SPECT evaluation of regional cerebral blood flow is commonly used to evaluate patients who are thought to have one of the dementias. The most common of these is Alzheimer's disease (AD), which is characterized by posterior superior parietal areas of regional cerebral blood flow reduction, which are often symmetrical and comprise the most reliable SPECT diagnostic sign of AD. There are other causes of such flow abnormalities, which can produce errors in diagnosis. To the list of possible causes of posterior flow abnormalities must be added the affective disorders, including unipolar and bipolar depression, which may cause posterior inferior regional cerebral blood flow deficits. Two cases, occurring in young persons, illustrate this finding.
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