In vivo evidence of glutamate toxicity in multiple sclerosis

Objective There is increasing evidence that altered glutamate (Glu) homeostasis is involved in the pathophysiology of multiple sclerosis (MS). The aim of this study was to evaluate the in vivo effects of excess brain Glu on neuroaxonal integrity measured by N-acetylaspartate (NAA), brain volume, and clinical outcomes in a large, prospectively followed cohort of MS subjects. Methods We used multivoxel spectroscopy at 3T to longitudinally estimate Glu and NAA concentrations from large areas of normal-appearing white and gray matter (NAWM and GM) in MS patients (n = 343) with a mean follow-up time of 5 years. Using linear mixed-effects models, Glu was examined as a predictor of NAA decline, annualized percentage brain volume change, and evolution of clinical outcomes (Multiple Sclerosis Functional Composite [MSFC], Paced Auditory Serial Addition Test-3 [PASAT], and Expanded Disability Status Scale). Glu/NAA ratio was tested as a predictor of brain volume loss and clinical outcomes. Results Baseline Glu_[NAWM] was predictive of accelerated longitudinal decline in NAA_[GM] (-0.06mM change in NAA_[GM]/yr for each unit increase in Glu; p = 0.004). The sustained elevation of Glu_[NAWM] was predictive of a loss of 0.28mM/yr in NAA_[NAWM] (p < 0.001) and 0.15mM/yr in NAA_[GM] (p = 0.056). Each 10% increase in Glu/NAA _[NAWM] was associated with a loss of 0.33% brain volume/yr (p = 0.001), 0.009 standard deviations/yr in MSFC z-score (p < 0.001), and 0.17 points/yr on the PASAT (p < 0.001). Interpretation These results indicate that higher Glu concentrations increase the rate of NAA decline, and higher Glu/NAA_[NAWM] ratio increases the rate of decline of brain volume, MSFC, and PASAT. This provides evidence of a relationship between brain Glu and markers of disease progression in MS.