Brain GABA and glutamate levels across pain conditions: A systematic literature review and meta-analysis of 1H-MRS studies using the MRS-Q quality assessment tool

BACKGROUND: A proposed mechanism of chronic pain is dysregulation between the main inhibitory (GABA) and excitatory (glutamate) neurometabolites of the central nervous system. The level of these neurometabolites appears to differ in individual studies of people with pain compared to pain-free controls across different pain conditions. However, this has yet to be systematically investigated. AIMS: To establish whether GABA, glutamate, glutamine and Glx levels differ across pain conditions when compared to pain-free controls. METHODS: Five databases were searched. Studies were included if they investigated: 1) A pain condition compared to control. 2) Reported GABA, glutamate, glutamine or glutamate/glutamine level. 3) Used 1H-Magnetic Resonance Spectroscopy (Prospero Project ID CRD42018092170). Data extracted included neurometabolite level, pain diagnosis, and spectroscopy parameters. Meta-analyses were conducted to establish the difference in neurometabolite level between participants with pain and pain-free controls for different pain conditions. The MRS-Q was developed from existing clinical consensus to allow for the assessment of quality in the included studies. RESULTS: Thirty-five studies were included investigating combinations of migraine (n = 11), musculoskeletal pain (n = 8), chronic pain syndromes (n = 9) and miscellaneous pain (n = 10). Higher GABA levels were found in participants with migraine compared to controls (Hedge's G 0.499, 95%CI: 0.2 to 0.798). In contrast, GABA levels in musculoskeletal pain conditions (Hedge's G -0.189, 95%CI: 0.530 to 0.153) and chronic pain syndromes (Hedge's G 0.077, 95%CI: 1.612 to 1.459) did not differ from controls. Results for other brain neurometabolites revealed significantly higher levels for glutamate in participants with migraine and Glx in chronic pain syndromes compared to controls. CONCLUSION: These results support the theory that underlying neurometabolite levels may be unique in different pain conditions and therefore representative of biomarkers for specific pain conditions.