An inability to recall information that is stored in the memory is called amnesia. There are three types of memory affected by amnesia including immediate memory, intermediate memory and long term memory. When the immediate memory is affected the person will have difficulty recalling the events that occurred in the preceding few seconds. Intermediate memory is affected when a person cannot recall events that happened from within a few seconds to a few days prior to the cause of the amnesia. With long term memory loss a person will be unable to recall events that occurred further back in time.

In most cases the memory is divided into short and long term. When there is damage or disease present in the area of the brain regions that are concerned with memory functions, amnesia occurs. There are several known causes of this type of damage including injuries to the head, degenerative disorders, infections such as encephalitis, thiamine deficiencies such as that found in Wernicke-Korsakoff syndrome, brain tumors, strokes, subarachnoid hemorrhage and some psychiatric illnesses. There are different types of amnesia that can affect a person including transient global amnesia, Wernicke-Korsakoff syndrome, Korsakoff's amnesia, retrograde amnesia and anterograde amnesia.

Transient global amnesia causes sudden and severe attacks of forgetful confusion for time, places and other people. This type of amnesia may occur only once in a persons life time or the attacks may be repeated. It is believed that this type of amnesia may be caused by reduced blood flow to the brain, heavy drinking, taking excessive amounts of tranquilizers or intermittent blockage of the small arteries in the brain. Wernicke-Korsakoff syndrome will cause acute confusion and a longer amnesia. This syndrome contains a combination of two disorders which result from brain dysfunction caused by a deficiency of thiamine. Korsakoff amnesia causes acute confusion that may be permanent when following severe or repeated attacks of the encephalopathy or alcohol withdrawal. When retrograde amnesia occurs the person will have a gap in memory that begins at the onset of the brain damage and extends back in time. They may find themselves unable to store new information after the damage as well. The gap in memory that results from this is called anterograde amnesia and it extends from the time of the onset of the amnesia until the time when the long term memory has resumed. In this case the gap in memory is usually permanent.

The prognosis for transient global amnesia is good with the confusion quickly clearing up and a total recovery. With Wernicke-Korsakoff syndrome patients are given thiamine intravenously to correct the problem. With Korsakoff's amnesia the administration of thiamine has been known to correct encephalopathy but not the amnesia. In some cases where alcohol is avoided and the illnesses that are contributing to the amnesia are treated this condition may gradually disappear. Other forms of amnesia may last for minutes or hours and then disappear without treatment. But in cases where severe brain injury has caused the amnesia, it could be permanent. Since the memory tends to be involved with many different brain functions, literally any type of brain damage is capable of causing a loss of memory.