Semantic dementia

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Overview
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 therefore often characterized (incorrectly) as a primary language disorder (a so-called progressive fluent aphasia).

SD is one of the three canonical clinical syndromes associated with frontotemporal lobar degeneration. 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 Professor Elizabeth Warrington in 1975 (Warrington EK. The selective impairment of semantic memory. Q J Exp Psychol. 1975 Nov;27(4):635-57) but was not given the name semantic dementia until 1989 (Snowden JS, Goulding PJ, Neary D. Semantic dementia: a form of circumscribed cerebral atrophy. Behav Neurol 1989;2:167-182.).

Signs and Symptoms
SD patients often present with the complaint of word-finding difficulties. On further questioning, patients often appear to have lost the meaning of certain words (e.g. asking "What is a fish?"). 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.

Neuropsychology
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 (Warrington EK, McKenna P, Orpwood L. Single word comprehension: a concrete and abstract word synonym test. Neuropsychological Rehabilitation 1998; 8: 143-154.) 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.

Imaging
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 (Chan et al. Patterns of temporal lobe atrophy in semantic dementia and Alzheimer's disease. Ann Neurol. 2001 Apr;49(4):433-42).

Pathology
The majority of patients with SD will have ubiquitin-positive, tau-negative inclusions although like all of the FTLD syndromes other pathologies have been described including Pick's disease and other tau positive pathology (Davies RR et al. The pathological basis of semantic dementia. Brain. 2005 Sep;128(Pt 9):1984-95.)

Genetics
Of all the FTLD syndromes SD is least likely to run in families and is usually sporadic (Goldman JS et al. Comparison of family histories in FTLD subtypes and related tauopathies. Neurology. 2005 Dec 13;65(11):1817-9.)

Management
There is currently no known curative treatment for this condition. Supportive care is essential in what is a greatly debilitating problem.