Semantic dementia

Semantic dementia
Classification and external resources
MeSH	D003704
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Semantic dementia (SD) is a progressive neurodegenerative disorder characterized by loss of semantic memory in both the verbal and non-verbal domains. The most common presenting symptoms are in the verbal domain however (with loss of word meaning) and it is characterized as a primary progressive aphasia.

SD patients sometimes show symptoms of surface dyslexia, a relatively selective impairment in reading low-frequency words with exceptional or atypical spelling-to-sound correspondences.

SD is one of the three canonical clinical syndromes associated with frontotemporal lobar degeneration (FTLD). SD is a clinically defined syndrome, but is associated with predominantly temporal lobe atrophy (left greater than right) and hence is sometimes called temporal variant FTLD (tvFTLD).

It was first described by Arnold Pick in 1904 and in modern times was characterised by Professor Elizabeth Warrington in 1975, but it was not given the name semantic dementia until 1989. The clinical and neuropsychological features, and their association with temporal lobe atrophy were described by Professor John Hodges and colleagues in 1992.

SD patients often present with the complaint of word-finding difficulties. Clinical signs include fluent aphasia, anomia, impaired comprehension of word meaning, and associative visual agnosia (inability to match semantically related pictures or objects). As the disease progresses, behavioural and personality changes are often seen similar to those seen in frontotemporal dementia although cases have been described of 'pure' semantic dementia with few late behavioural symptoms.

Patients perform poorly on tests of semantic knowledge. Published tests include both verbal and non-verbal tasks, e.g., The Warrington Concrete and Abstract Word Synonym Test, and The Pyramids and Palm Trees task (Howard and Patterson, 1992).

Testing will also reveal deficits in picture naming (with semantic errors being made e.g. "dog" for a picture of a hippopotamus) and decreased category fluency.

Structural MRI imaging shows a characteristic pattern of atrophy in the temporal lobes (predominantly on the left), with inferior greater than superior involvement and anterior temporal lobe atrophy greater than posterior. This distinguishes it from Alzheimer's disease.Meta-analyses on MRI and FDG-PET studies confirmed these findings by identifying alterations in the inferior temporal poles and amygdalae as the hotspots of disease - brain regions that have been discussed in the context of conceptual knowledge, semantic information processing, and social cognition. Based on these imaging methods, semantic dementia can be regionally dissociated from the other subtypes of frontotemporal lobar degeneration, frontotemporal dementia and progressive nonfluent aphasia.

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