Transient Semantic Amnesia: A New Syndrome?
ABSTRACT & COMMENTARY
Source: Hodges JR. Transient semantic amnesia: A new syndrome? J Neurol Neurosurg Psychiatry 1997;63:548-549.
Semantic memory refers to the permanent store of representative knowledge, which includes facts, concepts, words, and the meaning of objects (Hodges JR. Transient Amnesia: Clinical and Neuropsychological Aspects. London: JB Saunders; 1991). In its usual form, progressive, selective loss of semantic memory was recognized by Warrington (Q J Exp Psychol 1975;27A: 635-657) as a form of progressive degenerative dementia, associated with atrophy of the left inferolateral temporal lobe but not associated with apparent hippocampal damage. PET scanning in these patients has identified cerebral hypofunctioning in the same area.
Until a recent study by Hodget et al, however, transient semantic amnesia seems not to have been reported before. A 58-year-old man had a long history of hemicranial migraine leading into unilateral numbness of the face. Systemic health had been excellent. A migraine attack affected him at home one night while working on his automobile. He suddenly became unable to understand proper nouns of familiar objects. He also knew he had two children but could not recall their names. He was admitted to hospital wherein he could clearly recall his behavior for the preceding few hours but was unable to name or understand the jobs of top members of the government. A review the following day found him to be oriented, capable of anterograde memory, and able to identify 12 common objects. Nevertheless, his memory for animals remained measurably curtailed. CT scans and EEG were normal. One month later, he was interviewed by Prof. Hodges, at which time he fully recalled his inability to remember the names and nature of tested objects but accurately recalled his entire procedural activity during the attack.
Dr. Hodges, in his discussion, indicates that selective loss of semantic memory is usually progressive and is eventually associated with a severe anomia. This is associated with intact syntax, perception of objects, and preserved anterograde episodic memory. Structural and functional brain imaging in such cases has identified atrophy and reduced function in the left inferolateral neocortex with sparing of the hippocampus.
At first glance, one wonders why this man didn’t simply have transient ischemic attack affecting temporal lobe structures. Such would be unusual, however, given that the presumed locus of the attack originated in the inferotemporal lobe, an area rarely injured selectively by focal ischemia. The normal EEG also showed no evidence of a seizure. Symptomatically, the attack’s association with an acute migraine headache strongly suggests more than chance, just as full transient global amnesia episodes have a moderately elevated association with migraine. Time will tell, and, hopefully other neurologists may have an opportunity to evaluate whether transient semantic amnesia, like transient global amnesia, is most often an unwanted but not dangerous syndrome. fp