Retrograde amnesia: causes, mechanisms, and clinical presentation

Retrograde Amnesia

Amnesia is the loss of the faculty of memory and one of the common forms of brain disorders. It is caused by any organic causes like infections, psychological trauma's or physical injuries to the brain. Amnesia can either be associated with loss of recollection of the past or of the events that immediately follow the accident. In the former case it is called as retrograde amnesia while the later condition is referred to as anterograde amnesia. Retrograde amnesia is in general viewed as a problem with recovering remote information while anterograde amnesia represents problems with registering new information. Let us have a brief outlook into the diagnosis, cause and treatment of retrograde amnesia.

Types of Amnesia

There are different types of amnesia like retrograde, anterograde, lacunar amnesia, transient global amnesia and hysterical amnesia. Among these retrograde and anterograde amnesia are the two most prevalent conditions. Amnesia can be caused by brain damage due to accidents or infectious diseases and the condition can be temporary or more lasting depending on the severity of the damage. In general amnesia due to organic causes are of more permanent nature. The DSM defines retrograde amnesia as the, "Loss of memory of events that occurred before the onset of the etiological condition or agent." [Scottsdale] In retrograde amnesia the amnesiac looses long-term memory and he cannot recollect events that happened prior to the trauma. Before going into detail about retrograde amnesia it is necessary to have a brief outlook on the mechanism of the brain function.

Human brain is a highly complex network and divided into distinct regions such as frontal lobes, temporal lobes and parietal lobes etc. The information that we process everyday is either stored temporarily (discarded in a short while) or permanently embedded within out brain. For any particular information to be permanently stored there needs to be a permanent chemical change in the brain. This is automatically done when we use and reuse the same information again and again. This process by which, temporary data are converted and stored as permanent data is known as consolidation. The hippocampus located in the temporal area of the brain is found play a vital role in permanently wiring the information into the brain cells. As researcher Fanselow (2000) observes, "with the aid of the hippocampus, this memory becomes permanently stored in other, probably cortical, structures" As we have seen, the extant data demonstrate that in some circumstances," [Lynn Nadel] So any damage caused to the hippocampus region would directly affect this conversion process leading to loss of long-term memory.

Diagnostic Tests

Asides the normal symptoms of reported memory loss there are certain procedures that will help the physician in his diagnosis of amnesia. Cerebral angiography is one such procedure, which helps the physician identify the exact location of any abnormality in the arteries in the brain. Using a contrast dye the blood flow in the brain can be traced. CT scans may also be performed to identity any intra-cranial infarctions and to monitor the motor neuron activity within the brain. The Electro encephalogram is another test that is used to monitor the electrical activity within the brain and to identify any anomalies. [ADAM]

Causes of Amnesia

Amnesia is a symptomatic response to many different kinds of brain diseases. Amnesia may also result because of physical injury to the brain or infections and other kinds of degenerative brain disorders. Alzheimer's disease, dementia, brain tumor, encephalitis and brain infections are all leading causes of amnesia. Amnesia may also result from epileptic seizures. Electro conclusive therapy that is used to treat patients with chemical imbalances in the brain is also reported to be cause Amnesia. Similarly temporal lobe surgery (for tumors and other diseases) invariably results in some degree of amnesia.

Diagnosis of amnesia basically involves identifying the pattern of cognitive impairment and memory disorder. Brain lesions within particular regions can be attributed to specific symptoms and specific kind of amnesia. It was not until 1970, when computerized tomography became a reality that studying the exact locations of the lesions was possible. In general particular pattern of lesions are identified with particular type of organic amnesia. For example amnesia caused by encephalitis (brain infection) and korsakoff syndrome are largely due to lesions that are localized within a certain segment of the brain. However it is observed that even within a particular aetiological group the pattern of damage is much different. [Alan J. Parkin, 8]

Retrograde Amnesia (Some Facts)

Retrograde amnesia is a condition wherein there is an inability to retrieve remote or long-term memory. It can result due to a damage of different regions within the brain.

Retrograde amnesia may be temporally graded in which case only data pertaining to a certain period may be lost or it may involve the loss of entire memory prior to the accident. There are different types of retrograde amnesia namely autobiographical amnesia, Focal retrograde amnesia and semantic amnesia. These different conditions manifest as a result of different areas of damage within the brain.

Region of Brain Damage

As we discussed above the exact location of brain damage (lesions) is widely variant even within a particular "aetiological group." Studies have attested that damage caused by encephalitis may not be confined to hippocampus (the normal affected region) but may extend as far as orbitofrontal cortex and even into the thalamus. However recent neuroradiological systems have made it easier for us to track patterns of brain lesions to particular forms of cognitive impairment. Using MRI scans it is possible to study the brain structure in minute detail and to clearly identify the damaged areas. Positron emission tomography has further improved our understanding of the brain functions by enabling us to study the glucose metabolism of the brain.

Using the PET the cerebral blood flow can be directly observed using isotopes and tracing them within the brain region. Neuronal activity within the brain is a direct measure of the cerebral blood flow (CBF) and the level of oxygenation of blood. [Alan Parkin, 10] Using PET it is even possible to identify the nonfunctional areas within the brain, which do not show any structural abnormality. This kind of a problem is observed in areas of brain that are not directly affected by the damage but do not receive any signal input from the damaged regions.

The bilateral temporal lobe region is found to be damaged in most of the cases where the patient suffers from retrograde amnesia. Though retrograde amnesia is very rarely found in exclusion of anterograde amnesia there are still some rare cases where such conditions are manifest. Studies have identified that we possess discrete memory systems within our brain. For example episodic memory and semantic memory are two different types of memory systems located in different regions within the brain. However both these type of memories are dependent on specific area (neo-cortical zones) within the frontal and temporal lobes for information retrieval. Positron emission studies have also indicated that the right brain is predominantly used in recovery of episodic information while the left-brain is used in the retrieval of semantic details. This type of segregation was first studied by kapur et.al (1994) using PET. This is precisely the reason why some special cases of retrograde amnesiac's retain autobiographical memory while their semantic memory is severely impaired. So in cases where the left temporal lobe is affected there is marked fall in the ability to recollect facts. [Hans J. Markowitsch]

Focal Retrograde Amnesia

In general, amnesic syndrome is manifest as severe anterograde amnesia and mild to moderate retrograde amnesia. However there are certain special cases wherein there is little or no AA (anterograde Amnesia) but a clear loss of remote memory. (RA). This type of amnesic syndrome is what is referred to as Focal Retrograde amnesia. So in FRA there is remote memory deficit in the total absence of AA. (Or minimal AA) This presents a clear problem of understanding memory function as in the absence of remote memory recovery it opens the question as to how new information will be encoded for future purposes. Extensive studies in FRA have indicated that the "temporal lobe" has an important role to play in FRA. In many clinical cases of FRA the total dysfunction of the temporal lobes has been observed. [BabinsKy et al., 63]

The peculiarity of FRA cases is that though they loose all factual information that existed before the brain trauma they could relearn it later on. That is to say that in FRA condition remote information is lost but it could be relearned and encoded. This new information can be retrieved later on. In related clinical studies it was found that the damage to hippocampus region did not affect access to long-term memory. This further suggests that the role of Hippocampus is one of temporary nature (consolidation of memory) and hence after a period of time it does not affect remote memory. There is also no standard agreement regarding the RA for autobiographical amnesia. While some researchers regard it of temporal (limited to a minimum period) nature data collected indicates otherwise. [Lynn Nadel] The explanation for this is that autobiographical memory takes a long time (as much as a life time) to consolidate.

The extent of RA is directly dependent on the degree of damage. If the damage is limited to the hippocampus region then there is every likelihood that remote memory access (other than autobiographical memory) would be preserved. If the damage extends to the temporal cortex lobes then there would be a more pronounced loss of autobiographical or semantic memory depending on whether the right or left lobes are affected. [Reed JM]

Treatment for Amnesia

There is no specific treatment for amnesia although most people experience total or partial recovery over a period of time. Restoration of full memory is out of question especially in the case of organic amnesia. However amnesia induced by psychological trauma's have more probability of complete restoration. Treatment is generally based on rest and gradual recovery. There are many clinical cases, which have recorded the natural recovery of patients who suffered from retrograde amnesia due to brain injuries. In one particular case a patient had RA after a minor trauma in which he forgot all events that preceded the trauma but completely regained the memory of the old events in a short period of time. [Francois Sellall]. Researchers attribute this recovery to the pattern of brain connection mechanism. If the area in which the data is encoded is not affected but only the connective path is disrupted then as a natural process new connective mechanisms are developed in due course enabling the recovery of those data. Gloor (1990) states that, "the specific pattern of connectivity woven between the neurons which creates a distributed matrix of excitation and inhibition and is specific for the experience it represents" [Gloor, pg 1681]

Most memory disorders are common and therapists have evolved certain strategies that would help in compensating the memory loss. Using 'external aids' is a most popular strategy but it also incurs some kind of difficulty in that memory impaired people would find it difficult to manage the operations of such gadgets. Using other strategies such as mnemonic training and chaining (breaking tasks into simple steps) have proved to be effective under clinical tests. [Alan J. Parkin, 179]

Medical Intervention

There are some memory enhancing drugs such as RGH 5279, which have been tested to bring about a positive increase in memory. Similarly the alkaloid extract clausenamide derived from Clausena lansium has also proved effective in improving memory. Nooglutil, (N-5[hydroxynicotinoyl]-L-glutamic acid), is another antiamnesic drug that has been well tested. Acetylcholine one of the important neurotransmitters in the brain plays a vital role in memory. Some of the drugs like 'isatin' have been found to increase the concentration of acetylcholine in the brain, which has a direct effect on improving memory. [A. Scriabine]