Disorders of the brain are not uncommon afflictions in humans, who have the most highly developed brains of any creature to walk the earth. Brain disorders range from mild emotional or chemical imbalances to severe trauma and injuries. The most common brain disorders are those which affect a person's ability to interact with the world around him in effective and acceptable ways. These include autism, learning disabilities, depression, and attention deficit disorder (ADD).
The most common brain disorders which are not caused by injuries can have many different causes. The complex human brain, with its highly developed cognitive abilities, has many places and ways in which things can go wrong. Disorders can be rooted in abnormalities in the chemical or metabolic activities of the brain, as in ADD, bipolar disorder, and other dysfunctions. They can also affect the structure of the brain itself, such as in autism or Alzheimer's disease.
In autism, it is often the case that there is abnormally low blood flow to certain parts of the brain, resulting in fewer brain cells in these areas. These conditions almost always present in infancy. Alzheimer's, on the other hand, is a degenerative condition which mostly affects the elderly, and leads to the gradual loss of brain mass. It is often characterized by confusion and loss of memory, as well as an increasing inability to take care of oneself. Doctors do not yet know the exact causes of Alzheimer's, but have improved their ability to diagnose it and differentiate it from other conditions which have similar symptoms.
Many brain disorders are not themselves a lack of mental function, but can lead to it. Strokes and brain injury are among the more common brain disorders which happen suddenly, and are often caused by external factors. Brain injury is, of course, caused by events outside the brain. Apart from physical trauma, which by itself is very serious, conditions and illnesses in the rest of the body can cause injury to the brain. Heart disease, high blood pressure, and heatstroke are among these.
Stroke is the most common cerebrovascular disease, meaning a disease that affects blood flow to the brain. It occurs when an artery in the brain is blocked, or when hemorrhaging occurs within the brain. Both can lead to oxygen deprivation in the affected area, as well as infarction, the damaging of an area of tissue. All strokes lead to blood flow being denied to a certain part of the brain, but they can range in size from massive to barely noticeable. They can occur in any part of the brain, and therefore have an unlimited variety of possible effects.