FDG-PET in pathologically confirmed spontaneous 4R-tauopathy variants

The 4-repeat (4R)-tauopathies can be clinically heterogeneous and difficult to diagnose. An FDG-PET pattern of hypometabolism has been previously reported in clinically suspected 4R-tauopathies. Considering that pathological confirmation has not been used as inclusion criteria in these studies, however, the possibility exists that atypical cases were excluded. We studied pathologically confirmed cases of 4R-tauopathies to determine if FDG-PET patterns of hypometabolism different than those previously described exist. We identified all autopsy confirmed 4R-tauopathies with FDG-PET imaging performed between 2010 and 2013 within the Mayo Clinic database. Clinical features and FDG-PET imaging were compared to a group of normal controls. Ten patients, seven of which had autopsy-confirmed progressive supranuclear palsy (PSP), were identified. We also identified two cases with globular glial tauopathy (GGT) and one case of corticobasal degeneration (CBD). The overall predominant imaging findings included bilateral caudate hypometabolism in nine cases, mild asymmetric thalamic hypometabolism in eight,  midbrain hypometabolism in seven, and bilateral hypometabolism in the supplementary motor area in seven. No differences were observed between PSP and GGT. The one CBD case had asymmetric parietal hypometabolism that was not seen in the PSP and GGT cases. As previously described, 4R-tauopathies are associated with frontal, caudate, midbrain and thalamic hypometabolism on FDG-PET. This is the first report of FDG-PET in GGT, and although our series was limited, no features distinguish GGT from PSP. There was some evidence that parietal hypometabolism may be suggestive of CBD.