glycine, γ-aminobutyric acid, and alanine were 6.1±2.4, 16.3±7.5, 0.8±0.3, and 260.5±101.9 μM, respectively, in glaucoma and 5.2±2.3, 8.5±2.5, 0.6±0.2, and 159.5±54.9 μM in controls (P>.05 for all). None of the 16 amino acid concentrations measured showed a statistically significant difference between glaucoma and controls (P values between .06 and >.99). A power analysis indicated that a 1.8-fold elevation in the glutamate level was needed to reach significance. Main Outcome Measures: Vitreous amino acid concentrations. Conclusions: None of the 16 amino acids measured, including glutamate, were significantly elevated in the vitreous of glaucomatous eyes compared with controls. Our results are not consistent with the simple hypothesis of glutamate excitotoxicity in glaucoma. Instead, our findings indicate the dynamic nature of extracellular glutamate, whose concentration is dependent on complex mechanisms not yet fully understood. Further studies are needed to fully elucidate the role of glutamate in the pathogenesis of glaucoma.