Metabolic Diaschisis in Mild Traumatic Brain Injury

Neurophysiological diaschisis presents in traumatic brain injury (TBI) as functional impairment distant to the lesion site caused by axonal neuroexcitation and deafferentation. Diaschisis studies in TBI models have evaluated acute phase functional and microstructural changes. Here, in vivo biochemical changes and cerebral blood flow (CBF) dynamics following TBI are studied with magnetic resonance. Behavioral assessments, magnetic resonance spectroscopy (MRS), and CBF measurements on rats followed cortical impact TBI. Data were acquired pre-TBI and 1-3 hours, 2-days, 7-days, and 14-days post-TBI. MRS was performed on the ipsilateral and contralateral sides in the cortex, striatum, and thalamus. Metabolites measured by MRS included n-acetyl aspartate (NAA), aspartate (Asp), lactate (Lac), glutathione (GSH), and glutamate (Glu). Lesion volume expanded for 2 days post-TBI and then decreased. Ipsilateral CBF dropped acutely vs. baseline on both sides (-62% ipsilateral, -48% contralateral, p<0.05) but then recovered in cortex, with similar changes in ipsilateral striatum. Metabolic changes vs. baseline included increased Asp (+640% by Day 7 post TBI, p<0.05) and Lac (+140% on Day 2 post TBI, p<0.05) in ipsilateral cortex, while GSH (-67% acutely, p<0.05) and NAA decreased (-50% on Day 2, p<0.05). In contralateral cortex Lac decreased (-73% acutely, p<0.05). ANOVA showed significance for Side (p<0.05), Time after TBI (p<0.05), and interactions (p<0.005) for Asp, GSH, Lac, and NAA. Transient decreases of GSH (-30%, p<0.05, acutely) and NAA (-23% on Day 2, p<0.05) occurred in ipsilateral striatum with reduced GSH (-42%, p<0.005, acutely) in the contralateral striatum. GSH was decreased in ipsilateral thalamus (-59% ipsilateral on Day 2, p<0.05). Delayed increases of total Choline were seen in the contralateral thalamus were noted as well (+21% on Day 7 post TBI, p<0.05). Both CBF and neurometabolite concentration changes occurred remotely from the TBI site, both ipsilaterally and contralaterally. Decreased Lac levels on the contralateral cortex following TBI may be indicative of reduced anaerobic metabolism during the acute phase. The timing and locations of the changes suggest excitatory and inhibitory signaling processes are affecting post-TBI metabolic fluctuations.