This exploratory study indicated that the serum metabolome could be employed as an independent predictive factor to distinguish DAI from TBI patients. Serum metabolites (the combination of LPC 22:3 sn-2 and carnitine C8:1) presented high diagnostic accuracy for DAI (AUC = 0.944; sensitivity, 88.7%; specificity, 90.4%), but they showed relatively weak predictive ability for the 6-month GOSE in TBI patients.
DAI is a common primary injury in TBI, characterized histopathologically by focal axonal shearing and rupture[1, 21]. The injury mainly occurs in the corpus callosum, gray-white matter junction, upper brainstem, with disturbance of consciousness as a typical manifestation[22, 23]. Over 50 million people worldwide suffer from TBI each year, of which 25%-40% are complicated with DAI. Most DAI patients are in critical condition, with mortality rates as high as 40%-60%, and more than half of the survivors leave behind long-term sequelae or serious neurological deficits[3, 24–27]. The cognitive and other long-term neurological functions of DAI patients can be improved through early and effective treatment[8–10]. Therefore, the most crucial and challenging aspect currently is the early identification of DAI in the acute phase of traumatic brain injury.
Metabolism is the collective term for a series of orderly chemical reactions that occur within an organism to sustain life, encompassing both substance metabolism and energy metabolism[11, 12]. Metabolomics is a research approach that involves the quantitative analysis of all metabolites within an organism, aiming to identify the relative relationships between metabolites and physiological or pathological changes. It is widely applied in various fields closely related to human health, such as disease diagnosis and pharmaceutical research[11, 13].Thomas et al.[28] conducted a comprehensive metabolomics study on 716 TBI patients and non-TBI reference patients, they identified that choline phospholipids (sphingomyelins, ether phosphatidylcholines and lysophosphatidylcholines ) were negatively correlated with the severity of TBI and were among the strongest predictive factors for the prognosis of TBI patients. Banoei et al.[29] found that increased levels of glutamate, acylcarnitines, glucose, lactate, lysophosphatidylcholines and aromatic amino acids were associated with poor outcomes, and the serum metabolome on the first and fourth days post-injury showed high predictive value and accuracy for patient outcomes (AUC > 0.99). Two reviews on TBI metabolomics mentioned that changes in metabolites such as N-acetyl aspartate and propylene glycol in cerebrospinal fluid, serum, and urine are associated with the severity and prognosis of TBI[15, 16].
Current research has found that tau protein, neuron-specific enolase, S-100beta, eurofilaments, Peripherin and Hemopexin, myelin basic protein and beta-Amyloid precursor protein may be associated with DAI[2, 30–32]. However, some large molecular biomarkers face challenges in crossing the blood-brain barrier (BBB) into the bloodstream, making their collection difficult and hindering their widespread clinical application[3, 33]. The BBB consists of a lipid bilayer membrane structure of brain capillary endothelial cells that is lipophilic in nature[34]. Metabolomics can quantitatively analyze small molecules with a molecular weight under 1 kilodalton[16]. Therefore, lipid-soluble small molecule metabolites produced by damaged brain tissue are more likely to pass through the BBB, making them detectable in serum samples[15, 16].
Carnitine plays an important role in energy metabolism, participates in the process of fat catabolism in the body, and may have neuroprotective effects[35]. TBI and severe trauma may lead to a decrease in plasma free carnitine levels[36]. Other studies have shown that carnitine levels may decrease in the serum of TBI patients, possibly due to disruptions in energy metabolism and mitochondrial dysfunction associated with the injury[15, 28, 37]. Additionally, changes in carnitine levels in serum post-TBI have been linked to metabolic alterations and may serve as potential biomarkers for TBI severity and prognosis[38, 39]. Our study clearly identified significant differences in the levels of 27 metabolites in the serum of DAI and non-DAI patients, with subtypes of carnitine accounting for one-third, totaling 9. This underscores the importance of carnitine in predicting the appearance of DAI. Furthermore, our random forest analysis revealed that carnitine C8:1 had the highest MeanDecreaseGini value among the 27 differing metabolites, demonstrating its paramount importance in diagnosing DAI.
Lipids are essential components of brain structure and function. Lipidomics analysis revealed that LPC and cholesterol ester (CE) were the lipid families most affected by TBI in the hippocampal region. Additionally, LPC (16:0) was associated with hippocampal-dependent memory function[40]. Lipidomics analysis of a mouse model of repetitive mild TBI showed a significant increase in total LPC levels, leading to neuropathological and biochemical consequences[41]. Pasvogel et al.[42] dynamically monitored changes in phospholipid concentrations in the cerebrospinal fluid of TBI patients. The results showed that the median concentration of LPC was highest on the first day after brain injury, demonstrating the significant role of LPC in further membrane phospholipid breakdown in the central nervous system following TBI. In the 27 differing metabolites identified in the serum of DAI and non-DAI patients, there were 4 subtypes of LPC, which aligned with the findings of the aforementioned researchers, indicating that LPC may also serve as a potential biomarker for predicting DAI. Furthermore, our random forest analysis revealed that LPC 22:3 sn-2 had the second-highest MeanDecreaseGini value among the 27 differing metabolites, suggesting its significant role in diagnosing DAI.
Early diagnosis of DAI during the acute phase of TBI and the implementation of targeted treatment strategies could improve the long-term neurological function of DAI patients[8–10]. Therefore, many researchers were dedicated to exploring early diagnostic models for DAI[1, 2, 30, 32, 43–47]. Tomita et al.[45] measured serum tau levels in 40 suspected DAI patients within 6 hours of TBI. The ROC curve was used to evaluate its diagnostic ability for DAI, with an AUC of 0.690, sensitivity of 74.1%, and specificity of 69.2%. The results suggested that tau protein could be used as a biomarker for early diagnosis of DAI, but its accuracy was not high enough. Compared to the non-DAI group, DAI patients showed a significant decrease in serum high-density lipoprotein cholesterol (HDLc) levels one week after injury. Multivariate analysis indicated that high-density lipoprotein cholesterol was an independent predictive factor for DAI. The study suggested that plasma HDLc levels may be a feasible biomarker for predicting the presence and prognosis of DAI after TBI[43]. The ratio of serum NSE level to GCS score at admission may also be a useful model for early diagnosis of DAI[1]. However, in clinical patients, the limitations of low serum concentrations of large molecular biomarkers and the challenges in collecting cerebrospinal fluid restricted the application of biomarkers as early diagnostic indicators for DAI[2, 33].
Researchers were increasingly exploring the use of small molecule metabolites in TBI for this purpose[15, 28, 37, 39]. Oresic et al.[37] developed an algorithm based on metabolite concentrations of brain injury patients during hospitalization, which accurately predicts patient outcomes (with an AUC of 0.84 in the validation cohort). The addition of metabolites to the established clinical model (CRASH), including clinical and CT data, significantly improved the prediction of patient outcomes. Thomas et al.[28] conducted a comprehensive metabolomics study on 716 TBI patients and non-brain-injured reference patients. They identified a set of metabolites strongly associated with the severity of TBI and patient prognosis, which could be used to predict the outcomes of traumatic brain injury patients. Most current studies focus on TBI patients, lacking specific models for early diagnosis using serum metabolites in DAI patients. The early diagnostic model established using the two metabolites most correlated with DAI (LPC 22:3 sn-2 and carnitine C8:1) outperformed traditional clinical indicator prediction models (AUC: 0.944 vs. 0.744). However, its ability to predict 6-month neurological function was not as strong as the latter. Adding metabolites to the clinical model significantly improves the prediction of outcomes for DAI patients (AUC = 0.913).
Limitations
There are several limitations in our present study. First, the sample size was not large enough to evaluate the predictive value of metabolites for DAI and further research is needed in larger, confirmatory studies. Second, while our cohort included both DAI and non-DAI patients, the absence of a healthy control group limited our understanding of DAI, and including them in future studies would enhance our insights. Third, the blood samples we used, although easily accessible, primarily reflect overall metabolic changes in patients, but may not directly reflect metabolic disturbances in the brain. Forth, the results obtained through LC-MS methods were subject to inherent limitations of the technique, particularly in terms of sensitivity, separation, and/or extraction efficiency, which may introduce biases in the results[48].