Smell, disgust and Alzheimer’s

As I describe in Tattoo, my first symptom of Alzheimer’s occurred over fifteen years ago when I first noticed a decrease in my ability to smell and then began to have episodes of illusory odors called phantosmias.  The neuropathological changes of Alzheimer’s disease appear first in the locus coeruleus of the brainstem and then in olfactory parts of the brain including the olfactory bulb, anterior olfactory nucleus, and entorhinal cortex. I totally lost my ability to smell over the next few years.  Almost all people with Alzheimer’s have at least some impairment of olfaction, but most are not aware of it unless tested, probably because it comes on so gradually. 

I am often asked what it’s like to not be able to smell. Of course there are downsides. Food all tastes pretty much the same, not bad, but not very interesting. I really miss the smell of bacon frying in the pan. I wouldn’t be able to smell a gas leak, so there are safety issues. But there are a few advantages. I don’t waste money on expensive wine. I can’t smell a skunk. And I’m not grossed out by messy dog poop cleanup or changing a diaper. By the time my sense of smell was completely gone, I started to be aware of something else. I don’t get disgusted anymore. At first, I thought this was just because I could no longer smell disgusting things, but it seems to be more complicated than that.  I find that I have become a more tolerant person.  I will engage a homeless person in conversation, something I’m ashamed to admit I would not have done in the past. I don’t necessarily think I am more empathetic. People with Alzheimer’s usually have trouble with empathy. I think it is just that the protective barrier of disgust is no longer working, and maybe that results in something resembling empathy.

The insula is the part of our brain that registers disgust. It is located deep in the brain at the bottom of the lateral sulcus. It lights up on a functional MRI scan when a research subject sniffs a foul odor.  It even lights up when a subject watches the facial expression of someone else smelling a foul odor. Although amyloid plaques and neurofibrillary tangles eventually appear in the insula, it is usually not affected until later in the disease.

Don’t worry, there will not be a quiz.  I show this schematic just to illustrate how complex the pathways are connecting the olfactory sensory receptors in the nose (yellow) to the insula (red). As shown in the MRI image at the top of this post, my insula still looked pretty good four years ago.  There was no atrophy unlike in the adjacent temporal lobe.  My inability to register disgust probably lies elsewhere in this labyrinth of neuronal pathways, but that’s OK with me. I am actually quite happy to have a life without disgust.