Loss of smell in Covid-19 and Alzheimer’s

Two days ago, I read a fascinating article in the New York Times by restaurant critic Tejal Rao about her sudden loss of smell due to Covid-19 last December. “I noted that moment as it happened to me, stepping into the shower at my home in Los Angeles. At first, I mistook the lack of aromas for a new smell, a curious smell I couldn’t identify — was it the water itself? the stone tiles? — before realizing it was just a blank, a cushion of space between me and my world.” She describes her partial recovery of olfaction to date, a recovery she attributes at least in part to smell training.

Infection of the nose and sinuses is the most common cause of decreased ability to smell. Most of the time the anosmia (loss of smell) or hyposmia (decreased smell) is temporary and is mostly due to obstruction of the nasal passages, a stuffy nose.  The odors just can’t reach the olfactory receptors located high up in the sinuses. Some viruses and even some bacteria have an affinity for the olfactory receptors in the sinus and even the olfactory nerve that carries smell information to the brain. The impairment of smell due to these infections can be long lasting and sometimes permanent. Almost everyone who tests positive for the Covid-19 virus has olfactory impairment when tested, although some are not aware of it. It is only occasionally associated with a stuffy nose, is usually sudden in onset, and frequently is the very first symptom of Covid. I recommend a very recent review of the medical papers on this topic by Christopher Hawkes (follow the link and then click on PDF for a public access version of the entire paper).  

In contrast to Covid-19 induced anosmia, the loss of smell seen in Alzheimer’s disease is insidious in onset and slowly progressive. It does not go away. As in my case, it can begin ten years or more before the onset of cognitive impairment.  Many if not most people with Alzheimer’s disease don’t even notice the loss because it is so slowly progressive.  I don’t think I would have paid any attention to it had it not been for the illusory odor I experienced.  This was like the scent of baking bread mixed with perfume.  It initially would come on three or four times a week and last for at least a few minutes, sometimes as long as a half hour. There was never any olfactory stimulus, no external aroma. These phantosmias as they are called would just come out of the blue. My sense of smell gradually disappeared entirely over the next four or five years.  The phantosmias continued but became less frequent, finally disappearing altogether a few years ago.

Why does the loss of smell often improve after recovery from Covid-19 whereas the olfactory problems in Alzheimer’s only get worse with time? We are still learning more about Covid-19 almost every day, but the answer probably involves the location and type of injury. In Covid, the virus attacks olfactory receptors in the sinuses, the olfactory nerve, and probably the olfactory bulb, the first center in the brain for processing olfactory signals. The Covid-19 virus appears to cause inflammation and swelling in these areas, and this inflammation most likely resolves as the infection runs its course.  By contrast, smell loss in Alzheimer’s disease is due to damage of not only the olfactory bulb but also other olfactory centers higher in the brain such as the piriform cortex and orbitofrontal cortex. Amyloid plaques and tau-containing neurofibrillary tangles can be found in these sites before there has been any loss of memory or other cognitive impairments.  The damage progresses in these centers over the years and is probably irreversible.

For me the very gradual loss of my ability to smell due to Alzheimer’s has been relatively easy to adapt to. I like a lot of spice in my food because otherwise there isn’t much taste, and I don’t have to buy expensive wine, they all taste the same to me now. I don’t mind picking up dog poop or other smelly jobs. On the other hand, I sometimes really miss the aroma of bacon frying in the pan or an apple pie just out of the oven.