[Language impairment and semantic memory loss of semantic dementia]

Semantic dementia (SD) is a neurodegenerative disease characterized by atrophy of the anterior temporal regions and progressive loss of semantic memory. SD has recently been reported to be associated with a pathologic diagnosis of frontotemporal lobar degeneration (FTLD) with T^ar DNA-binding protein of 43 kDa (TDP-43) immunoreactive inclusions (FTLD-TDP) type 2 by Mackenzie. In the first several years of the disease, SD patients, especially those with left hemisphere-dominant temporal atrophy, present with primary progressive aphasia, in which language deterioration is obvious; however, they do not have other cognitive and behavioral impairments. The language impairment in SD is termed as word meaning aphasia, in which patients experience both word finding difficulties and word recognizing difficulties (two-way anomia). Phonemic cues are not effective in improving anomia. In addition, SD patients do not experience a sense of familiarity with words that they cannot find or recognize. While reading and writing Japanese words, SD patients, except those who also have motor neuron disease, exhibit well-preserved kana (phonogram) processing. However, in the case of kanji, they often exhibit surface dyslexia while reading and also exhibit phonetic miswriting. In the aphasic stage, SD patients can explain what the objects are and can use them appropriately; however, they cannot find or recognize the names of the objects. On progressing to the semantic memory impairment stage, the patients do not exhibit any familiarity with the objects whose names they cannot find or recognize and are unable to appropriately use these objects. Semantic memory impairment in SD is attributed to damage of gray matter and of superior and inferior white matter connections in the anterior temporal lobe.