Transient global amnesia (TGA) is a somewhat enigmatic, but benign, neurological syndrome in which a person loses the ability to form new memories for up to 24 hours. Due to its rarity, clinicians presented with a case of TGA in the emergency department may conduct a series of neuropsychological assessments to make sure that the amnesia being experienced doesn’t have a more serious underlying cause. These tests range from simply asking someone to repeat a series of numbers to more thorough tests that involve answering multiple questions on different topics. 

In this article, we will break down some commonly used memory and cognitive assessments that may be used in the diagnosis of TGA and explain how they work and why they are important. 

Introduction

Transient global amnesia, or TGA for short, is a syndrome in which a person suddenly becomes unable to create new memories for a short period of time of up to 24 hours. This results in a type of short-term memory loss; a person may remember their name and long-term memories, but will not remember things such as where they are or how they got there. 

TGA is not well-characterised as it is relatively uncommon. It is estimated that TGA affects around 3-10 people per 100,000, with this number rising to 23.5 per 100,000 in those above the age of 50.¹ It is set apart from other forms of amnesia (memory loss) as it does not occur as a result of an underlying neurological condition (like amnesia resulting from an epileptic seizure or stroke).¹ 

As TGA is a short-lived and poorly understood syndrome, there is currently no treatment for it on the market. Neuropsychological assessments during or after TGA episodes may be performed to evaluate cognitive function during episodes or to rule out other neurological conditions for which treatment is available.

Key characteristics of a TGA episode

Below, we will delve into the symptoms that are indicative of a TGA episode. These include more information regarding the memory problems, spared cognitive functions and recovery patterns of TGA episodes. 

Memory deficits

During an episode of TGA, a person becomes unable to create new memories, so they cannot remember the recent past. This is referred to as anterograde amnesia. Examples of this include a person not knowing where they are, how they got there or asking the same questions over and over again. In anterograde amnesia, the person experiencing it can remember things from before the episode, including things like personal information or the identity of loved ones. TGA episodes are also associated with a mild reduction in long-term (retrograde) memory.² 

Sometimes, emergency department doctors can interview witnesses of the TGA episode to gain some more information. This is a completely normal procedure, and provides information that can help to distinguish whether the witnessed amnesia is TGA, or whether there could have been another causative event, such as an accident or seizure.³ 

Spared cognitive functions

What makes TGA unique is the fact that it is characterised by short-term memory loss, with the sparing of other cognitive functions. This means that a person will not experience the following during an episode:³

• Clouding of consciousness or brain fog
• Loss of identity
• Difficulties with speech or movement
• Recent history of brain trauma 
• Recent history of a seizure

Recovery patterns

A key characteristic of TGA is the resolution of the episode within 24 hours. The course of recovery is as follows:⁴

1. Repetitive questioning stops
2. Awareness of time and place is regained
3. Short-term and general knowledge memory improves

After 24 hours, when short-term memory is close to baseline, memory of what has occurred before, during or after an episode may remain blurry. The ability to recall information from long-term memory not related to the episode may also be impaired.⁴ 

The biology behind TGA

The involvement of the brain

Research suggests that TGA is linked to a part of the brain called the hippocampus, which plays a key role in learning and memory. It is thought that TGA is caused by a short-lived change in the way neurons fire and communicate with the body.⁴ Some studies point to the presence of lesions in the brains of TGA patients, which show up in MRI scans.⁵ 

Possible triggers

In around a third of cases of TGA, the amnesia is triggered by some sort of precipitating event. These are typically highly emotional or stressful situations, which include:³ 

• Intense cold 
• Intense pain
• Strenuous physical activity
• Sexual intercourse
• Being robbed
• Hearing bad news
• Medical procedures

Neuropsychological assessments in TGA

In cases of TGA, neurological assessments are carried out in order to rule out other neurological disorders. These aim to assess the experienced amnesia against the key characteristics of TGA outlined above and can be used to track recovery from episodes. These can be split into memory evaluation and cognitive testing. The type of tests used varies from clinic to clinic. 

An extensive list of neurological and cognitive tests can be found here. 

Memory evaluation

Due to the rarity of the syndrome, there is limited research on the types of memory tests that are best for cases of TGA. This means that the memory tests used by clinicians can range from simply asking a patient to repeat a 7-digit number to more rigorous scale tests that encompass all aspects of immediate memory (auditory, visual and working memory).^1,6 Below, we will take a look at some example memory tests that may be used in the diagnosis of TGA. 

Famous faces and events tests

In some instances, a famous faces and famous events test may be performed.⁷ This allows doctors to investigate whether the long-term (retrograde) memory is affected by the episode and consists of showing the patient the faces of famous people and asking them to identify them, or by inquiring about recent famous events. 

Crovitz’s cue-word and autobiographical memory tests 

The Crovitz cue-word and autobiographical memory tests have also been used in the assessment of patients with TGA to determine the quality of their autobiographical memory.⁷ These tests also look at the memory of past events, but those that apply to the patient. For example, memories from childhood or the recent past. In these tests, the patient is presented with cue words and they must link those words to a memory in their lives. In the Crovitz cue-word test, these words are neutral, for example, “beach” or “train”.⁸ In the autobiographical memory test, these words are linked to emotions, for example, “happy” or “sad”. 

Wechsler memory scale

The Wechsler memory scale test is a comprehensive memory test, which assesses the following memory types:⁹

• Verbal memory
• Visual memory
• Visual working memory
• Immediate memory
• Delayed memory

This test is composed of subtests, like visual puzzles and sequence tests, which aim to give a score indicative of the extent of memory loss in that category. This test is not commonly used in TGA, as it is too extensive, but it may be helpful in fully assessing the extent of memory loss and to monitor recovery from an episode. 

Cognitive function tests

Those experiencing TGA episodes should not have severe deficits in cognitive functions other than memory. This is why a doctor may conduct cognitive function tests to rule out other possible diagnoses. “Cognitive function” is a broad term used to describe practically every brain function. Examples of cognitive functions include:

• Attention
• Processing speed
• Reasoning
• Planning
• Problem solving
• Multitasking

Cognitive function tests do not require you to have any additional subject knowledge. 

Montreal cognitive assessment test

The Montreal cognitive assessment test is used in the diagnosis of mild cognitive impairment in diseases such as Alzheimer’s disease, Parkinson’s disease, depression or multiple sclerosis, among others. It is a 10-minute 30-point test involving:

• Short-term memory recall tasks
• Clock drawing task
• Image copying task
• Language tasks
• Evaluation of orientation in time and space - This refers to questions like “What date is it?” and “What city are we in?”

Mini-mental state examination

The mini mental state examination is a 5-10 minute examination of cognitive function aimed at testing for cognitive impairments. This examination involves questions assessing:

• Knowledge of time and place
• Language - For example, following simple verbal instructions, repeating sentences or writing sentences
• Attention - For example, by spelling a word backwards
• Short-term memory recall

Trail making test

Trail-making tests are one of the most commonly used cognitive function tests in the assessment of executive functions in ageing. As TGA is most commonly seen in the older population, it would be a useful test to conduct. 

The trail-making test only requires the patient to draw a trail in the order of the numbered or lettered dots on a page. 

Summary

Neuropsychological assessments play a critical role in the understanding and diagnosis of transient global amnesia (TGA). Memory evaluations, like the famous faces test and the Crovitz cue-word test, assess a patient's ability to recall both general and personal past events. The Wechsler memory scale can offer a more detailed examination of verbal, visual and working memory. Cognitive function tests, including the Montreal cognitive assessment, mini-mental state examination and trail making test, help to rule out other neurological disorders by evaluating attention, reasoning and problem-solving skills, which should be unaffected in TGA. These tests not only assist in confirming a TGA diagnosis but may also provide valuable insight into the recovery process of TGA.