In neurology, semantic dementia (SD), also known as semantic variant primary progressive aphasia (svPPA), is a progressive neurodegenerative disorder characterized by loss of semantic memory in both the verbal and non-verbal domains. However, the most common presenting symptoms are in the verbal domain (with loss of word meaning). Semantic dementia is a disorder of semantic memory that causes patients to lose the ability to match words or images to their meanings. However, it is fairly rare for patients with semantic dementia to develop category-specific impairments, though there have been documented cases of it occurring. Typically, a more generalized semantic impairment results from dimmed semantic representations in the brain. SD is one of the three canonical clinical syndromes associated with frontotemporal lobar degeneration (FTLD), with the other two being frontotemporal dementia and progressive nonfluent aphasia. 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). SD is one of the three variants of primary progressive aphasia (PPA), which results from neurodegenerative disorders such as FTLD or Alzheimer's disease. The other two variants of primary progressive aphasia (PPA) are logopenic variant PPA (lvPPA) and confluent-agrammatic variant PPA (navPPA). There are distinctions between Alzheimer's disease and semantic dementia with regard to types of memory affected. In general, Alzheimer's disease is referred to as a disorder affecting mainly episodic memory, defined as the memory related to specific, personal events distinct for each individual. Semantic dementia generally affects semantic memory, which refers to long-term memory that deals with common knowledge and facts. SD was first described by Arnold Pick in 1904 and in modern times was characterized 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.
Presentation The defining characteristic of SD is decreased performance on tasks that require semantic memory. This includes difficulty with naming pictures and objects, single word comprehension, categorizing, and knowing uses and features of objects. SD patients also have difficulty with spontaneous speech creation, using words such as "this" or "things" where more specific and meaningful words can be used. At earlier stages, patients may replace words with vague terms; as an example, 'crow' may become 'bird', then 'animal', before ultimately becoming 'thing'. Syntax is spared, and SD patients have the ability to discern syntactic violations and comprehend sentences with minimal lexical demands. SD patients have selectively worse concrete word knowledge and association, but retain knowledge and understanding of abstract words. SD patients are able to retain knowledge of numbers and music, but have more difficulty with concrete concepts with visual associations. Impairments of processing of phonemic structure and prosodic predictability have also been observed.
Hemispheric lateralization and modality-specific deficits Research has shown that the lateralization of temporal lobe atrophy in semantic dementia produces distinct patterns of semantic memory impairment. A study was done on a cohort of 41 patients with semantic dementia, where it was shown that performance deficits occurred in accordance with the predominantly affected hemisphere. Patients with predominantly left temporal lobe damage (31 patients) exhibited poorer naming and comprehension abilities. Conversely, patients with predominantly right temporal lobe damage (10 patients; a smaller subgroup) exhibiting poorer facial recognition, picture interpretation, and comprehension of visual information. This hemispheric dissociation indicates that semantic memory is organized in a modality-specific manner. While semantic dementia affects both the right and left temporal lobe, the incongruence of atrophy will play a role in the clinical symptoms displayed.
Genetics The majority of SD patients have ubiquitin-positive, TDP-43 positive, tau-negative inclusions, although other pathologies have been described more infrequently, namely tau-positive Pick's disease and Alzheimer's pathology. Of all the FTLD syndromes, SD is least likely to run in families and is usually sporadic.
Alzheimer's disease and semantic dementia Alzheimer's disease is related to semantic dementia, which both have similar symptoms. The main difference between the two being that Alzheimer's is categorized by atrophy to both sides of the brain while semantic dementia is categorized by loss of brain tissue in the front portion of the left temporal lobe. With Alzheimer's disease in particular, interactions with semantic memory produce different patterns in deficits between patients and categories over time which is caused by distorted representations in the brain. For example, in the initial onset of Alzheimer's disease, patients have mild difficulty with the artifacts category. As the disease progresses, the category specific semantic deficits progress as well, and patients see a more concrete deficit with natural categories. In other words, the deficit tends to be worse with living things as opposed to non-living things.
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