Olfactory Deficit in Alzheimer's Disease?

To the Editor: I read with great interest the article by Dr. Devanand et al. Although this is a fascinating and important study that is in general accordance with another recent prospective study (1), there are several errors that should be pointed out.

First, the authors stated that deficits in olfactory identification are not consistently seen in patients with Huntington’s disease. In fact, olfactory loss is common in patients with Huntington’s disease once the clinical signs of the disorder are manifest. For example, in one study (2), 25 of 25 Huntington’s disease patients exhibited Pennsylvania Smell Identification Test scores below those of matched comparison subjects and at-risk asymptomatic relatives. This is nearly equivalent to the finding reported by my colleagues and me in 1987 (3) that 23 of 25 patients with early-stage Alzheimer’s disease who were capable of psychophysical testing scored below matched normal comparison subjects on the Pennsylvania Smell Identification Test. In accordance with the general thesis of Dr. Devanand et al., we found in this early study that only two of 34 patients with Alzheimer’s disease were aware of their deficit.

Second, the authors stated that the findings relative to olfactory losses for Parkinson’s disease are “equivocal.” In fact, the prevalence and magnitude of olfactory losses of patients with Parkinson’s disease are indistinguishable from those seen in early-stage Alzheimer’s disease, both in terms of scores on the Pennsylvania Smell Identification Test and threshold values (4, 5).

Dr. Devanand et al., as well as others, have asked whether many older patients with olfactory losses and marginal cognitive impairment already have Alzheimer’s disease that has not progressed clinically to the point at which it can meet the criteria of the National Institute of Neurological and Communicative Disorders and Stroke and the Alzheimer’s Disease and Related Disorders Association. If so, then a redefinition of the operational criteria for establishing the presence of Alzheimer’s disease at its earliest stages may be in the offing, perhaps incorporating criteria determined from both olfactory tests and magnetic resonance imaging (MRI) (6).