Prolonged hypoxia induces neuronal cell death via apoptosis, resulting in a hypoxic brain injury.

While HIE is associated in most cases with oxygen deprivation in the neonate due to birth asphyxia, it can occur in all age groups and is often a complication of cardiac arrest.

Mild symptoms include difficulties with complex learning tasks and reductions in short-term memory.

[citation needed] Cerebral hypoxia can be caused by any event that severely interferes with the brain's ability to receive or process oxygen.

Mild and moderate forms of cerebral hypoxia may be caused by various diseases that interfere with breathing and blood oxygenation.

[citation needed] Severe cerebral hypoxia and anoxia is usually caused by traumatic events such as choking, drowning, strangulation, smoke inhalation, drug overdoses, crushing of the trachea, status asthmaticus, and shock.

Hypoxic-anoxic events may affect the fetus at various stages of fetal development, during labor and delivery, and in the postnatal period.

[citation needed] Problems during labor and delivery can include umbilical cord occlusion, torsion or prolapse, rupture of the placenta or uterus, excessive bleeding from the placenta, abnormal fetal position such as the breech position, prolonged late stages of labor, or very low blood pressure in the mother.

Problems after delivery can include severe prematurity, severe lung or heart disease, serious infections, trauma to the brain or skull, congenital malformations of the brain or very low blood pressure in the baby[17] and due to suffocation in cases of Münchausen syndrome by proxy.

[18] The severity of a neonatal hypoxic-ischaemic brain injury may be assessed using Sarnat staging, which is based on clinical presentation and EEG findings, and also using MRI.

[19] Signs and symptoms of HIE may include:[citation needed] Cerebral hypoxia is typically grouped into four categories depending on the severity and location of the brain's oxygen deprivation:[20] Cerebral hypoxia can also be classified by the cause of the reduced brain oxygen:[23] For newborn infants starved of oxygen during birth, there is now evidence that hypothermia therapy for neonatal encephalopathy applied within 6 hours of cerebral hypoxia effectively improves survival and neurological outcome.

[citation needed] A deep coma will interfere with the body's breathing reflexes even after the initial cause of hypoxia has been dealt with; mechanical ventilation may be required.

[8] Severe cerebral hypoxia can also cause seizures, which put the patient at risk of self-injury, and various anti-convulsant drugs may need to be administered before treatment.

[citation needed] There has long been a debate over whether newborn infants with cerebral hypoxia should be resuscitated with 100% oxygen or normal air.

[28] Research by Ola Didrik Saugstad and others led to new international guidelines on newborn resuscitation in 2010, recommending the use of normal air instead of 100% oxygen.

Recent research suggests this may be due to an autoimmune response caused by carbon monoxide-induced changes in the myelin sheath surrounding neurons.