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.
The pace of the symptoms and length of disease can vary dramatically from person to person.
In general, each type of FTD follows a pattern where the symptoms seen in the mild stage become more pronounced and disabling over a course of 8-10 years.
People with early semantic dementia that is predominantly on the left side of the brain, usually complain of a hard time coming up with the word or name for something.
Words that the person uses a lot may remain available, but more unusual words may be replaced by “thingy” or “you know.”
The tone, rhythm and fluctuations of pitch (prosody) generally sound normal. Memory for day-to-day events is usually spared.
The early signs of SD in people with asymmetric right-sided damage include a decline in empathy or awareness of other people’s emotions.
After two to three years, the people with left sided damage and those with right sided damage tend to look more similar, as the disease typically progresses to involve both sides.
With moderate SD, most people show at least some of the behavioral problems that are similar to the behavioral variant of FTD.
People with moderate semantic dementia will have immense trouble understanding you. They may also have increasing difficulty recognizing the names and faces of people – even friends and family.
Reading and writing, mostly likely, will have declined noticeably.
The person may still be able to use numbers, colors and shapes – the brain functions responsible for these skills are organized in a different area of the brain from words.
After four to five years of SD, the disease is usually quite advanced, which means the person’s language skills have significantly eroded, making communication very difficult while the behavioral problems have significantly increased.
Typical behaviors seen in late stage SD include disinhibition, apathy, compulsions, impaired face recognition, altered food preference and weight gain.
People with left-sided damage tend to show more interest in visual or non-verbal things while people with right-sided damage tend to prefer games with words and symbols.
The time from diagnosis to the end typically takes about six years, although this can vary significantly from person to person.
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.