Cerebral hypoxia refers to a reduced supply of oxygen to the brain. Prolonged cerebral hypoxia eventually causes nerves in the brain to die, leading to hypoxic brain injury. When the oxygen supply to the brain is cut off completely, the condition is referred to as cerebral anoxia.
There are four types of cerebral hypoxia and these include:
Diffuse cerebral hypoxia – A reduced blood oxygen level causes a mild to moderate reduction in brain function.
Focal cerebral ischemia – This occurs when a cerebral blood vessel becomes blocked by a clot such as an aneurysm, thrombus or embolism. Many cases of stroke are due to focal cerebral ischemia and usually involve occlusion of the middle cerebral artery.
Global cerebral ischemia – This refers to a severe reduction in blood flow to widespread areas of the brain, such as in cardiac arrest cases.
Massive cerebral infarction – This is a dangerous form of ischemic stroke caused by reduced blood flow depriving multiple brain areas of oxygen.
Causes and pathology
Prolonged oxygen deprivation to the brain due to hypoxia or anoxia causes brain injury referred to as hypoxic/anoxic injury (HAI).
Hypoxic ischemic encephalopathy (HIE) is a condition caused by prolonged deprivation of adequate oxygen to the brain and is usually associated with cases of asphyxia in newborns. However, HIE can occur at any age and is a common complication of cardiac arrest.
Cerebral hypoxia can be classified according to how the brain becomes deprived of oxygen.
The different causes include:
Hypoxic hypoxia – This is caused by a limited oxygen supply in the environment and is a risk faced by mountaineers, divers and aviators, for example.
Hypemic hypoxia – This arises due to an inadequate level of oxygen in the blood even though environmental oxygen levels are sufficient. Common causes of this hypoxia include anemia and carbon monoxide poisoning.
Ischemic hypoxia – Also called stagnant hypoxia, this refers to inadequate blood flow to the brain, which becomes deprived of oxygen. This is seen in stroke, heart attack and cardiac arrest.
Histotoxic hypoxia – Here, the brain oxygen level is sufficient but the brain tissue cannot metabolize it. Cyanide poisoning is an example of histotoxic hypoxia.
Symptoms of cerebral hypoxia
Mild symptoms include a reduction in short term memory and the ability to learn difficult things.
Continued hypoxia can lead to disruption of cognitive ability, reduced motor control, increased heart rate and blue colouring of the skin.
Eventually, continued hypoxia leads to a loss of consciousness, seizures, coma and even brain death.
The primary aim of treatment is restoration of the brain’s oxygen content. For mild to moderate cases of hypoxia, simply eliminating the cause of the hypoxia and providing oxygen may be enough to restore the oxygen content. In severe cases, however, life support may be needed.
Reviewed by Sally Robertson, BSc