Diffuse Axonal Injury: Definition, Signs & Symptoms

Instructor: Patricia Jankowski

Patricia has a BSChE. She's an experienced registered nurse who has worked in various acute care areas as well as in legal nurse consulting.

In this lesson, you'll learn about a diffuse axonal injury. We'll explore signs and symptoms of this a type of traumatic brain injury and discuss the prognosis of the typical patient.

Definition and Pathology

''Operator, I was driving along I-94 and I saw a car roll over several times and land upside-down on the shoulder of the freeway. I pulled over and checked the car and saw one driver and one passenger inside. Neither one seems to be bleeding from the head, but I think they're both unconscious! Better send an ambulance, fast!''

Rollover accident
Automobile crash

Diffuse axonal injury (DAI) is a type of traumatic brain injury that occurs with a closed head injury. Although the skull and outer layers of skin are not necessarily broken, the brain itself is bounced around pretty badly, just like in the rollover accident in the introduction. This results in damage to the axons, which are extensions of the brain cells, or neurons. But let's begin at the beginning, and check out the anatomy of a brain cell, or neuron.

A neuron consists of axons, dendrites, and the cell body itself with all its internal components, like the nucleus, the mitochondria, etc. The axons carry nerve impulses away from the cell body. There is only one axon for each neuron. Dendrites carry nerve impulses toward the cell body.

Brain tissue can be divided into two categories: cortical, or tissue found on the brain's surface, and subcortical, or tissue that is found deeper within the brain. Cortical tissue is gray matter, while subcortical tissue is white matter. These two types of tissue are both made up of neurons, but subcortical tissue has the basic neuron structure, plus a myelin sheath on its axon. This myelin sheath enables the neurons to carry nerve impulses faster. Myelin is white in color, which is why they call the myelin-covered axons ''white matter''.

Neuron with Axon
Neuron with axon

The neurological impulses that the axons of white matter carry enable the entire body to function normally, and they are widespread within the brain. A DAI is called ''diffuse'' because the axons that are damaged cover a wide area, not just one isolated spot. What is seen in this type of brain injury are axonal swellings, or varicosities. These are buildups of cell flow material that occur proximal, or down from, the actual site of injury. These often happen at the nodes of Ranvier, which are spaces along the axon where there is no myelin sheath to protect the nerves. Nobody is sure yet exactly how this occurs, but it is suspected that the normal intracellular ion balance that exists in the axon is compromised, and when it does happen, vital nerve impulse transmission is interrupted and the patient is in trouble.


Since much of the pathology that happens in a DAI is on a microscopic level, a computed tomography (CT scan) or magnetic resonance imaging (MRI) are not very useful in diagnosing it. It's just too small to see! Instead, the accumulation of cell proteins in the axonal swellings can be tracked. This is done by using histology, or the examination of tissue samples under a microscope. The histological marker that is considered most accurate for measuring the presence of DAI is called amyloid precursor protein (APP). Normally, this protein travels quickly along the axon, but in DAI, it gets backed up along the axon and accumulates at the varicosities. If done correctly, APP can be used to diagnose a DAI within an hour or so.

Signs and Symptoms

The symptoms of a DAI can be devastating, since the injury of the axons is often widespread, affecting the transmission of nerve impulses globally. They can also be permanent, since most nervous tissue cannot regenerate itself. However, the effects of the injury can vary since every case and every patient is unique.

Unconsciousness is the most common sign of DAI. This may last for a long time, or it may be permanent, leading to a vegetative state. In a vegetative state, the basic motor functions such as breathing and heartbeat continue along with some reflexes, but the patient has no awareness of his surroundings or of himself. This state can go on for some time, or can lead to death. The term coma implies a state of unconsciousness that is more temporary, although it may be as deep.

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