Semantic dementia

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). 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. It is important to note the distinctions between Alzheimer’s Disease and Semantic dementia with regard to types of memory affected. In general, Alzheimer’s Disease is referred to as 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.3 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). 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. It is important to note the distinctions between Alzheimer’s Disease and Semantic dementia with regard to types of memory affected. In general, Alzheimer’s Disease is referred to as 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.3 It 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. 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. 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. Structural and functional MRI imaging show 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. Selective hypometabolism of glucose has been observed in the anterior temporal lobe, as well as the medial temporal lobe and limbic areas. Damage to white matter tracts connecting the anterior temporal cortex to the inferior longitudinal, arcuate, and uncinate fasciculi, which are regions of the language network, is also seen using diffusion tensor imaging. Imaging also shows the integrity of the axonal connections from the anterior temporal cortex to frontal and posterior associative areas to be altered. Functional abnormalities have also been observed in hippocampal structures, the ventromedial prefrontal cortex, and the cingulate cortex. SD patients generally have difficulty generating familiar words or recognizing familiar objects and faces. 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, behavioral and personality changes are often seen similar to those seen in frontotemporal dementia. SD 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. Testing also reveals deficits in picture naming (e.g. 'dog' for a picture of a hippopotamus) and decreased category fluency. The question 'What is a Stapler?' has been used as a primary diagnostic technique for discerning how SD patients understand word meaning.