The term cerebral hypoxia technically refers to lack of oxygen supply to the cerebral hemispheres (the outer portion of the brain). However, it is more typically used to refer to a lack of oxygen supply to the entire brain.

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

• Drowning
• Drug overdose
• Asphyxiation caused by smoke inhalation
• Very low blood pressure
• Strangulation
• Injuries during birth
• Cardiac arrest (when the heart stops pumping)
• Carbon monoxide poisoning
• High altitudes
• Choking
• Compression of the trachea
• Complications of general anesthesia
• Diseases that paralyze the respiratory muscles

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

This is an emergency condition and the sooner medical attention is given and the oxygen supply restored, the lower the chances of severe brain damage and death.

In mild cases, hypoxia causes only inattentiveness, poor judgment, and uncoordinated movement. Severe cases result in a state of complete unawareness and unresponsiveness (coma) -- brain stem reflexes, including response to light and the breathing reflex, stop. Only blood pressure and heart function are maintained. If this persists, brain death is inevitable.

If the lack of oxygen to the brain is limited to a very brief period of time, coma may be reversible with varying levels of return to function, depending on the extent of injury. Sometimes seizures may occur, which may be continuous with no stop between them (status epilepticus).

Cerebral hypoxia can usually be guessed from the clinical history and examination. Testing is targeted at determining the cause of the hypoxia. Tests may include:

• Magnetic resonance imaging (MRI), which provides high-quality pictures of the brain
• Electrocardiogram (EKG), a test of the electrical activity of the heart
• Echocardiogram (a test that allows the doctor to see images of the working heart)
• Blood tests, including arterial blood gases and blood sugar level
• Electroencephalogram (EEG), a test of brain waves, needed when seizures are suspected
• Evoked Potentials, a test that determines whether certain sensations like vision and touch reach the brain, causing an electric response

Treatment depends on the underlying cause. Most importantly, basic life-support has to be ensured.

• Mechanical ventilation must be used
• Blood pressure must be supported with fluids, blood products, or medications
• Heart rate must be controlled
• Seizures, if they occur, must be treated

If seizures occur, medications including phenytoin, phenobarbital, valproic acid, and general anesthetics are used to suppress them (with variable degree of success).

Sometimes cooling with blankets (hypothermia) is used, because much of the brain damage in hypoxia is caused by heat. Those who suffer the condition (if it is caused by drowning, for instance) in cold temperatures generally survive longer than those who are affected in higher temperatures. However, the benefit of this treatment remains to be established.

Experimental drugs called neuroprotectants have not shown a significant benefit so far.

The outlook depends on the extent of the hypoxic brain injury, which is determined by how long the brain was deprived of oxygen. Most people who make a full recovery are unconscious only briefly.

The longer the patient is unconscious, the higher the chances of death or brain death, and the lower the chances of 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 there is no consciousness or response to the environment.

Prolonged vegetative state is usually followed by death in less than 1 year, although, rarely, it may last longer. Other complications depend upon the degree of neurologic function and include lung infections (pneumonia), improper nutrition, bed sores, and clots in the veins (deep vein thrombosis).

Cerebral hypoxia is a medical emergency, when every moment is critical for a chance of meaningful survival. Call 911 IMMEDIATELY if someone you are with is losing consciousness or has other symptoms consistent with cerebral hypoxia.

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

Knowledge of cardiopulmonary resuscitation (CPR) can be lifesaving, especially when they are started soon after the onset of symptoms.