Primary and secondary brain injury are ways to classify the injury processes that occur in
brain injury. In
traumatic brain injury (TBI), primary brain injury occurs during the initial insult, and results from displacement of the physical structures of the brain.
Secondary brain injury occurs gradually and may involve an array of cellular processes.
Secondary injury, which is not caused by mechanical damage, can result from the primary injury or be independent of it.
The fact that people sometimes deteriorate after brain injury was originally taken to mean that secondary injury was occurring.
It is not well understood how much of a contribution primary and secondary injuries respectively have to the clinical manifestations of TBI.
Primary and secondary injuries occur in instances other than a TBI, such as
spinal cord injury and
stroke
A stroke is a medical condition in which poor blood flow to the brain causes cell death. There are two main types of stroke: ischemic, due to lack of blood flow, and hemorrhagic, due to bleeding. Both cause parts of the brain to stop functionin ...
.
Primary
In TBI, primary injuries result immediately from the initial trauma.
[
] Primary injury occurs at the moment of trauma and includes
contusion, damage to
blood vessels, and axonal
shearing
Sheep shearing is the process by which the woollen fleece of a sheep is cut off. The person who removes the sheep's wool is called a '' shearer''. Typically each adult sheep is shorn once each year (a sheep may be said to have been "shorn" or ...
, in which the
axons of
neurons are stretched and torn.
The
blood brain barrier and
meninges
In anatomy, the meninges (, ''singular:'' meninx ( or ), ) are the three membranes that envelop the brain and spinal cord. In mammals, the meninges are the dura mater, the arachnoid mater, and the pia mater. Cerebrospinal fluid is located in th ...
may be damaged in the primary injury, and neurons may die.
[
] Cells are killed in a nonspecific manner in primary injury.
[
] Tissues have a deformation threshold: if they are deformed past this threshold they are injured.
Different regions in the brain may be more sensitive to mechanical loading due to differences in their properties that result from differences in their makeup; for example,
myelinated tissues may have different properties than other tissues.
Thus some tissues may experience more force and be more injured in the primary injury.
The primary injury leads to the secondary injury.
Secondary
Secondary injury is an ''indirect'' result of the injury. It results from processes initiated by the trauma.
It occurs in the hours and days following the primary injury
and plays a large role in the brain damage and death that results from TBI.
[
] Unlike in most forms of trauma a large percentage of the people killed by brain trauma do not die right away but rather days to weeks after the event.
[
] In addition, rather than improving after being hospitalized as most patients with other types of injuries do, about 40% of people with TBI deteriorate.
[
] This is often a result of secondary injury, which can damage neurons that were unharmed in the primary injury. It occurs after a variety of brain injury including
subarachnoid hemorrhage
Subarachnoid hemorrhage (SAH) is bleeding into the subarachnoid space—the area between the arachnoid membrane and the pia mater surrounding the brain. Symptoms may include a severe headache of rapid onset, vomiting, decreased level of consci ...
, stroke, and
traumatic brain injury and involves
metabolic
Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run cell ...
cascade
Cascade, Cascades or Cascading may refer to:
Science and technology Science
*Cascade waterfalls, or series of waterfalls
* Cascade, the CRISPR-associated complex for antiviral defense (a protein complex)
* Cascade (grape), a type of fruit
* Bioc ...
s.
Secondary injury can result from complications of the injury.
These include
ischemia
Ischemia or ischaemia is a restriction in blood supply to any tissue, muscle group, or organ of the body, causing a shortage of oxygen that is needed for cellular metabolism (to keep tissue alive). Ischemia is generally caused by problems wi ...
(insufficient blood flow); cerebral
hypoxia
Hypoxia means a lower than normal level of oxygen, and may refer to:
Reduced or insufficient oxygen
* Hypoxia (environmental), abnormally low oxygen content of the specific environment
* Hypoxia (medical), abnormally low level of oxygen in the tis ...
(insufficient oxygen in the brain);
hypotension (low
blood pressure
Blood pressure (BP) is the pressure of circulating blood against the walls of blood vessels. Most of this pressure results from the heart pumping blood through the circulatory system. When used without qualification, the term "blood pressure" r ...
);
cerebral edema (swelling of the brain); changes in the
blood flow to the brain; and raised
intracranial pressure (the pressure within the skull).
If intracranial pressure gets too high, it can lead to deadly
brain herniation
Brain herniation is a potentially deadly side effect of very high pressure within the skull that occurs when a part of the brain is squeezed across structures within the skull. The brain can shift across such structures as the falx cerebri, the te ...
, in which parts of the brain are squeezed past structures in the skull.
Other secondary injury include
hypercapnia (excessive carbon dioxide levels in the blood),
acidosis (excessively acidic blood),
[
] meningitis
Meningitis is acute or chronic inflammation of the protective membranes covering the brain and spinal cord, collectively called the meninges. The most common symptoms are fever, headache, and neck stiffness. Other symptoms include confusion or ...
, and
brain abscess.
In addition, alterations in the release of
neurotransmitter
A neurotransmitter is a signaling molecule secreted by a neuron to affect another cell across a synapse. The cell receiving the signal, any main body part or target cell, may be another neuron, but could also be a gland or muscle cell.
Neuro ...
s (the chemicals used by brain cells to communicate) can cause secondary injury. Imbalances in some neurotransmitters can lead to
excitotoxicity, damage to brain cells that results from overactivation of
biochemical receptors
In biochemistry and pharmacology, receptors are chemical structures, composed of protein, that receive and transduce signals that may be integrated into biological systems. These signals are typically chemical messengers which bind to a recept ...
for excitatory neurotransmitters (those that increase the likelihood that a
neuron will
fire). Excitotoxicity can cause a variety of negative effects, including damage to cells by
free radicals, potentially leading to
neurodegeneration
A neurodegenerative disease is caused by the progressive loss of structure or function of neurons, in the process known as neurodegeneration. Such neuronal damage may ultimately involve cell death. Neurodegenerative diseases include amyotrophic ...
. Another factor in secondary injury is loss of cerebral autoregulation, the ability of the brain's blood vessels to regulate
cerebral blood flow.
Other factors in secondary damage are breakdown of the
blood–brain barrier
The blood–brain barrier (BBB) is a highly selective semipermeable membrane, semipermeable border of endothelium, endothelial cells that prevents solutes in the circulating blood from ''non-selectively'' crossing into the extracellular fluid of ...
, edema, ischemia and hypoxia.
[
] Ischemia is one of the leading causes of secondary
brain damage
Neurotrauma, brain damage or brain injury (BI) is the destruction or degeneration of brain cells. Brain injuries occur due to a wide range of internal and external factors. In general, brain damage refers to significant, undiscriminating t ...
after head trauma.
[
] Similar mechanisms are involved in secondary injury after ischemia, trauma, and injuries resulting when a person does not get enough oxygen.
After stroke, an
ischemic cascade, a set of
biochemical cascades takes place.
Prevention
Since primary injury occurs at the moment of trauma and is over so rapidly, little can be done to interfere with it other than prevention of the trauma itself.
However, since secondary injury occurs over time, it can be prevented in part by taking measures to prevent complications such as hypoxia (oxygen deficiency). Furthermore, secondary injury presents opportunities for researchers to find drug therapies to limit or prevent the damage. Since a variety of processes occur in secondary injury, any treatments that are developed to halt or mitigate it will need to address more than one of these mechanisms.
[
]
Thus efforts to reduce disability and death from TBI are thought to be best aimed at secondary injury, because the primary injury is thought to be irreversible.
See also
*
Wallerian degeneration
Wallerian degeneration is an active process of degeneration that results when a nerve fiber is cut or crushed and the part of the axon distal to the injury (i.e. farther from the neuron's cell body) degenerates. A related process of dying back o ...
References
{{Reflist, 2
Neurotrauma
Brain disorders