by André Delacourte
Semantic dementia is characterised by the inability to match certain words with their images or meanings (semantic memory). However, patients with this disorder retain the ability to speak quite fluently, as well as the ability to remember day-to-day events (episodic memory). The cognitive locus of this syndrome appears to lie in the permanent store of long-term memory representing general world knowledge-semantic memory.
Symptoms and course
This begins with loss of knowledge about the world, which often presents as problems with language. Although people can still speak fluently they lose the words for certain items and also lose the knowledge of the meaning of the word. For example, someone may not only forget the word hippopotamus when shown a picture, but also loses all the knowledge they once had about this (e.g. that it is an African animal that lives in rivers). However, unlike Alzheimer's disease, memory for day-to-day events may be good. People may also have difficulty recognising what things are. At later stages, personality is often affected.
SD is gradually progressive and after 5 years, it develops into FTD.
The average age of onset is usually 55 (+/- 10 years). The duration of the disease from diagnosis is 6 to 8 years (longer than AD).
People with primary progressive aphasia are fighting against a condition in which they will continue to lose their ability to speak, read, write, and/or understand what they hear.
Causes and risk factors
The causes and risk factors are unknown.
Very rare disease.
Magnetic resonance imaging (MRI) of the brain can aid physicians in distinguishing semantic dementia from Alzheimer's disease, two neurodegenerative disorders that are hard to differentiate in their early stages.
In patients with semantic dementia, the loss of brain tissue was mostly confined to the left side of the brain and particularly to the front portion of the left temporal lobe.
In patients with Alzheimer's disease, the degree of atrophy was equivalent on both sides of the brain, with no evidence to suggest greater atrophy in the front portion, compared to the back portion, of the temporal lobes.
Semantic impairment, hypoperfusion of the temporal cortex, bilateral but with a left predominance.
Care and treatment
Cholinesterase inhibitors are not useful. No prevention.
Last Updated: Friday 09 October 2009