The cortical topography of temporal lobe hypometabolism in early Alzheimer's disease

Alzheimer's disease (AD) is characterized by a pathological process with specific predilection for association neocortex and the mesial temporal lobes. Recently developed high-resolution positron emission tomographs (PET) are able to quantitate regional cerebral metabolic rates for glucose (rCMRglc) in these brain regions and map the distribution of the metabolic consequences of Alzheimer pathology. In order to evaluate the relative involvement of mesial and neocortical temporal lobe brain regions in AD, we studied 22 AD patients, 11 of whom were mildly demented and 11 of whom were moderately demented in comparison to 8 age-matched control subjects. We used a PET instrument with 2.6 mm in-plane resolution and quantitated rCMRglc in anterior, middle, and posterior temporal neocortex, visual association cortex, primary visual cortex, and mesial temporal cortex. Although the moderately demented AD patients showed significantly lower metabolic rates than controls in visual association cortex and all temporal lobe regions except right anterior temporal neocortex, the mildly demented patients were different from the controls in only middle temporal neocortex. Considerable variability was found in the relative involvement of mesial temporal lobes and temporal neocortex in the AD patients, however, as shown by greater variance of a ratio of mesial temporal lobe rCMRglc to temporal neocortical rCMRglc (MES/NEO ratio) in the AD patients than the controls. A series of stepwise multiple regressions showed that this ratio was related to patient cognitive symptomatology, with more severely memory-impaired patients showing lower MES/NEO ratios, while patients with visuospatial disturbances showed higher MES/NEO ratios. In addition, the only biological variable that was related to this ratio was patient age, with older patients showing lower MES/NEO ratios. These results indicate that mesial temporal lobe structures are not invariably the earliest nor the most severely metabolically involved brain regions in AD and that the relative involvement of the mesial and neocortical temporal lobe is related to the patient's cognitive symptoms and age.