In reply (1998)

Editors note: The above letter was referred to the authors of the original paper, and their reply follows. In reply: We thank Drs. Noto and Ott for their interest in our results. Indeed, we are in agreement with them that SPECT may have a possible role in the evaluation of patients with suspected AD when combined with other clinical and, perhaps, biological measures. The optimum algorithm for deriving an early clinical diagnosis of AD remains an evolving question to be resolved over the next few years. However, we are clearly not in agreement regarding their specific assertions concerning the utility of SPECT used alone in clinical diagnosis of AD. While there have been claims that SPECT is “a useful diagnostic tool” in evaluating such patients, the fact remains that our study and many others show unacceptably low sensitivity and specificity for SPECT to improve in any way on the clinical evaluation of physicians. Even the critical study by Holman et al., quoted by Drs. Noto and Ott, was re-analyzed by Claus et al.1 and was similarly found to have a very low sensitivity and specificity. Drs. Noto and Ott suggest that the reason for our negative results may lie in our use of a single head gamma camera for SPECT scanning. Indeed this was a conscious choice on our part because the majority of SPECT facilities available to referring geriatricians and neurologists across North America utilize such single head gamma cameras. We consider it an empirical claim to be tested: would the results have been any better with more sensitive dedicated head scanners or multiple head cameras. Nine of the subjects used in our study had concurrent evaluation with O15-H2O PET scanning (probably the gold standard in functional neuroimaging of blood flow) and the baseline blood flow images derived with PET were identical in terms of Holman's visual classification to our HM-PAO SPECT results or the gamma camera. Investigators who have evaluated gamma cameras versus multiple head cameras directly have found no particular difference in the pattern of results in AD according to the Holman classification (J. Claus, personal communication). For these reasons we doubt that the problem lies with the sensitivity of our imaging instrument. Finally, some clarification regarding terminology is necessary. The patients we examined were those who met NINCDS criteria for probable or possible AD. The latter label is not the same as saying they had, as Noto and Ott suggest, “a possible diagnosis of AD.” The label possible AD refers to individuals who meet criteria for Alzheimer's disease but have some atypical factors in their clinical presentations. Of the 11 individuals in our series receiving the label of possible AD, nine were so labeled because they had more marked verbal than nonverbal cognitive impairment, and two others had a notable plateau in their course. The clinical pathological correlation in such individuals remains virtually the same as that in individuals meeting criteria for probable AD by the NINCDS-ADRDA criteria. Furthermore, contrary to most other studies, all of our patients were followed for at least 12 months in order to confirm the diagnosis, making our diagnostic accuracy even stronger than many other dementia studies.