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. 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.
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. 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.
Diagnosis
SD patients generally have difficulty generating familiar words or recognizing familiar objects and faces. Clinical signs include fluent aphasia, anomia (inability to recall names of persons or things), 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. and The Pyramids and Palm Trees task.
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. Moreover, it is important to maintain that these tests must be compared to nonmusical domain tests, as music cognition is not often measured in semantic dementia patients (less data available).
Physical changes
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 (including hippocampal formation and amygdala) atrophy greater than posterior. As the disease progresses, this atrophy spreads to the ventromedial prefrontal cortical regions. 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. Moreover, several dementia patients, all with varied musical experience and knowledge, all demonstrated an understanding of the fundamental governing rules of western music. Essentially, it was found that superordinate knowledge of music, such as the rules of composition, may be more robust than knowledge of specific music.
Regarding the neurobiological correlates for this study, it was determined, via lesion studies, that bilateral (but especially the left-side of the brain) fronto-temporoparietal areas are significant in the associative processing of melodies. No preventative measures for SD are recognized.
While no pharmacological interventions have been introduced, there have been behavioral interventions that have shown success in experimental settings. These experimental approaches focus on vocabulary retraining in order to improve word retrieval. Studies also show that semantic dementia patients can improve their ability to name objects through targeted cognitive intervention. Notably, these improvements do have a tendency to fade over time if the intervention practice is not maintained, and longevity of treatment is variable among patients depending on the progression of the disease.