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Anoxic Hypoxic Brain Injury - Fact Sheet
 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
relative lack of oxygen, with anoxic brain injury occurring when there is an
absolute lack of oxygen. Causes include incidents such as respiratory arrests,
drowning, heart attacks, carbon monoxide inhalation and poisoning.
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. The damage
ensuing from lack of oxygen leads to serious problems with cognition, emotions
and movement.
Types of Anoxic / Hypoxic
Injury
There have been various classifications for
these types of injuries, however the standard system is the four categories
introduced by Barcroft, who coined the terms  Anoxia and Hypoxia:
- 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 blood
carrying oxygen to the brain. This can occur due to loss of blood or
insufficient oxygen being carried in the blood. Acute haemorrhage, obstructed
arteries, carbon monoxide poisoning and chronic anaemia are common causes of
this type of injury. Acute haemorrhages can occur due to open head injuries or
a burst aneurysm. Obstructions often occur due to clots, such as in a stroke.
Chronic anaemia occurs when there are persistent low red blood cells or
haemoglobin, the chemical which carries oxygen and gives blood its red colour.
Carbon monoxide poisoning is seen in suicide attempts using the exhaust of
cars, but can also occur in home or industrial accidents including
malfunctioning fuel-burning appliances. Carbon monoxide poisoning also appears
to selectively damage areas of the brain such as the basal ganglia, caudate
nucleus, putamen, globus pallidus, and central white matter. These brain areas
are important for the control of movement, and injury to central white matter
frequently results in general Cognitive impairment and fatigue.
- Stagnant (ischaemic) Anoxia -
critical reduction of Cerebral blood flow or pressure, resulting in
insufficient oxygen being carried into the brain. Also called hypoxic-ischaemic
injury, or HII. Injury can be localised or generalised, but typically causes
general, diffuse damage to the Cerebral Cortex and Cerebellum. Areas of the brain
that are very sensitive to lack of oxygen include the hippocampus (a region
critical for memory), border zone areas of the cerebral cortex (the
parieto-occipital and fronto-parietal regions), cerebellum, basal ganglia, and
spinal cord (thoracic region). Perhaps the most common cause of ischaemic
anoxia is cardiac arrest.
- Toxic anoxia - toxins or
metabolites that may interfere with oxygen utilisation. 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. Unfortunately, accidents under anaesthesia is a common cause of toxic
anoxia.
As with traumatic brain injury (TBI),
hypoxic/anoxic brain injury may occur along a spectrum from very mild to very
severe.
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. This will
tend to be evenly spread over the surface of the brain but, as a rule, will
also involve areas within the brain.
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
consolidation.
Effects
The effects can vary depending upon the
part of the brain injured. Typical symptoms of hypoxic/anoxic brain injury
include cognitive deficits, abnormal movements, weakness in arms and legs, lack
of coordination and visual problems.
Some of the major cognitive problems are:
- Impaired short-term memory;
- Problems with reasoning, making
judgments, and analysing information;
- Communication disorders;
- Visual problems.
Movement disorders are quite common,
including lack of coordination, Spasticity (involuntary muscle tightness),
tremors and impaired ability to adjust the body’s position. Where the lack of oxygen
and/or blood is caused by a localised problem, such as a stroke, movement
disorders are often unilateral – on one side of the body only. This is known as
Hemiparesis if movement is impaired, or Hemiplegia if there is total paralysis
of the limbs.
As with other types of brain injury, there
can be behavioural problems, although they may be more severe and possibly
include psychotic, delusional and personality disorders.
Recovery
Recovery is similar to that of other types
of brain injury, however because an anoxic/hypoxic injury usually impact upon
cognitive skills and results in diffuse damage throughout the brain, the
prognosis may not be as good. Indicators such as length of time unconscious or
in a Coma, and speed of recovery in the first month, tend to be reliable
indicators of how much recovery can be expected.
Rehabilitation will depend upon the
deficits that are identified, and are likely to include physiotherapy and
occupational therapy for movement disorders, speech pathology for communication
difficulties, and cognitive rehabilitation if that is available.
References and further
information
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Anoxic hypoxic brain injury 
