Cerebral Hypoxia

Cerebral hypoxia technically refers to the brain’s cerebral hemispheres, but the term is often used to refer to a reduced supply of oxygen to the entire brain or any part of it. (When there is a complete lack of oxygen reaching the brain, this effect is called cerebral anoxia.) The word “cerebral” also differentiates this type of hypoxia from other forms that can occur and affect other tissues in the body.

There are many possible causes for cerebral hypoxia, relating to the respiratory, circulatory, or nervous systems. It can be caused by a temporary but sustained interruption in breathing or issues that prevent blood (which carries oxygen) from reaching the brain. The nervous system requires oxygen to send nerve signals and messages through the body for voluntary and involuntary processes. Cerebral hypoxia is a life-threating emergency, and anyone experiencing it needs immediate medical care to restore the flow of oxygen to the brain. Brain cells are extremely sensitive to a lack of oxygen, and they begin to die less than four minutes after losing their supply of oxygen.

Healthy adults typically have a blood oxygen saturation (abbreviated SpO2) between 95% and 100%, reflecting the percentage of hemoglobin in the blood that carries oxygen. The risk of brain damage begins when blood oxygen saturation drops below 80%, and it becomes severe when this measurement falls below 60%.

Cerebral hypoxia can happen to anyone, and it can occur any time something interrupts the flow of oxygen to the brain. It can be caused by any of the following issues:

• Cardiac arrest (the most common cause in the United States)
• Cardiac arrhythmia (irregular heartbeat)
• Severe anemia
• Systemic hypotension (low blood pressure)
• Systemic hypoxia (low oxygen levels in other body tissues)
• Smoke inhalation or carbon monoxide poisoning
• Choking, strangulation, suffocation, or other compression of the windpipe
• Head injury, including traumatic brain injuries
• Near-drowning
• High altitude exposure
• Electrocution
• Heart attack
• Prolonged seizure
• Severe blood loss or hemorrhage
• Drug overdose or misuse of inhalants
• Stroke
• Complications of general anesthesia
• Chronic obstructive pulmonary disease (COPD)
• Diseases that paralyze the breathing muscles, such as amyotrophic lateral sclerosis (ALS)
• Injuries to a newborn before, during, or soon after birth, such as cerebral palsy

It is possible to experience mild hypoxia, where oxygen flow to the brain is only slightly below a healthy level. For example, this can occur when a person is visiting a high-altitude location, experiencing sleep apnea, or when there is inadequate ventilation in a room. This leads to mild symptoms, which might be difficult to recognize:

• Shortness of breath
• Feeling easily winded
• Fatigue
• Headache 
• Inattentiveness
• Poor judgment

In these cases, the condition often can be counteracted by getting more breathable air, for example by returning to a lower elevation, getting treatment for sleep apnea, or improving indoor ventilation.

Moderate to severe cerebral hypoxia is extremely dangerous, however. Symptoms vary depending on how long the brain goes without sufficient oxygen. Within a few minutes, the initial symptoms may include the following:

• Ataxia (a loss of muscle control)
• Difficulty concentrating
• Dizziness
• Feeling lightheaded
• Feeling euphoria for no apparent reason
• Rapid, shallow breathing
• Sensations of numbness and tingling

As hypoxia continues, the symptoms become more obvious and increasingly serious:

• Confusion
• Agitation
• Difficulty speaking
• Drowsiness
• Muscle twitches
• The skin, lips, or nails turn a bluish color
• Seizures
• Loss of consciousness

However, prolonged cerebral hypoxia of more than a few minutes can lead to coma. Permanent brain damage can lead to post-coma unresponsiveness (also known as unresponsive wakefulness syndrome, defined by complete unawareness and unresponsiveness). The most severe potential complication of cerebral hypoxia is death. 

Cerebral hypoxia can usually be diagnosed based on a physical exam and the consideration of a patient’s medical history. To determine the cause, a healthcare provider can perform various tests:

• Angiography (reviewing x-ray images of the brain’s blood vessels)
• Blood tests
• CT scan or MRI of the head to look for signs of stroke, bleeding in the brain, brain swelling, or other brain damage
• Electroencephalogram (EEG), a test of brain waves that can identify seizures 
• Electrocardiogram (ECG), a test to measure the heart’s electrical activity 
• Echocardiogram, a test that uses ultrasound to view the heart
• Evoked potentials, a test that determines if sensations, like vision and touch, reach the brain 

Cerebral hypoxia is an emergency condition that needs to be treated immediately. The sooner that healthcare providers can identify the cause and restore the flow of oxygen and/or oxygen-rich blood to the brain, the lower the patient’s risk for severe brain damage and/or death. Basic life support is the top priority. 

Depending on the cause of the hypoxia, one or more of these treatments will be the first course of action to restore oxygen supply: 

• Breathing assistance provided by mechanical ventilation
• Regulating heart rate and rhythm
• Raising blood pressure if it is too low, using fluids, medications, or other products
• Medications or general anesthetics to calm seizures, if applicable

The outlook for patients depends on the extent of brain injury, based on how long the brain lacked oxygen and additionally whether blood flow (containing necessary nutrients) to the brain was also affected. Brain damage begins within four minutes of the start of cerebral hypoxia. The longer the brain lacks oxygen, the more likely it is that a person will experience brain damage.  

Most people who make a full recovery are those who only briefly lose consciousness. If the brain lacks oxygen only for a few minutes, a person is more likely to be able to regain full or partial brain function. Sometimes individuals who recover from cerebral hypoxia have some lasting effects such as twitching or jerking muscle movements or seizures. The longer a person remains unconscious, the lower the chances of recovery. 

Individuals who experience cerebral hypoxia and remain unconscious for an extended period have a high risk of death or severe brain damage that leaves them completely unresponsive and lacking awareness. Many in this state pass away within a year, but those who survive will not regain brain function and will require caretaking for the rest of their lives. They also face potential risks such as bed sores, deep vein thrombosis, pneumonia, and malnutrition. 

Given the seriousness of these outcomes, it is imperative to call 911 or your region’s emergency number immediately if someone is experiencing the symptoms of cerebral hypoxia.