Concussion and mild traumatic brain injury (mTBI) are terms often used interchangeably, leading to a great deal of confusion in both diagnosis and management. Despite efforts to clarify these terms, such as the 2012 Zurich Consensus Statement and the 2013 American Academy of Neurology guidelines, universal agreement remains elusive.
What Happens in a Concussion?
Concussions are caused by acceleration and deceleration forces acting on the brain. This mechanical force leads to stretching of neurons, glial cells, and blood vessels, affecting membrane permeability. The axons, which are extensions of nerve cells, are particularly susceptible to damage.
The Neurometabolic Cascade
Concussions induce a sudden release of neurotransmitters and rapid changes in ion concentrations, referred to as the “neurometabolic cascade.” This series of events results in a heightened metabolic demand in the face of reduced blood flow, leading to a cellular energy crisis. Pathological changes, such as diffuse axonal injury (DAI) and microhemorrhages, can occur post-concussion.
Tools for Detection
Traditional imaging techniques like CT scans and MRIs are limited in detecting these structural changes. However, specialized imaging techniques like diffusion tensor imaging (DTI) and susceptibility-weighted imaging (SWI) are more promising in capturing the subtle alterations in brain structure.
Diffuse Axonal Injury (DAI)
DAI is a key pathological feature of traumatic brain injury, caused by shearing forces that damage axons. These injuries can be detected within hours after trauma and are indicative of the severity of the injury. DTI has emerged as a useful tool for detecting DAI.
The Role of Microglia and Inflammation
Microglia, the brain’s immune cells, play a crucial role in mediating inflammation post-injury. They rush to the site of injury and potentially form a protective barrier. However, it is unclear whether modulating this inflammatory response has therapeutic benefits.
Dendritic and Spinal Changes
TBI leads to increased dendritic arborization and synaptogenesis, indicating a regenerative response. Proteins like GAP-43 have been linked to neurite sprouting post-injury.
Other Protein Markers: TDP-43 and Tau
TDP-43 and Tau proteins have been linked to various neurodegenerative diseases and also appear to play a role in concussion and mTBI. Elevated levels of Tau protein in cerebrospinal fluid (CSF) and plasma are being investigated as potential markers for TBI.
Special Considerations in Youth and Athletes
Second Impact Syndrome (SIS) and Juvenile Head Trauma Syndrome are conditions affecting young individuals, leading to severe cerebral edema and potentially fatal outcomes. SIS occurs when a second injury happens before the first has fully healed.
The Inflammatory Response
Post-injury, various inflammatory markers like IL-1, IL-6, and TNF-α are elevated. These markers could potentially serve as indicators for the severity and outcome of mTBI.