Frontotemporal Lobar Degeneration
Frontotemporal lobar degeneration is a heterogeneous group of clinicoanatomical subtypes with various underlying pathologies. These include tau and ubiquitin inclusions, 'fused in sarcoma' pathology, 'dementia lacking distinctive histology' and even AD-type pathology. This group of conditions is as common as AD below the age of 65 years, and can be categorised into behavioural variant frontotemporal dementia and primary progressive aphasia. Primary progressive aphasia is itself divided into three subtypes, non-fluent progressive aphasia, semantic dementia and logopenic progressive aphasia.
Behavioural Variant Frontotemporal Lobe Dementia
This variant is characterised by atrophy of the frontal and temporal lobes (including medial temporal structures such as the hippocampus) and largely sparing the parietal lobe (figure 9), with an anterior to posterior gradient, and often asymmetric, with the left side more atrophic (Table 3).
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Figure 9.
Structural imaging in behavioural variant frontotemporal lobar degeneration. Axial and coronal CT images demonstrating marked frontal atrophy along with asymmetrical medial temporal atrophy, most marked on the left.
Tc-HMPAO SPECT shows reduced perfusion of the frontal and anterior temporal lobes (figure 10). The frontal hypometabolism and perfusion are asymmetrical and can be predominantly left or right dominant.
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Figure 10.
Tc-HMPAO SPECT abnormalities in behavioural variant frontotemporal lobar degeneration. Source images (A), and statistical parametric mapping (B) glass brain and (C) surface rendered representations showing a predominant pattern of bifrontal hypoperfusion.
Non-fluent Progressive Aphasia
This is marked by atrophy in the left inferior frontal regions of the pars opercularis and a small region of the pars orbitalis (figure 11). There is usually obvious grey matter loss in the left precentral gyrus, and more anteriorly in the middle frontal gyrus. F-FDG PET and Tc-HMPAO SPECT show asymmetric left frontal hypometabolism/perfusion, often involving the insular region (figure 12).
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Figure 11.
Structural imaging changes in non-fluent progressive aphasia. Coronal and axial CT images demonstrating left-sided peri-insular volume loss.
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Figure 12.
Tc-HMPAO SPECT abnormalities in non-fluent progressive aphasia. Note the left frontal and peri-insular hypoperfusion.
Semantic Dementia
This is marked by bilateral atrophy in the medial and lateral portions of the anterior temporal lobes, including amygdala/anterior hippocampus, anterior inferior, and middle and superior temporal gyri and the anterior fusiform gyri. The atrophy is commonly asymmetrical and maximal on the left but with progressive changes on the right also. Again, there is an anterior to posterior gradient (figure 13). F-FDG PET and Tc-HMPAO SPECT in semantic dementia shows prominent anterior temporal hypometabolism or perfusion, left greater than right. (figure 14).
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Figure 13.
Structural imaging in semantic dementia. Axial and coronal CT images show asymmetrical medial temporal atrophy, maximal on the left, with associated anterolateral atrophy as well. A coronal FLAIR MRI in another (semantic dementia) patient (right) shows similar structural changes.
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Figure 14.
Tc-HMPAO SPECT abnormalities in semantic dementia. Source images (A) and statistical parametric mapping (B) glass brain and (C) surface rendered representations demonstrating bilateral anterior temporal hypoperfusion, maximal on the left.
Logopenic Progressive Aphasia
This is atrophy involving the posterior temporal cortex and inferior parietal lobule. There may also be marked atrophy in the left anterior hippocampus, in the right angular gyrus and in the precuneus. 18F-FDG PET and Tc-HMPAO SPECT show the greatest metabolic and perfusion lesions in the left parietal lobe (inferior lobule) and posterolateral temporal lobes (superior and middle temporal gyri) (figure 15).
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Figure 15.
Tc-HMPAO SPECT abnormalities in logopenic aphasia. Source images (A) and statistical parametric mapping (B) glass brain and (C) surface rendered representations. Note, hypoperfusion in the left inferior parietal lobule and posterolateral temporal lobes.