Under the Umbrella -- Semantic Dementia

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Overview

Semantic dementia (SD) is also known as fluent Progressive Aphasia. It is a disorder of language in which patients demonstrate a progressive deterioration of semantic knowledge (understanding and recognizing words), while other cognitive faculties remain remarkably spared. Specifically, patients with SD retain the ability to produce fluent speech, but this speech becomes increasing meaningless. SD patients also lose the ability to recognize the meaning of specific words, or to spontaneously name familiar, everyday objects. As with all FTDs, as the disorder progresses and the primary symptoms (in the case of SD, language deficits) worsen, the patient may also develop other FTD features, including behavioral, social, or motor difficulties.


Key Clinical Features

* Difficulty generating or recognizing familiar words An example of this would be when a patient is shown a picture of a cat, and he can neither come up with the word “cat” nor does he recognize the word when it is provided as the appropriate label for the picture.
* Fluent Spontaneous Speech is retained. Especially in early stages of SD, patients may be able to “talk around” the meaning of a specific word they are unable to generate.
* Word-finding pauses in speech
* Difficulty naming familiar objects
* Visual recognition deficit is experienced by some SD patients, who have increasing trouble recognizing familiar objects and faces.
* Neuroimaging studies demonstrate reduced neural activity and decreased blood flow in the left temporal lobe of SD patients.

In later stages, clinical features may include ones found more commonly in other FTD subtypes. These include:
* Behavioral Symptoms
* Emotional Symptoms
* Neurological Symptoms
* Psychiatric Symptoms
* Cognitive Symptoms


Key Pathologic Features

It is important to note that Semantic Dementia is a clinical diagnosis, defined by the symptoms the patient displays. There is no single pathology (meaning a specific physical finding in the brain) associated with the clinical disorder known as SD. Indeed, upon autopsy, a variety of pathological features have been found in the brains of SD patients, many of which are associated with other FTD subtypes. These include: the inclusions seen in nerve cells (Pick bodies) characteristic of Pick's disease; nerve cell loss in the cerebral cortex and basal ganglia seen in Corticobasal Degeneration; and Dementia Lacking Distinctive Histopathology (DLDH), which is a pattern of neural damage seen in several types of FTD. DLDH is characterized by neuronal loss, gliosis (overgrowth, or development of tumors), and spongiosis (swelling with excess fluid).

In patients for whom language dysfunction is the most severe feature, it is common to see cells in the temporal lobe most affected by these disease changes.


Cause

Scientists have recently linked DLDH to abnormally low levels of the protein tau


Genetics

The majority of Semantic dementia is not hereditary.


Treatment

Currently, there is no specific treatment that has been proven effective in SD patients.


Management and Prognosis

Semantic Dementia is a progressive disorder of language, which usually begins with an inability to generate familiar words, and progresses with increasing difficulties in word comprehension, generation, and recognition. In later stages of the disorder, some SD patients will develop symptoms common to the other FTD subtypes, including behavioral, social, or motor difficulties.

During early stages of SD, patients may benefit from speech therapy. A word book, which the patient can use to point to words they are unable to generate spontaneously, may be of some use, as well.



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Material prepared by Jennifer M. Farmer, MS, CGC, Website Clinical Consultant and Susan L-J Dickinson, MS, CGC, Website Medical and Science Writer.