Alterations in regional cerebral blood flow, with increased temporal interhemispheric asymmetries, in the normal elderly: an HMPAO SPECT study.

Single photon emission computed tomography (SPECT) with the tracer 99Tcm-hexamethylpropyleneamine oxime (HMPAO) provides images allowing semiquantitative estimation of regional cerebral blood flow (rCBF). Despite its widespread use there is little data on patterns of rCBF obtained using this tracer in normal elderly subjects, although other methods of measurement suggest a fall in cerebral blood flow with age. Furthermore, the detection of interhemispheric asymmetries on HMPAO SPECT is often used to identify areas of pathological abnormality yet there is little data on the prevalence of asymmetries in the normal elderly. An increased prevalence of asymmetries in the elderly may explain the difficulties recently reported in using functional imaging in the diagnosis of Alzheimer's dementia. Patterns of HMPAO uptake were compared in 10 young (mean age 24.9 years; range 21-34 years) and 10 elderly (mean age 74.1 years; range 70-76 years) normal subjects. Percentage interhemispheric asymmetry ratios were calculated and found to be greater in the elderly, particularly for the temporal cortex (young 0.87%, elderly 3.73%, P < 0.001). The proportion of injected HMPAO taken up by the head was 29% higher in the younger age group. Analysis of regional uptake revealed that this trend towards reduced uptake in the elderly was a global phenomenon affecting all brain regions. The increased interhemispheric asymmetries seen in the elderly imply that a higher threshold for interpreting asymmetries as abnormal must be used in the elderly, particularly for the temporal cortex.