(continued)
Cytoskeleton disruption
Immunoreactive axonal profiles are observed as either granular (B,G,H) or more elongated, fusiform (F) swellings in the corpus callosum and the brain stem (H) at 24h post traumatic brain injury. Example of APP immunoreactive neurons (arrow heads) observed in the cortex underneath the impact site (E,G). No APP staining was observed in healthy control animals (D). Axons are normally elastic, but when rapidly stretched they become brittle, and the axonal cytoskeleton can be broken. It is thought that integrins connected to the extracellular matrix outside the cell and to the cytoskeleton within it can transmit forces from the matrix to the cytoskeleton and cause the latter to tear when the axon is stretched.
Misalignment of cytoskeletal elements after stretch injury can lead to tearing of the axon and death of the neuron. Axonal transport continues up to the point of the break in the cytoskeleton, but no further, leading to a buildup of transport products and local swelling at that point. When it becomes large enough, swelling can tear the axon at the site of the break in the cytoskeleton, causing it to draw back toward the cell body and form a bulb. This bulb is called a retraction ball, the hallmark of diffuse axonal injury.
When the axon is transected, Wallerian degeneration, in which the part of the axon distal to the break degrades, takes place within one to two days after injury. The axolemma disintegrates, myelin breaks down and begins to detach from cells in an anterograde direction (from the body of the cell toward the end of the axon), and nearby cells begin phagocytic activity, engulfing debris.
Calcium influx
While sometimes only the cytoskeleton is disturbed, frequently disruption of the axolemma occurs as well, causing the influx of Ca2+ into the cell and unleashing a variety of degrading processes. An increase in Ca2+ and Na+ levels and a drop in K+ levels is found within the axon directly after injury. Possible routes of Ca2+ entry include sodium channels, pores torn in the membrane during stretch, and failure of ATP-dependent transporters due to mechanical blockage or lack of energy. High levels of intracellular Ca2+, the major cause of post-injury cell damage, destroy mitochondria, contribute to the generation of reactive oxygen species and trigger phospholipases and proteolytic enzymes that damage Na+ channels and degrade or alter the cytoskeleton and the axoplasm. Excess Ca2+ can also lead to damage to the blood brain barrier and swelling of the brain.
One of the proteins activated by the presence of calcium in the cell is calpain, a Ca2+-dependent non-lysosomal protease. About 15 minutes to half an hour after the onset of injury, a process called calpain-mediated spectrin proteolysis, or CMSP, begins to occur. Calpain breaks down a molecule called spectrin, which holds the membrane onto the cytoskeleton, causing the formation of blebs and the breakdown of the cytoskeleton and the membrane, and ultimately the death of the cell. Other molecules that can be degraded by calpains are microtubule subunits, microtubule-associated proteins, and neurofilaments.
Generally occurring one to six hours into the process of post-stretch injury, the presence of calcium in the cell initiates the caspase cascade, a process in cell injury that usually leads to apoptosis, or "cell suicide".
Mitochondria, dendrites, and parts of the cytoskeleton damaged in the injury have a limited ability to heal and regenerate, a process which occurs over 2 or more weeks. After the injury, astrocytes can shrink, causing parts of the brain to atrophy.
Diagnosis
Diffuse axonal injury after a motorcycle accident. MRI after 3 days: on T1-weighted images the injury is barely visible. On the FLAIR, DWI and T2* weighted images a small bleed is appreciated.DAI is difficult to detect since it does not show up well on CT scans or with other macroscopic imaging techniques, though it shows up microscopically. However, there are characteristics typical of DAI that may or may not show up on a CT scan. Diffuse injury has more microscopic injury than macroscopic injury and is difficult to detect with CT and MRI, but its presence can be inferred when small bleeds are visible in the corpus callosum or the cerebral cortex. MRI is more useful than CT for detecting characteristics of diffuse axonal injury in the subacute and chronic time frames. Newer studies such as Diffusion Tensor Imaging are able to demonstrate the degree of white matter fiber tract injury even when the standard MRI is negative. Since axonal damage in DAI is largely a result of secondary biochemical cascades, it has a delayed onset, so a person with DAI who initially appears well may deteriorate later. Thus injury is frequently more severe than is realized, and medical professionals should suspect DAI in any patients whose CT scans appear normal but who have symptoms like unconsciousness.
MRI is more sensitive than CT scans, but MRI may also miss DAI, because it identifies the injury using signs of edema, which may not be present.
DAI is classified into grades based on severity of the injury. In Grade I, widespread axonal damage is present but no focal abnormalities are seen. In Grade II, damage found in Grade I is present in addition to focal abnormalities, especially in the corpus callosum. Grade III damage encompasses both Grades I and II plus rostral brain stem injury and often tears in the tissue.
Treatment
DAI currently lacks a specific treatment beyond what is done for any type of head injury, including stabilizing the patient and trying to limit increases in intracranial pressure (ICP).
Potential treatments
Polyethylene glycol acts as a membrane sealant, and may serve to prevent the aforementioned devastating calcium influx. Rats treated with polyethylene glycol immediately following DAI induction showed no cytotoxic edema on diffusion weighted MRI 7 days later unlike controls.
History
The idea of DAI first came about as a result of studies by Sabina Strich on lesions of the white matter of individuals who had suffered head trauma years before. Strich first proposed the idea in 1956, calling it diffuse degeneration of white matter, however the more concise term "Diffuse Axonal Injury" was eventually preferred. Strich was researching the relationship between dementia and head trauma and asserted in 1956 that DAI played an integral role in the eventual development of dementia due to head trauma. The term DAI was introduced in the early 1980s.
Notable examples
Top Gear presenter Richard Hammond suffered a DAI as a result of the Vampire Dragster Crash in 2006
Champ Car World Series driver Roberto Guerrero suffered a DAI as a result of a crash during testing at the Indianapolis Motor Speedway in 1987.
Formula 1 driver Jules Bianchi suffered a DAI as a result of an accident at the 2014 Japanese Grand Prix.