Semantic dementia

Last revised by Daniel J Bell on 22 Jun 2021

Semantic dementia, also known as semantic variant primary progressive aphasia, is generally considered to be one of three subtypes of primary progressive aphasia, along with progressive non-fluent aphasia and logopenic dementia.

Patients with semantic dementia typically present with a gradual worsening of both expressive and receptive language function. Patients complain of difficulty in remembering the names of places, people or objects or attributing the correct function to named objects 2,3.

Speech fluency is typically preserved but associated anomia leads to the common clinical ascription of "fluent empty speech". Patients will often also exhibit dyslexia, dysgraphia and difficulty recognizing faces or objects 2.

Early on the deficits are largely isolated to temporal lobe function, and especially language. Later frontal symptoms also develop indistinguishable from the frontal variant of frontotemporal dementia, thus supporting the inclusion of semantic dementia as a frontotemporal lobar degeneration 3.

In contrast to Alzheimer disease, episodic memory is usually unimpaired, with patients retaining a good memory of life events 2,3.

The following diagnostic criteria have been proposed 4:

  • clinical diagnosis of semantic dementia
    • both of the following:
      1. impaired confrontation naming
      2. impaired single-word comprehension
    • and at least 3 of the following:
      1. impaired object knowledge, particularly for low-frequency or low-familiarity words
      2. surface dyslexia or dysgraphia
      3. spared repetition
      4. spared speech production (grammar and motor speech)
  • imaging-supported semantic dementia
    • both of the following:
      1. clinical diagnosis of semantic variant PPA
      2. predominant anterior temporal lobe atrophy OR predominant anterior temporal hypoperfusion or hypometabolism on SPECT or PET
  • semantic dementia with definite pathology
    • both of the following:
      1. clinical diagnosis of semantic variant PPA
      2. histopathologic evidence of specific neurodegenerative pathology (eg FTLD-tau, FTLD-TDP, AD, other) OR presence of a known pathogenic mutation

MRI is the modality of choice to assess the pattern of atrophy, although volumetric CT with multiplanar reconstructions can be very useful.

The pattern of atrophy in semantic dementia is relatively characteristic and helps distinguish these patients from those with Alzheimer's disease although there is considerable overlap with other primary progressive aphasias including logopenic dementia (which is an Alzheimer variant). Features include 1-3:

  • temporal lobe atrophy: asymmetric (left more than right)
    • atrophy is more marked anteriorly with relative preservation of volume posteriorly
    • preferentially involves the
      • temporal pole
      • amygdala
      • parahippocampal gyrus (including the entorhinal cortex)
      • inferior and middle temporal gyri
      • fusiform gyrus
    • enlargement of the temporal horn of the lateral ventricle 
  • hippocampal atrophy
    • asymmetric (left more than right)
    • atrophy is more marked anteriorly with relative preservation of volume posteriorly 

It is important to note that although semantic dementia is one of the so-called frontotemporal dementias, frontal atrophy is not really a notable feature, at least not early on in the disease.

The main clinical differential (early on) is that of Alzheimer's disease, and a number of features on imaging are helpful.

  • atrophy is predominantly of the entorhinal cortex, hippocampus, and amygdala
  • no anterior to posterior gradient of atrophy (i.e the whole hippocampus is affected)
  • usually symmetric
  • generalized atrophy also present

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Cases and figures

  • Case 1
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  • Case 2
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