Although only two percent of our body weight, the brain consumes 20% of the body’s oxygen supply. Brain injury can occur quickly once there is insufficient oxygen being supplied.

Hypoxic brain injury is caused by a reduction in oxygen supply to the brain and Anoxic brain injury when there is a complete lack of oxygen to the brain. Both conditions can occur despite sufficient blood supply to the brain. Some causes include stroke, near drowning, heart attack, drug overdose, strangulation, accidents involving anaesthesia, carbon monoxide inhalation and poisoning. Hypoxia can also occur as a secondary injury following a traumatic brain injury, for example, when there is serious blood loss resulting in low blood pressure or as a result of brain swelling that restricts oxygen supply to areas of the brain [1].

Oxygen is crucial to the brain as it is used to metabolise glucose, which provides energy for all body cells. Most of the brain’s glucose is used to send impulses and keep cells alive. Brain cells are sensitive to the effects of restricted oxygen supply and may begin to die within minutes of oxygen restriction [2]. The immediate outcome of severe oxygen restriction is often coma and in very severe cases brain death. Long term outcomes can be problems with cognition, emotions and movement.

Types of Anoxic / Hypoxic Injury

• Anoxic Anoxia - inadequate oxygen to be breathed in and absorbed by the body, for example altitude sickness or suffocation in a non-ventilated space.
• Anaemic anoxia – inadequate oxygen supply to the brain resulting from a decrease in total haemoglobin or change in the haemoglobin’s ability to carry oxygen [3].
• Stagnant (ischaemic) Hypoxia (or hypoxic-ischaemic injury) – inadequate oxygen supply to the brain due to reduction of Cerebral blood flow or blood pressure. Injury can be localised or generalised, but typically causes general, diffuse damage to the Cerebral Cortex and Cerebellum. Causes of this type of brain injury include stroke, heart attack and brain haemorrhage. Pressure on the brain caused by hydrocephalus or brain haemorrhage can also be a cause of stagnant hypoxia [4].
• Toxic anoxia - toxins or substances that interfere with oxygen utilisation [5]. Also known as histotoxic anoxia. Carbon Monoxide poisoning also falls into this category, but the classic example is cyanide poisoning. Toxic anoxia can also be caused by exposure to narcotics, alcohol, formaldehyde, acetone, toluene, and certain anaesthetic agents.

How Long Before Injury Occurs?

Generally speaking, injury will set in after a lack of blood flow to the brain for around three to four minutes, with longer restriction of oxygen leading to more severe brain damage. This is why it is so important the emergency medical team quickly re-establish normal oxygen supply to the brain upon contact. Later a ventilator may be used to maintain breathing and oxygen in the intensive care unit [1].

What are the possible effects of a hypoxic brain injury?

The overall effects of a hypoxic/anoxic brain injury vary depending upon the the severity of damage. Areas of the brain particularly vulnerable to lack of oxygen include the Purkinje’s fibres of the cerebellum and the parieto-occipital cortex, which play a large role in coordination and movement, and the hippocampus which is one of the major structures responsible for memory [5.] A significant hypoxic brain injury may result in coma and possibly post-coma unresponsiveness. Symptoms following return to consciousness may include cognitive deficits, specifically memory difficulties, abnormal movements, weakness in arms and legs, lack of coordination and visual problems [5]. Movement disorders are quite common, including lack of coordination, Spasticity (involuntary muscle tightness), tremors and impaired ability to adjust the body’s position [5].

As with other types of brain injury, challenging behaviours may be presented. Individuals may also experience emotional problems including, depression, agitation and a reduced ability to tolerate stress and frustration.

Outcomes and recovery

Recovery is similar to that of other types of brain injury, however because a hypoxic injury usually results in diffuse damage to the brain, the outcome may not be as good. However, the level of recovery can not always be predicted, as some patients make a better recovery than expected, despite serious injury.

A number of factors can provide an indication of the level of recovery to expect following a hypoxic injury. Such factors include how long oxygen supply to the brain was restricted, age of the person and the severity of the resulting brain damage [6]. If someone only experiences a brief duration of unconsciousness, generally a good outcome can be expected. Other strong indicators of severity of the injury and therefore outlook for the individual include duration of coma and post-traumatic amnesia following hypoxic brain injury. One study found length of coma and post-traumatic amnesia to predict daily life functioning and quality of life 2-7 years after hypoxic injury caused by cardiac arrest [7].

Length of time in rehabilitation will depend upon the issues that are identified. To maximise recovery it is important that the individual receives a holistic level of support, meaning that all areas of the person’s functioning are being assisted in the recovery process. This means having help from a physiotherapist and occupational therapist for movement disorders, speech pathologist for communication difficulties, and a neuropsychologist to assess for cognitive deficits and how rehabilitation can be provided for these problem areas [5]. Support from a good team of specialists and family and friends will mean an optimal recovery.

More Information

For more information on Anoxic and Hypoxic Brain Injury visit the following websites-

Hypoxic-Anoxic Brain Injury- http://www.caregiver.org/caregiver/jsp/content_node.jsp?nodeid=575
Hypoxic Brain Injury Treatment Rehabilitation- http://brainstimulant.blogspot.com/2008/01/hypoxic-brain-injury.html
National Institute of Neurological Disorders and Stroke- NINDS Cerebral Hypoxia Information Page- http://www.ninds.nih.gov/disorders/anoxia/anoxia.htm

References

[1]Powell, T. (1994). Head Injury: A Practical Guide. United Kingdom: Winslow Press (IN BIAQ LIBRARY)

[2]Family Caregiver Alliance. (2004). Hypoxic-Anoxic Brain Injury. Retrieved April 1, 2008, from http://www.caregiver.org/caregiver/jsp/content_node.jsp?nodeid=575

[3]The Free Dictionary. (2007). Hypoxia. Retrieved April 1, 2008 from http://medical-dictionary.thefreedictionary.com/hypoxia

[4] Wikipedia. (2008). Cerebral Hypoxia. Retrieved April 1, 2008 from http://en.wikipedia.org/wiki/Cerebral_hypoxia

[5] Brain Injury Association of America. Frequently Asked Medical Questions. (2008). Anoxia. Retrieved April 1, 2008, from http://www.biausa.org/Pages/askthedoctor.html#anoxia

[6] The National Institute of Neurological Disorders and Stroke (NINDS). (2007). NINDS Cerebral Hypoxia. Retrieved April 1, 2008, from http://www.ninds.nih.gov/disorders/anoxia/anoxia.htm

[7] Middelkamp, w., Moulaert, V. R., Verbunt, J. A., van Heugten, C. M., Bakx, W. G., & Wade, D. T. (2007). Life after survival: long-term daily life functioning and quality of life of patients with hypoxic brain injury as a result of cardiac arrest. Clinical Rehabilitation, 21, 425-431