Olfactory Deficit in Alzheimer's Disease?

To the Editor: The report by Dr. Devanand and colleagues confirmed prior work (1, 2) but confused some terms and concepts. The authors cited evidence (2) that “impaired olfactory detection has been observed in first-degree relatives of patients with Alzheimer’s disease” (p. 1399). In fact, the study to which they referred demonstrated significantly reduced olfactory identification scores, not odor detection scores, in these relatives. A prior study (1) noted impaired odor identification at the earliest phase of Alzheimer’s disease but intact detection until at least moderate disease progression. This means that a patient with early Alzheimer’s disease or someone who has preclinical Alzheimer’s disease (is at risk) may have an impaired ability to choose the correct name of an odorant but can smell the presence of that odor as well as any age-matched comparison subject. Because these patients can detect odors normally, they probably report no problem smelling. Thus, their negative response to the question about problems smelling does not relate to their reduced ability to name odorants; this cannot, then, be considered anosognosia.

The authors also supported their findings by noting a prior report of the lack of awareness of loss of smell in 74% of patients with Alzheimer’s disease compared to 8% of subjects with sinusitis (3). The latter ailment affects peripheral portions of the olfactory system, leading to diminished detection, which is likely to be perceived as loss of smell. Alzheimer’s disease patients, however, have damaged medial temporal lobes, leading to poor identification but no change in detection. In summary, the study by Dr. Devanand et al. mixed the “aromas” of apples and oranges.