H-1-MRS evidence of neurodegeneration and excess glutamate plus glutamine in ALS medulla

Objective: To determine whether short echo-time (TE) proton magnetic resona nce spectroscopic imaging (H-1-MRSI) can detect in vivo differences in sign al intensities of specific metabolites in the medulla of patients with ALS compared with healthy individuals and whether these metabolites could be us eful surrogate markers of disease. Background: H-1-MRSI can detect N-acetyl aspartate + N-acetylaspartylglutamate (abbreviated NAx), which is localized to neurons, and glutamate (Glu) + glutamine (Gln), abbreviated Glx, which may be important in ALS pathogenesis. The medulla is an ideal region to stu dy ALS because of its high density of nuclei and fiber tracts that frequent ly undergo degeneration, even when more rostral brain regions show minimal pathology. Methods: Ten patients with ALS and seven healthy control subject s underwent short TE H-1-MRSI on a 1.5 T clinical imaging system. Signal in tensities of NAx and Glx were normalized to creatine-phosphocreatine and co mpared between groups. Results: Compared with normal subjects, the medulla of patients with ALS had 17% lower NAx(p = 0.03) and 55% higher Glx (p = 0. 02) signals. Bulbar symptoms, represented by the ALS Functional Rating Scal e, correlated with Glx (r = -0.68, p = 0.03) but not NAx (r = 0.22, p = 0.5 3). Conclusion: There is in vivo H-1-MRSI evidence of neuronal degeneration or loss and excess Glu + Gin in the medulla of patients with ALS. Although this cross-sectional study cannot identify which change occurred first, th e higher Glx signal in the medulla of patients with more dysarthria and dys phagia is consistent with the hypothesis of Glu excitotoxicity in ALS patho genesis. Longitudinal H-1-MRSI studies of the medulla (and other brain regi ons) in more patients with ALS are required to confirm these findings and t o determine whether such metabolite changes will be useful in monitoring di sease progression, in clinical diagnosis, and in understanding the pathogen esis of ALS.