The New York University Langone Medical Center explains that brain cells begin to die after about four minutes without oxygen. Brain damage caused from a lack of oxygen is called anoxic damage, while hypoxia describes a low-oxygen condition. In other words, someone suffering from a lack of oxygen is said to be hypoxic, but any damage sustained is anoxic damage.
Hypoxic conditions can be caused from a lack of blood flow to the brain, or when blood reaches the brain but does not carry enough oxygen, according to NYU Langone Medical Center. Shock, heart problems and strokes may prevent blood from reaching the brain, while lung disease, carbon monoxide poisoning and traumatic events, such as drowning, cause the blood to become oxygen-depleted, even though it can reach the brain.
While severe hypoxia often causes patients to fall into a coma or die, mild hypoxia can produce less serious symptoms. According to Heathline, temporary memory loss, difficulty moving the body and concentration problems may occur during mild hypoxia. Doctors usually diagnose brain hypoxia through a combination of a medical history and physical examination, however, tests may be used as well. For example, doctors who suspect hypoxia may also order blood work, magnetic resonance imaging or computerized tomography.