A 68-year-old lady with a background of hypertension presents to her GP with sudden-onset memory loss that began earlier that day. Her symptoms had started immediately after lifting a heavy box at home. Her husband reports that she had asked the same questions repeatedly every five minutes. He also says that at the time of the episode, she was unable to recall events from earlier that day. This episode lasted for two hours before resolving gradually.
Her husband reports no speech disturbance or limb weakness during this time. She is now back to her normal self, but cannot recall anything that happened from the onset of the episode until it resolved. Both the patient and her husband are concerned she may have had a stroke.
On examination she appears well. Neurological examination of her cranial nerves and of her limbs is unremarkable. She scores 10/10 on an abbreviated mental test. The GP reassures the patient that she has not had a stroke and that the episode was most likely to have been transient global amnesia. She goes home without any further investigation but is advised to return if she has a further episode in the future.
The problem
Transient global amnesia (TGA) is a benign amnestic syndrome, lasting less than 24 hours. It is relatively uncommon, with an incidence of 5-11 per 100,000 people per year. The mean age of onset is 60 years.1
The aetiology of TGA is unknown. Proposed causes include migraine, venous flow abnormalities, focal ischaemia and epilepsy.
Features
TGA is characterised by sudden-onset, transient anterograde and retrograde amnesia. During the episode, patients will typically ask the same questions repeatedly as they are unable to retain new information. The ability to perform complex tasks such as cooking and driving is preserved during an attack.
Several triggers have been identified, such as sudden changes in temperature, emotional distress or a Valsalva manoeuvre (as in this case). The episode should last less than 24 hours and in most cases will last only a few hours.
After the episode, the patient will return to normal but will have no memory of the period during the attack and may not recall the hours preceding symptom onset. Most patients only ever suffer one episode of TGA.
Diagnosis
Diagnosis is based entirely on the history. A witness is required as the patient will not recall anything from the episode.
A diagnosis of TGA can be made if there is an episode of sudden-onset anterograde amnesia lasting several hours, with repetitive questioning in the absence of any other impairment.
The following diagnostic criteria for TGA are widely used:2
• Acute onset of anterograde amnesia.
• Attack witnessed by an observer.
• No change of consciousness or loss of self-awareness.
• Cognitive impairment limited to amnesia.
• No recent history of head trauma or seizures.
• Duration of symptoms of one to 24 hours.
• No neurological symptoms besides dizziness, vertigo or headache.
The differential diagnosis of TGA includes migraine, transient epileptic amnesia and a transient ischaemic attack (TIA). It is the presence of the very focal deficit in memory that differentiates TGA from a TIA.
Features from the history that would not be typical for TGA and are suggestive of alternative diagnoses are:
• Recurrent events.
• Brief duration (of less than five minutes).
• Impairment of consciousness.
• Automatisms.
• Any motor deficit or dysphasia.
It has been increasingly recognised that some patients who have had an episode of TGA have changes on diffusion-weighted brain MRI within 48 hours of symptom onset. Although these findings have intensified the debate surrounding the aetiology of TGA, they are not useful diagnostically and TGA remains a clinical diagnosis.3
Management
Once a diagnosis of TGA has been made, reassurance is all that is required. Many patients and relatives are anxious that the patient has suffered a stroke, but they can be reassured that this is not the case. Unlike with a TIA, patients who have had TGA are not at an increased risk of stroke.
The patient and their relative should be advised to seek medical help if the symptoms recur, as it is very uncommon for someone to suffer an episode of TGA more than once, and repeated episodes are suggestive of transient epileptic amnesia instead.
If there is uncertainty about the diagnosis, the patient should be discussed with a neurologist.
Dr Tim Wilkinson is a specialist registrar in neurology, and Dr Chris Derry is a consultant neurologist, both based at the Western General Hospital, Edinburgh
References
