Anoxic Brain Injury

The brain makes up only about two percent of body weight but uses around 20 percent of the body’s oxygen. It has almost no reserves. When the oxygen supply stops, a person loses consciousness within seconds, and after about four to five minutes brain cells begin to die. The resulting damage is called anoxic or hypoxic brain injury. It is one of the most serious forms of acquired brain injury, and its effects range from mild memory problems to a lasting disorder of consciousness.

Anoxic, Hypoxic, Hypoxic-Ischemic: What the Terms Mean

• Anoxic brain injury: the brain receives no oxygen at all.
• Hypoxic brain injury: the brain receives too little oxygen.
• Hypoxic-ischemic brain injury: the oxygen shortage is combined with a lack of blood flow, as in cardiac arrest. This is the most common form in adults.

In everyday medical language the terms are often used interchangeably. Unlike a traumatic brain injury, there is no outside force – the damage is caused by the missing oxygen itself.

The classic four types

Older textbooks distinguish four mechanisms that are still helpful for understanding:

• Anoxic anoxia: not enough oxygen in the air, for example at high altitude or in enclosed spaces.
• Anemic anoxia: the blood cannot carry enough oxygen – after severe blood loss, in severe anemia or in carbon monoxide poisoning, because carbon monoxide binds to hemoglobin far more strongly than oxygen.
• Toxic anoxia: poisons prevent the cells from using oxygen, as with cyanide.
• Stagnant (ischemic) anoxia: the blood does not reach the brain – in cardiac arrest, severe heart rhythm disorders or a stroke.

Common Causes

• Cardiac arrest – the most common cause in adults. More than 350,000 cardiac arrests occur outside hospitals in the US each year, according to the American Heart Association.
• Drowning – a leading cause of death in young children.
• Choking on food or objects.
• Drug overdose, especially opioids such as fentanyl, which suppress breathing.
• Carbon monoxide poisoning from faulty heaters, generators or car exhaust in closed rooms.
• Severe asthma attacks and other respiratory failure.
• Strangulation and pressure on the neck.
• Complications during anesthesia or birth.

Which Parts of the Brain Are Most Affected

Not all brain cells are equally sensitive. Areas with a particularly high energy demand are damaged first:

• Hippocampus: essential for forming new memories – hence the frequent memory problems.
• Basal ganglia: control movement; damage leads to stiffness, tremor or involuntary movements.
• Cerebellum: coordinates movement and balance.
• Cerebral cortex: especially the areas at the borders between the large arteries (“watershed areas”), as well as the visual cortex in the back of the head.

Symptoms and Consequences

In the first days

After a severe lack of oxygen, many patients are in a coma. Some wake up within days; others pass through a vegetative state (unresponsive wakefulness syndrome) or a minimally conscious state, in which they show only limited signs of awareness.

Long-term effects

• Memory: difficulty learning and retaining new information is the most common lasting problem.
• Thinking: problems with attention, planning, judgment and problem solving.
• Language: trouble finding words and understanding complex sentences.
• Vision: problems recognizing objects or faces, in rare cases cortical blindness.
• Movement: weakness, poor coordination, spasticity, tremor.
• Myoclonus: sudden muscle jerks. A special form is Lance-Adams syndrome, in which jerks appear during intended movements weeks after the event, when the patient has woken up.
• Behavior and mood: personality changes, irritability, depression, lack of drive.
• Seizures

Diagnosis

Doctors combine several examinations, because no single test gives a reliable answer:

• Neurological exam: pupil and corneal reflexes, response to pain, breathing.
• CT scan: quickly shows swelling and rules out bleeding.
• MRI with diffusion-weighted imaging: detects damaged areas more precisely, usually a few days after the event.
• EEG: measures brain activity and finds seizures, which may not be visible from the outside.
• Somatosensory evoked potentials (SSEP): test whether signals from the arm nerves reach the cerebral cortex.
• Blood test for neuron-specific enolase (NSE): high levels indicate extensive damage to nerve cells.

Treatment in the First Days

After a cardiac arrest, the care team stabilizes breathing, circulation, blood sugar and blood pressure. Targeted temperature management is part of intensive care: fever is avoided because it increases brain damage; many centers keep the body temperature controlled for at least the first days.

A key rule concerns the prognosis. International guidelines from the American Heart Association and the European Resuscitation Council recommend assessing the outlook no earlier than 72 hours after the return to normal body temperature and after sedation has worn off. Early judgments can be wrong, because sedatives and cooling affect the test results. Families should ask the care team which findings the assessment is based on.

Prevention: What Saves the Brain

• Bystander CPR: chest compressions keep blood flowing to the brain until help arrives. Hands-only CPR – call 911, then push hard and fast in the center of the chest at 100 to 120 compressions per minute – can be learned in minutes.
• AED: automated external defibrillators in public places talk the user through each step and can restore a normal heart rhythm.
• Naloxone: the nasal spray reverses opioid overdoses and is available without a prescription in US pharmacies.
• Carbon monoxide detectors on every level of the home and near sleeping areas.
• Water safety: constant supervision of children, pool fences, swimming lessons and life jackets.

Rehabilitation and Recovery

Recovery after anoxic brain injury is often slower than after a traumatic brain injury and very different from person to person. Most progress happens in the first months, but improvement can continue for a long time. Patients who are medically stable usually move to an inpatient rehabilitation hospital. The team typically includes:

• Physical therapy: strength, balance, walking.
• Occupational therapy: everyday activities such as dressing, eating and personal care.
• Speech-language therapy: communication, swallowing, cognitive training.
• Neuropsychology: testing memory and thinking, strategies such as calendars, checklists and smartphone reminders.
• Counseling and support for patients and families.

Many patients have to relearn basic activities. Patience and a structured daily routine help. Information on rehabilitation settings can be found in the article Rehabilitation hospital services explained.

Frequently Asked Questions

What is the difference between anoxic and hypoxic brain injury?

In anoxic brain injury, the brain receives no oxygen at all; in hypoxic brain injury, it receives too little. If blood flow is also interrupted, as in cardiac arrest, doctors speak of hypoxic-ischemic brain injury.

How long can the brain survive without oxygen?

Consciousness is lost within seconds. After about four to five minutes without oxygen, brain cells begin to die, and the damage grows with every further minute.

When can doctors say how well someone will recover?

Guidelines recommend assessing the prognosis after cardiac arrest no earlier than 72 hours after the return to normal body temperature, using several tests together.

How can bystanders help?

By calling 911, starting chest compressions immediately and using an AED. In a suspected opioid overdose, naloxone should be given.