For a stretch I helped research Alzheimer's effects the olfactory system -- one result of the cascade of Alzheimer's described in the article is that olfactory sensory neurons start mistargeting their downstream glomeruli [1] [2]. Alzheimer's patients correspondingly perform poorly on odor identification tests, though the confounding factor of dementia makes it a poor diagnostic tool.

More generally, while this article mostly discusses treatment, the physical symptoms of plaques, tangles, and neural degeneration occur before a patient starts displaying dementia. Assuming an Alzheimer's treatment can't reverse the damage done, it would have to be coupled with an early diagnostic, something which doesn't exist yet.

1. http://en.wikipedia.org/wiki/Olfactory_receptor_neuron#media...

2. http://www.nature.com/ncomms/journal/v3/n8/fig_tab/ncomms201...