Ischaemic hypoxic brain injuries are complicated injuries to the brain that are caused by ischaemia (a lack of blood flow to the brain), hypoxia (a lack of oxygen to the brain, or cytotoxicity (toxins that harm brain cells). The hypoxia/anoxia can be so caused by an individual suffering a cardiac arrest, respiratory arrest, near hanging experience, near drowning experience, exposure to carbon monoxide, near suffocation, exposure to toxic gases and perinatal lack of oxygen (during the birth process). These things expose the entire brain to a lack of oxygen. Some areas of the brain, however, are more sensitive to a loss of oxygenation than others.

While the terms "hypoxic" and "ischaemic" are used together, they are actually two different things. Hypoxia refers to a lowering of the oxygen level in the brain, which can be due to a variety of things. In its more severe form, you get "anoxia" or a lack of blood flow to the brain altogether. With ischaemia, there is a lack of blood flow to the brain. Without blood flow, you don't get any oxygen whatsoever. Between hypoxia and ischaemia, ischaemia is much worse. Hypoxia is associated with things like strangulation and near drowning, with intact circulation and rapidly diminishing concentrations of oxygen that result in death if the lowering of oxygen results in anoxia. In many cases, if the patient is salvaged from their event, the impact to the brain is transient.

Hypoxic brain injury happens during times of lung dysfunction, interferences by carbon monoxide or other toxic gases, or really low haemoglobin levels, such as you find in severe bleeding. You can also get it when the blood flow is partially or completely blocked on a temporary basis. When this happens, you get both a lack of oxygen and a lack of glucose, which is the fuel the brain uses for its metabolic functions. The energy within the cell will decrease and there will be swelling of the brain cells as the cell membranes fail to process water within the cell. There will be an increase in neurotransmitters, especially the excitatory neurotransmitters and an increase in oxygen free radicals - both of which are dangerous to brain cells.

The difference between a stroke and Ischaemic Hypoxic Brain injury is one of scope. A stroke affects one or more focal areas of the brain, whereas ischaemic hypoxic injuries are the result of a global phenomenon to the brain. The upper brainstem, cerebellum, white matter and subcortical structures are more sensitive to brain injury from hypoxia. These are areas that are in the watershed areas between major blood vessel structures.

The main consequences of a cerebral ischaemic or hypoxic event are seizures, which can be a one-time thing or recurrent for years after the ischaemia takes place. There can also be problems with sensorimotor abilities, behavioral changes, cognitive changes and emotional disturbances that can last for a long time to come. About a third of all people having an ischaemic hypoxic event will have at least one seizure within twenty four hours of the injury. These seizures can return for up to two weeks later before they stop. Many seizures are what are called "partial seizures" in which the patient maintains consciousness but has abnormal, uncontrollable movements of their body. The presence of seizures doesn't necessarily mean the event was particular bad. If there is status epilepticus or status myoclonic epilepticus, which involves a "never-ending" or difficult to treat seizures, the outcome is almost always fatal. It means that the injury to the brain was particularly serious and that there is likely swelling of the brain cells from which the patient does not recover.

Many people with ischaemic hypoxic brain injury develop a movement disorder. In children, this movement disorder is called cerebral palsy. Adults can get post-hypoxic Parkinson's disease or dystonia. The type of movement disorder adults get looks very similar to cerebral palsy. There is rigidity and spasticity of the muscle groups and they don't have the common tremor of many other cases of Parkinson's disease. These post ischaemic hypoxic phenomena often happen weeks, months or even years after the event. They are also much more difficult to treat when compared to other cases of Parkinson's disease or dystonia. This is because the cells that cause these diseases have died rather than simply becoming ill and cannot be targeted with medications.

Simple motor function can be damaged with ischaemic hypoxic injury to the brain. This happens when deeper structures of the brain or the spinal cord are affected. There can be various degrees of motor loss, including complete quadriplegia that affects all extremities and the trunk. One unique motor problem seen in ischaemic hypoxic brain injury is called bibrachial paresis. This involves weakness of the proximal parts of the arms with relative sparing of the lower extremities. This happens because there are portions of the brain that receive the watershed of larger arteries and tend to be more vulnerable to oxygen deficit. Such individuals and those with other ischaemic hypoxic conditions need physical and rehabilitative therapy to counteract spasticity, weakness, contractures, and gait problems.

Following an ischaemic hypoxic injury to the brain, the person can be in a coma, can be in varying degrees of a vegetative state, or can be in a "minimally conscious state". Impairments can occur in attention to stimuli and in processing the information given to the injured person. There can be personality changes and abnormalities of speech that can improve over time or remain stagnant despite therapy.

In some cases of ischaemic hypoxic brain injury, the person appears to recover fully only to develop a severe demyelinating syndrome similar to multiple sclerosis. There can be delayed damage to the brain that can lead to psychosis, delirium, Parkinson's disease, mutism or weakness of all extremities. It is especially common in cases where the person has suffered an injury to the brain as a result of carbon monoxide poisoning. No one knows exactly why or how this happens following an ischaemic injury.