Alzheimer’s disease and olfaction

In Alzheimer’s disease and olfaction can be impaired. Indeed this is so characteristic that Solomon et al (1998) suggested that the Pocket Smell Test could be used as a diagnostic tool to distinguish between the condition and major depression.

Doty reviewed 13 studies investigating olfaction in Alzheimer's patients compared to age-matched controls and identified that the  majority of studies found significantly decreased function in Alzheimer's patients.

Nordin and Murphy also documented impaired olfaction in patients with possible Alzheimer's.

Morgan et al. reported that odor identification tasks could be useful in diagnosing Alzheimer's and that possible language deficits in this group were not responsible for the difficulties in smell identification.

Anosmia - lack of ability to smell
For some people wih Alzeimers the sense of smell can disappear completely. Solomon GS: Anosmia in Alzheimer's disease. Percept Mot Skills 1994; 79:1249—1250,/ref>

Neuroscience
A postmortem study by Reyes et al (1993) has demonstrated increased numbers of neurofibrillary tangles and neuritic plaques in the olfactory cortex of Alzheimer's patients versus age-matched control subjects. Doty has suggested that the magnitude of the olfactory deficit parallels the progression and severity of the Alzheimer's process.