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Dr Asheesh Mehta Internal Medicine Specialist February 07, 2019
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Amnesia is a symptom denoting a defect in recall or formation of memories by the brain. In its more severe forms amnesia is extremely incapacitating and causes substantial distress. The deficit in memory may be of different types. When there is a failure to recall memories before the onset of the problem it is called retrograde amnesia while failure to lay down new memories is termed anterograde amnesia. Both forms may coexist. When there is retrograde amnesia the memory loss may be selective for recent memories or for distant memories. Memory loss may develop gradually or acutely depending on the underlying cause. Also, the amnesia may be static or progressive and this too depends on the cause of amnesia. The severity of amnesia may vary from mild to severe. Whether amnesia is permanent or temporary too depends on the underlying lesion in the brain causing amnesia. Sometimes there are specific types of amnesia such as a failure to remember faces.

Memory is a very complex process and how we store and recall memories is still not fully clear. There are two main types of memory, implicit memory and explicit memory. Implicit memory is related to skills, habit and behavior. This is the type of memory for performing daily activities carried out more or less automatically such as brushing teeth, combing hair, tying shoelaces, riding a bicycle, driving a car, etc. Once one learns how to do these activities one carries them out more or less automatically without conscious thought. Repetition and practice of these activities are required to carry them out efficiently. The amount of practice required to embed the memory and allow the activity to be carried out without conscious thought depends on the complexity of the activity. However, once learned properly, memory of how to carry out these activities is unlikely to be lost. Many different areas in our brain including the basal ganglia and cerebellum are involved in implicit memory. The other main type of memory is explicit memory. This relates to memory of facts and events. Examples include names of people, their address or telephone number, what one ate for lunch, what one did the previous day, etc. This type of memory is consciously recalled. It is easily acquired and also easily forgotten. Explicit memory can be further divided into immediate memory, working memory, short-term memory and long-term memory. Immediate memory is what comes into play in alertness or attention to things around us. Working memory is for dealing with day to day activities and interactions. However, the capacity for working memory is limited and it gets wiped out regularly. Short-term memory allows us to remember items such as during school lessons or phone number or address. This memory can be converted to long-term memory. The important areas of the brain involved in explicit memory are the hippocampus, cerebral cortex and amygdala.  In people developing amnesia loss of memory is almost always restricted to explicit memory and they are still able to carry out learned behavior of the automatic type dependent on implicit memory.

Memory is a very complex process and how we store and recall memories is still not fully clear. There are two main types of memory, implicit memory and explicit memory. Implicit memory is related to skills, habit and behavior. This is the type of memory for performing daily activities carried out more or less automatically.
Disease affecting areas of the brain involved in memory may lead to amnesia. Since very often the same memory is stored in multiple locations of the brain, we are protected to a certain degree from amnesia. In spite of these mechanisms memory loss does occur when the damage sustained by the brain is severe enough. Memory loss may occur in isolation or it may be just one of the components of a wider problem. Isolated memory loss may not interfere significantly with intellect and the affected person will then be able to understand and solve problems, have normal alertness and intelligence and also normal judgment. Memory loss is an accompaniment of all types of dementia including Alzheimer’s disease which is the commonest type. In dementia the main problem is a deterioration of intellect and memory loss is only one of the components. In most such cases recent memory is most affected while distant memory is often well preserved till late. Thus, the affected person may not be able to remember the current day or even the year but very often is able to recall with a fair degree of clarity events that occurred in childhood.

Amnesia may occur when areas of the brain involved in memory are affected by any disease process. Vascular problems which result in a stroke is one of the commoner such problems and both haemorrhagic and ischaemic strokes could be responsible. The brain is highly vulnerable to lack of oxygen and glucose and interference with delivery of either of these two vital metabolic inputs has the potential to cause substantial brain damage including causing amnesia. Oxygen supply to the brain may suffer on account of conditions like cardiac arrest or carbon monoxide poisoning. Glucose deprivation of the brain may occur due to insulinoma which is an insulin producing tumour causing sudden and severe drops in blood glucose level or more commonly due to low blood glucose as a result of overenthusiastic diabetes therapy. Infections such as viral encephalitis can damage large areas of the brain and are another cause of amnesia developing. Cancers of the brain, if affecting critical areas involved in memory, too can cause amnesia. Degenerative diseases like Alzheimer’s are important causes of memory loss but there is more often a general deterioration of intellectual function rather than that of memory alone. Sometimes medications like sedatives and tranquilizers when taken in excessive doses could be responsible for amnesia and a similar problem may also be encountered with use of illicit drugs like cocaine, ecstasy and cannabis. The loss of memory with the latter group of drugs may at times be irreversible. The commonly used statin group of drugs indicated for high cholesterol levels occasionally causes amnesia. Fortunately, this side effect of these very useful group of drugs is quite rare and is reversible. Head injuries are another cause of amnesia with this idea used innumerable times in popular literature, television and cinema. Transient amnesia is not uncommon but lasting severe amnesia is usually only with severe head injuries.

Among other causes of amnesia, Wernicke-Korsakoff syndrome, psychogenic amnesia and transient global amnesia merit specific mention. The first is a vitamin B1 (thiamine) deficiency disorder. This is usually seen in chronic alcoholics who are prone to malnutrition. It may also occur with severe nutritional deficiency from any other cause such as after weight loss surgery, starvation and severe wasting illnesses like cancer or advanced tuberculosis. The onset is often sudden with involuntary eye movements, confusion and also amnesia. Early recognition and prompt treatment with thiamine may lead to recovery but it is not at all uncommon for patients to be left with substantial mental incapacity and many of the affected people require long-term specialized care. Psychogenic amnesia is also known as dissociative amnesia and arises as a result of emotional stress. This type of amnesia is not too common and amnesia usually lasts only a short time. In transient global amnesia there is a temporary loss of memory in the absence of specific neurological disorder such as stroke or epilepsy. The episode lasts usually less than 24 hours and personal identity is remembered. However, other memories such as where one is, how they got there, past events or learned facts are not recalled. People with migraine are at increased risk for transient global amnesia. There are no other abnormal neurological findings and recovery soon occurs. The condition can be frightening but is otherwise considered to be fairly benign with no increased risk for stroke or other major brain disorders. Most affected people will have only one or at most two such episodes in their entire life.

Diagnostic tests depend on the suspected cause of amnesia and generally include imaging tests for the brain such as CT or MRI scan. The underlying cause may need specific treatment measures. Early treatment is particularly important in Wernicke-Korsakoff syndrome and even suspicion regarding this being the diagnosis warrants immediate thiamine injection rather than waiting for confirmation. Delay in treatment results in irreversible damage to the brain. Whatever the cause, there is no specific treatment for the amnesia itself. Most cases improve with time and in many cases there is more or less complete recovery of memory. Basically, the degree of improvement depends on how much the brain recovers from whatever caused amnesia. People with significant persistent memory loss face major obstacles in getting on with their life and seriously impaired memory loss may require long-term special care.

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