Hypoxic-ischaemic encephalopathy (HIE) is brain injury caused by a combination of insufficient oxygen (hypoxia) and reduced blood flow (ischaemia). It is the most serious complication of cardiac arrest and the leading cause of death and disability in survivors who are initially resuscitated.
What happens to the brain
During cardiac arrest, blood flow to the brain stops completely. Brain cells have very limited energy reserves and begin to fail within minutes. The longer the brain is deprived of oxygen and glucose, the greater the damage. When blood flow is restored, a secondary wave of injury can occur through reperfusion, which triggers inflammation, free radical production, and programmed cell death (apoptosis) in affected neurons.
Severity and outcome
HIE exists on a spectrum. At its most severe, it results in a persistent vegetative state or brain death. At a milder level, it can cause subtle cognitive difficulties, memory impairment, or personality changes that only become apparent once the person has physically recovered. Factors that influence severity include the duration of the cardiac arrest, the time until CPR was started, the time until defibrillation, and the quality of post-resuscitation care.
Assessment
Doctors use a range of investigations to assess the degree of brain injury, including neurological examination, EEG (to monitor brain electrical activity and detect seizures), CT and MRI brain scans, and blood tests for markers of brain cell damage such as neuron-specific enolase (NSE).
Recovery
Some degree of recovery is possible, even after significant HIE, because the brain retains an ability to reorganise itself (neuroplasticity). Cognitive rehabilitation, structured therapy, and time are important components of recovery for many survivors.
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