Hypoxic Brain Injury

This cerebral hypoxic type of brain injury occurs when some oxygen is received by the brain but not enough to enable the body to function without impairment.
If the brain is absolutely deprived of oxygen, it is called a cerebral anoxic injury.  There are four types of hypoxia in the brain.  There is diffuse cerebral hypoxia or DCH, focal cerebral ischemia, global cerebral ischemia and cerebral infarction.  Of these, the global cerebral ischemia is considered the worst.  If there is prolonged lack of enough oxygen, neuronal cell death is induced via apoptosis.  This results in a hypoxic brain injury.

In cases of complete oxygen deprivation or anoxia, this can be hypoxic in origin with reduced oxygen availability, or ischemic in origin (secondary to a disruption in blood flow).  Brain injuries because of oxygen deprivation that is due to hypoxic or anoxic mechanisms are called hypoxic/anoxic injuries.  Hypoxic ischemic encephalopathy is a problem that happens when the brain is deprived of enough oxygen. The deprivation, though, is not complete.  This condition often happens in situations of birth asphyxia. It is also a common finding in patients who have sustained a cardiac arrest.  These patients recover their oxygenation but changes occur within the damaged cells themselves that lead to programmed cell death or apoptosis in certain cells.  When this happens, those cells do not recover and there is permanent disability.

Classification of Cerebral Hypoxic Injury
There are four categories of cerebral hypoxic injuries; they depend on the location and severity of the oxygen deprivation of the brain.
Anoxic Anoxia is when the body simply cannot take in enough oxygen such as on high mountains or in tight and inadequately ventilated spaces.
Anemic anoxia resulting from a decrease in total hemoglobin in the blood.
Toxic Anoxia resulting from toxic substances in the blood that restrict utilization of oxygen. Could result from formaldehyde, certain narcotics, carbon monoxide or certain poisons.

Stagnant Hypoxia resulting from very low blood pressure or reduction of cerebral flow. Could happen from a stroke or heart attack.

Causes of Hypoxic Injury
Hypoxic injures can be caused by an event that interferes with brain tissue’s ability to receive oxygen or to process it.  The actual event can be internal to the body or external.  Mild to moderate forms of hypoxic injury to the brain can be caused by things like anemia and asthma, which interfere with breathing and blood oxygenation ability.  Other causes include flying in an unpressurized cabin, flying at high altitudes, ascent from a deep water dive, status epilepticus, and intense exercise at high altitudes without being acclimatized to the altitude.

Severe instances of hypoxic injury to the brain are caused by drug overdoses, smoke inhalation, shock, status asthmaticus, choking, strangulation, drowning, and crushing of the trachea.  It is also seen in self-induced blockage of the carotid artery and in erotic asphyxiation.

A transient ischemic attack or TIA is sometimes called a mini-stroke.  It involves having transient neurological findings without true infarction.   It can affect the brain, the spinal cord, or the retina.  It usually resolves without sequelae after a few minutes.  The symptoms are exactly like a stroke but they pass because the blockage resolves itself.  The symptoms usually pass within 24 hours but the brain injury itself is just a few minutes.  A TIA becomes a risk factor for having a stroke in the future.
A silent stroke is one in which there are no outward symptoms and the affected individual does not know that they’ve had a stroke.  Even so, there is brain damage going on and this is an individual at risk of having a stroke in the future.  In one study in 1998, it was estimated that more than 11 million people have had a stroke in the US.  About 770,000 of these were without symptoms and 11 million were silent infarcts found on MRI.  They were either infarcts or hemorrhages. Women are at a greater risk for having a silent stroke.  Predisposing factors are high blood pressure and cigarette smoking.

Pre-and Postnatal Hypoxic Injury
Hypoxic injury can affect a fetus during the various stage of fetal development, during the labor process and in the postnatal period.  Problems predisposing a fetus to this condition include maternal diabetes, preeclampsia, congenital infections, severe fetal anemia, drug or alcohol abuse, maternal vascular disease, cardiac disease, problems affecting blood flow to the placenta or cardiac disease.

Trouble during the labor and delivery process can involve umbilical cord occlusion, umbilical cord torsion or prolapse, rupture of the uterus, rupture of the placenta, bleeding from behind the placenta, abnormal fetal position, very low blood pressure in the mother, or prolonged late stages of labor.  Problems occurring after the delivery is over with include congenital malformations of the brain, very low blood pressure in the baby, trauma to the fetal skull or brain, serious infections in the newborn, severe lung or heart disease in the baby or extreme prematurity.

It is possible to assess the degree of neonatal hypoxic-ischemic brain trauma using what’s called Sarnat Staging. This staging is based upon MRI findings and EEG results.

Signs and Symptoms of Hypoxic Brain Injury
The body responds to a lowering of the oxygen by sending more blood to the brain and less blood to peripheral tissues.  Cerebral blood flow is increased up to twice the normal amount.  If the increased blood flow is enough, then there will be no symptoms.  If, on the other hand, the twice blood flow is not enough, then symptoms of cerebral hypoxia begin.  The first things to go are complex learning tasks and short term memory skills.  If the oxygen deprivation is allowed to continue, there will be cognitive disturbances and deficits in motor control.  The skin may by cyanotic (bluish) and the heart rate goes up.  Even more oxygen deprivation and there is fainting, coma, loss of consciousness, seizures, loss of brainstem function and death.
Blood oxygen saturation can be used in situations of hypoxic hypoxia but it means nothing in other types of hypoxia.  Normal pulse ox reading is 95-100 percent.  Any reading below 86 percent is considered severe hypoxia.
Cerebral hypoxia is a reference to the oxygen levels in the brain’s tissue and not in the blood.  In many cases, blood oxygenation will be normal in situations of ischemic, hypemic and hystoxic cerebral hypoxic injuries.    The degree of oxygenation in the brain tissue will depend upon how well the body copes with reduced oxygen content in the blood.

Treatment of Hypoxic Injuries to the Brain
For newborns starved of oxygen during the birth process, the latest treatment is hypothermic therapy.  It is done within 6 hours of the cerebral hypoxia and improves neonatal outcomes and survivability.  This is a less effective treatment in adults, however, and the major objective is to restore oxygen to the brain as quickly as possible.  If the hypoxia is mild, simply removing the cause can be sufficient in clearing up the problem.  Life support measures and measures to reduce damage to the brain are also important.  Doctors can apply inhaled oxygen to bring up the oxygen tension.
A person with a deep coma will have problems with the body’s breathing reflexes even after the initial hypoxia has been taken care of and mechanical ventilation might be necessary.  In severe cases of hypoxia, there is an elevated heart rate or even failure of the heart.  CPR, epinephrine, defibrillation and atropine may be used in the resuscitation efforts.  Because of the probability of seizures following severe cerebral asphyxia, anticonvulsants may be necessary before they occur.

Brain damage is possible after oxygen deprivation as well as during it.  Cells develop acidosis during oxygen deprivation and free radicals build up. When the oxygen comes back into the tissues that were hypoxic, the cellular material mixes with oxygen to create a great deal of oxidants that interfere with normal brain chemistry.  This is called a reperfusion injury and creates more problems for the brain.

Some techniques used to prevent damage to brain cells are being researched. Things like hypothermia for neonatal hypoxia are an evidence-based treatment.  Antioxidant drugs and controlling blood glucose can help as well as thinning of the blood.  Some doctors are studying drug-induced hypertension.  Hyperbaric oxygen might work in some circumstances as it might reduce the overall systemic inflammatory response.  Brain cells are very sensitive to reduced oxygen levels and when they are deprived of oxygen, they will begin to die within 5 minutes.  This is why it is important to act as soon as possible after the hypoxic event.
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