Transient epileptic amnesia (TEA) is a rare but likely underdiagnosed neurological condition which manifests as relatively brief and generally recurring episodes of amnesia caused by underlying temporal lobe epilepsy. Though descriptions of the condition are based on fewer than 100 cases published in the medical literature, and the largest single study to date included 50 people with TEA, TEA offers considerable theoretical significance as competing theories of human memory attempt to reconcile its implications.
Symptoms A person experiencing a TEA episode has very little short-term memory, so that there is profound difficulty remembering events in the past few minutes (anterograde amnesia), or of events in the hours before the onset of the attack, and even memories of important events in recent years may not be accessible during the amnestic event (retrograde amnesia). FT zemanTEA1998 Some people report short-lived retrograde amnesia so deep that they do not recognize their home or family members, though personal identity is preserved. The amnestic attack has a sudden onset. Three-fourths of cases are reported upon awakening. In attacks that begin when an individual is fully alert, olfactory hallucinations or a "strange taste" or nausea have been reported. Somewhat less than half the cases include olfactory or gustatory hallucinations, and slightly more than a third involve motor automatisms. A quarter of attacks involve a brief period of unresponsiveness. Frequently, however, there is no warning.
During the attack the person's cognitive functions are not generally impaired; perception, communication, attention are normal for most of the duration of the event. In half the cases reported, behavior includes repetitive questioning to attempt to orient experience as the brain fails to lay down new memories or recall a range of recent experiences. The website of the UK-based organization The Impairment of Memory in Epilepsy (TIME) describes an attack this way:
Attacks typically last 20 to 60 minutes. Some attacks may be less than five minutes in duration. Much longer attacks have been reported; in the 2007 study of 50 TEA cases, one lasted four days and one lasted two days. Such unusual presentations "may be due to ongoing seizure activity (non-convulsive status epilepticus) or persistent post-ictal dysfunction of memory-related brain structures."
As the amnesia resolves, the person may recall very little about it, although some memory may be retained of the fact of an episode. Of greater consequence than this brief gap are three common persistent memory complaints among people who have experienced TEA: difficulty recalling knowledge recently gained (accelerated forgetting); difficulty recalling events in one's personal life over a period of decades (autobiographical amnesia); and difficulty with spatial memory, recalling routes or places and the navigational cues that are associated with them (topographical amnesia), discussed below.
Diagnosis Transient amnesia can be the principal manifestation of epilepsy. This diagnosis, however, is "seldom suspected by clinicians and remains controversial". TEA is "almost always misdiagnosed" according to a leading authority. In the largest study to date (2007) "Epilepsy was the initial specialist diagnosis in only 12 of 50 cases." Diagnosis is further complicated by the fact that only somewhat more than a third of cases present positive EEG readings after the attack. However, as TEA tends to recur at a median rate of 12 times a year, witnesses and clinicians may be able to recognize the condition retrospectively; the median delay to diagnosis of TEA in the 2007 study was 12 months.
TEA is a form of focal seizure, which is "the most common variety of adult-onset epilepsy" as opposed to the stereotypical tonic-clonic or grand mal seizure in which affected persons suffer a loss of consciousness and convulse. Diagnostic criteria for the disorder were adopted in the 2007 study of 50 case emphasized clinical features that distinguish TEA from transient global amnesia (TGA), with which TEA is often compared: A history of recurrent witnessed episodes of transient amnesia. In some people, they are very infrequent (less than one per year) whereas others experience them as often as once a week. The episodes are usually very similar to each other. Cognitive functions other than memory are judged to be intact during typical episodes by a reliable witness. In addition to the complex learned behaviors cited above by the TIME organization, reports include people with TEA sight-reading music and playing piano, translating languages, driving or sailing a yacht in the open sea. Evidence for a diagnosis of epilepsy based on one or more of the following: Epileptiform abnormalities on electroencephalography (EEG). In a survey of all known cases, 43.6% have epileptiform abnormalities localised over the temporal or frontotemporal region on electroencephalography following the attack. Of these, 31.7% were left-sided, 12.2% were right-sided and 56.1% were bilateral. The concurrent onset of other clinical features of epilepsy (e.g., lip-smacking or other involuntary automatic behaviors, olfactory hallucinations). Approximately 40% of people with TEA report one of these symptoms in at least some attacks. A clear-cut response to anticonvulsant therapy. Attacks ceased in 44 of 47 treated patients in one study.
Neuroimaging during events An individual in the midst of a prolonged TEA attack was given a PET scan which revealed "dramatic and circumscribed hypermetabolism in the left medial temporal lobe" and his "fluid-attenuated inversion-recovery MRI scan revealed high signal in the left hippocampus". Surface EEG recording during an amnesic attack was performed in ten TEA cases. "All recordings showed seizure activity, which in 8/10 cases involved both temporal lobes and in the others remained unilateral (1 left and one right-sided)." "Another patient was having an EEG when an amnesic event occurred; the trace showed a one-minute burst of left temporal spikes, followed by normalization of the EEG. MRI or CT scans have been reported in 62 TEA cases, with a very low incidence of clinically significant findings. Those focal brain lesions that have been detected have involved the medial temporal lobes. Typically, MRI and CT findings are not remarkable.
Other transient amnestic syndromes TEA is, at first, a challenge to distinguish its salient features during the event from transient global amnesia and dissociative amnesia, though other forms of transient amnesia can include reactions to various medications, closed head injury, and migraine. (Other sources of amnestic symptoms include herpes encephalitis, hypoxia, vascular or basal forebrain lesions, deep midline tumors, early dementia, and Korsakoff syndrome which is secondary to thiamine deficiency, most often the result of alcohol use disorder.)
Typical presentation TEA TGAAge at onsetearly 60searly 60sGender67% male46% malePrecipitating factors70%:sleep/waking80%: stress, exercise, cold waterHistoryepilepsymigraineProcedural memory intactintactRecognizes family, homeusually yesDuration1–60 minutes 2–8 hoursEEGduring: abnormalafter: 40%+abnormalduring: normal after: 7% develop epilepsyOther occasional symptomsbrief unresponsivenessnausea, headache Autonomic actions Yes (40%) noDegree of amnesia of event44% have partial recall no recall Personal identityintactintact Persistent memory loss 80%+: ALFautobiographical amnesia topographical amnesia 30%+: ALFautobiographical amnesia. Hallucination42%: olfactory and gustatory no Treatment anticonvulsant medication noneRecurrence12-13/yearrare
Epidemiology TEA characteristically starts in late middle-age. A sample of validated individuals with TEA had a mean age of 62 and a range of 44 to 77 years of age for the first attack. Research samples have been predominantly male by a ratio of two-to-one. The IQ of people diagnosed with TEA tends to be in the high average to superior range, perhaps due to selection bias.
Persistent memory effects Memory difficulties are among the most common issues for people with epilepsy, and "persistent memory impairment is reported by about 75% of patients with TEA." Other studies suggest the rate exceeds 80%. People who have had TEA attacks frequently report three kinds of persistent problems with memory: accelerated long term forgetting remote memory loss topographical amnesia Importantly, these types of memory difficulty are not detected by standard neuropsychological tests.
Accelerated long-term forgetting (ALF) Accelerated long-term forgetting (ALF) may be described as the abnormally rapid loss of recently learned material from memory. People who have experienced TEA often have difficulty in recalling information over days to weeks, even if this information was well learned initially. Even though accelerated memory loss is reported in 44% of TEA patients, it is undetected by standard tests of memory. Typically these tests examine the ability to store information for up to 30 minutes but the problem of accelerated long-term forgetting in TEA patients is not generally noticeable at this point. It becomes apparent over the following days and weeks. A recent study indicated that acquisition of recently everyday events can be normal after 30 minutes delay but that within 24 hours patients recall significantly less information than age and IQ matched control participants. Several non-mutually exclusive hypotheses have been advanced to explain why TEA patients experience accelerated forgetting. Seizure activity Seizures in TEA patients commonly occur upon waking suggesting a link between TEA and sleep. It is possible that abnormal electrical activity during sleep disrupts the process of memory consolidation which normally occurs when we sleep. On-going subclinical seizure activity has implications for theories of memory consolidation, discussed below. Brain pathology It is possible that a pre-existing condition affecting the temporal lobe may both cause attacks of TEA and interfere with the handling of memories. "Alternatively, episodes of TEA might themselves give rise to structural damage within the hippocampus or related structures, which could disrupt long term consolidation." One study found "subtle hippocampal volume loss" in patients with TEA, but the atrophy did not correspond to measures of either ALF or autobiographical memory loss, suggesting "a more diffuse physiological basis rather than being a consequence of structural damage." Anticonvulsant medication Antiepileptic drugs sometimes affect memory. However, this is unlikely to be the cause of ALF in TEA, as patients tend to report memory problems before they start taking the antiepileptic drugs and can experience improvement in their memory after taking them. Psychological mechanisms Research has reported that low mood and poor self-esteem are associated with memory problems. These are important factors when investigating patients with epilepsy in general. However, in TEA they do not appear to play an important role in accelerated forgetting.
Remote memory loss 70% of people with TEA notice a patchy but persistent loss of memories of events from their past personal experiences and this autobiographical amnesia has been reported in all cases in which accelerated forgetting was also present. This memory loss can occur in people whose ability to acquire new memories is intact. Studying this 'autobiographical amnesia' or 'focal retrograde amnesia' or 'delayed onset amnesia' has been challenging because people do not always realize they have forgotten events or periods of time until they have difficulty in retrieving memories of specific significant events, and find they cannot form a coherent recollection. At first they might think they have forgotten some isolated events due to normal forgetting, and not realize they have forgotten larger blocks of past time: