Semantic dementia

Semantic variant primary progressive aphasia, also known as semantic dementia (SD) is one of the clinical neurodegenerative diseases associated with fronto-temporal lobar degeneration (FTLD). It is a subtype of thelanguage variant front-temporal dementias.

Diagnostic criteria⁴

Clinical diagnosis of semantic variant PPA:

Both of the following:

1. Impairedimpaired confrontation naming
2. Impairedimpaired single-word comprehension

At least 3 of the following:

1. Impairedimpaired object knowledge, particularly for low-frequency or low-familiarity words
2. Surfacesurface dyslexia or dysgraphia
3. Sparedspared repetition
4. Sparedspared speech production (grammar and motor speech)

Imaging-supported semantic variant PPA:

Both of the following:

1. Clinicalclinical diagnosis of semantic variant PPA
2. Predominantpredominant anterior temporal lobe atrophy OR predominant anterior temporal hypoperfusion or hypometabolism on SPECT or PET

Semantic variant PPA with definite pathology:

Both of the following:

1. Clinicalclinical diagnosis of semantic variant PPA
2. Histopathologichistopathologic evidence of specific neurodegenerative pathology (eg FTLD-tau, FTLD-TDP, AD, other) OR presence of a known pathogenic mutation

Clinical presentation

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 objects or attributing the correct function to named objects ^2-3.

 SpeechSpeech 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 recognising faces or objects².

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

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

Radiographic features

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

MRI

The pattern of atrophy in semantic dementia is relatively characteristic and is helpful in distinguishing these patients from those with Alzheimer's disease with prominent semantic symptoms (see below). Features include ^1-3:

• temporal lobe atrophy: asymmetric (L>R(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 (L>R(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.

Differential diagnosis

The main clinical differential (early on) is that ofAlzheimer'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
• generalised atrophy also present