Cerebral hypoxia technically means a lack of oxygen supply to the outer part of the brain, an area called the cerebral hemisphere. However, the term is more typically used to refer to a lack of oxygen supply to the entire brain.

There are many causes of cerebral hypoxia. These include, but are not limited to:

• Asphyxiation caused by smoke inhalation
• Carbon monoxide poisoning
• Cardiac arrest (when the heart stops pumping)
• Choking
• Complications of general anesthesia
• Compression of the windpipe (trachea)
• Diseases that cause a loss of movement (paralysis) of the breathing muscles
• Drowning
• Drug overdose
• High altitudes
• Injuries before, during, or soon after, birth (See: Cerebral palsy)
• Strangulation
• Stroke
• Very low blood pressure

Brain cells are extremely sensitive to oxygen deprivation. Some brain cells actually start dying less than 5 minutes after their oxygen supply disappears. As a result, brain hypoxia can rapidly cause death or severe brain damage.

Symptoms of mild cerebral hypoxia include:

• Change in attention (inattentiveness)
• Poor judgment
• Uncoordinated movement

Symptoms of severe cerebral hypoxia include:

• Complete unawareness and unresponsiveness (coma)
• No breathing
• No response to light

If only blood pressure and heart function remain, then the brain may actually be completely dead.

Cerebral hypoxia can usually be diagnosed based on the person's medical history and a physical exam. Tests are done to determine the cause of the hypoxia, and may include:

• Blood work, including arterial blood gases and blood sugar level
• Echocardiogram
• Electrocardiogram (ECG), a measurement of the heart's electrical activity
• Electroencephalogram (EEG), a test of brain waves that can identify seizures and show how well brain cells work
• Evoked potentials, a test that determines whether certain sensations such as vision and touch reach the brain
• Magnetic resonance imaging (MRI)

Cerebral hypoxia is an emergency condition that requires immediate treatment. The sooner the oxygen supply is restored to the brain, the lower the risk of severe brain damage and death.

Treatment depends on the underlying cause of the hypoxia. Basic life support is most important. Treatment involves:

• Breathing assistance (mechanical ventilation)
• Controlling the heart rate
• Fluids, blood products, or medications to control blood pressure
• Medications including phenytoin, phenobarbital, valproic acid, and general anesthetics to calm seizures

Sometimes cooling the person with cold blankets is used, because cooling slows down the activity of the brain cells and decreases their need for oxygen. However, the benefit of such treatment has not been firmly established.

The outlook depends on the extent of the brain injury, which is determined by how long the brain lacked oxygen.

If the brain lacked oxygen for only a very brief period of time, a coma may be reversible and the person may have some return of function. However, this depends on the extent of injury. Some patients recover many functions, but have abnormal movements such as twitching or jerking. Seizures may sometimes occur, and may be continuous (status epilepticus).

Most people who make a full recovery were only briefly unconscious.

The longer the person is unconscious, the higher the risk for death or brain death, and the lower the chances for a meaningful recovery.

Complications of cerebral hypoxia include prolonged vegetative state -- basic life functions such as breathing, blood pressure, sleep/wake cycle, and eye opening may be preserved, but the person is not alert and does not respond to their surroundings. Such patients usually die within a year, although some may survive longer.

Length of survival depends partly on how much care is taken to prevent other problems. Major complications may include:

• Bed sores
• Clots in the veins (deep vein thrombosis)
• Improper nutrition
• Lung infections (pneumonia)

Cerebral hypoxia is a medical emergency. Call 911 IMMEDIATELY if someone is losing consciousness or has other symptoms of cerebral hypoxia.

Prevention depends on the specific cause of hypoxia. Unfortunately, hypoxia is usually unexpected. This makes the condition somewhat difficult to prevent.

Cardiopulmonary resuscitation (CPR) can be lifesaving, especially when it is started right away.