Loss of Smell – Profound Sensory Loss

Loss of Smell is a profound sensory loss that subtly can impact the joy of life, disrupt taste and leave a survivor with ongoing disability.

By Rebecca Martin

A recent article in the New York Times about “Super Taster”, Michele Crippa, and his battle to recover his sense of taste after Covid brought up many questions in my own mind about our sense of smell and ultimately taste. How does it all work and what are the real consequences of losing one’s sense of smell? Is there hope for recovery and how does the recovery process work? While loss of smell from Covid has a different pathological process than a traumatically caused loss of smell, the functional loss is comparable. For the symptoms of carbon monoxide poisoning, including neurological deficits, click here.

I remember a family member who, after surviving a severe brain injury, announced at every meal his disappointment in not being able to taste his food. I At the time, it seemed such an insignificant complaint in light of his more significant injuries. Yet, years later I was able to experience the impact of “loss of taste” firsthand during chemotherapy, which enlightened me on this significant sensory loss.

Loss of Smell a Huge Sensory Loss

Chemotherapy, though aimed at killing cancer cells, can also kill healthy cells such as taste buds. Taste buds are extremely susceptible to the effects of chemo. As a cancer patient I educated myself on what to eat during this period of time, but nothing prepared me for the extreme change of taste which occurred. I can only describe it from my own experience, which turned every meal into something best described as rotten cardboard which had gone rancid.

Loss of Smell Impacts Taste

“Loss of taste” turned out to be much more complex than losing one’s taste buds. Chemotherapy and radiation can commonly cause changes in how we perceive taste because taste buds grow very quickly after treatment is over and this rapid growth confuses the brain and its interpretation of what signals it is receiving.

loss of smell and taste

Loss of Smell is a profound sensory loss that subtly can impact the joy of life, disrupt taste and leave a survivor with ongoing disability.

To this day, there are many things I vividly remember the taste of from a lifetime of experience, but the memories do not match my experience of them anymore. The comfort of old favorites has been replaced by new staples which often feature more pronounced flavors. During chemotherapy such a loss of taste was very emotional and resulted in a desperate search for familiar taste experiences. Over time, my brain has adjusted and the memories of what things should taste like and what I experience have reconciled as those memories fade. But I am still disappointed when something so simple as that first bite of pizza fails to match the memory I have stored of what it should be like.

What about those experiencing long term loss of smell and taste due to brain injury or to Covid? Loss of smell is the number one complaint of Covid survivors. With loss of smell, there is also a disruption in taste. We talked about memories in the previous blog and how many memories use many different parts of the brain for storage and how memories are retrieved from storage the same way, by reconstructing the bits and pieces to make a coherent memory. Smell and taste are part of that method of accumulating memories and also vital in retrieving memories. I talked previously about how all animals with brain cells and neurons have some form of memory. Even the most basic animals have the ability to retain a memory of how a certain condition is favorable to finding food. One of the basic building blocks of life is the ability to taste things for our survival.

According to Fifth Sense fifthsense.org.uk an organization started in 2012 by Duncan Boak, who had lost his sense of smell due to a brain injury:

 “To lose the sense of smell is to lose a rich, powerful and emotional way of experiencing the world, something that is very difficult to understand without personal experience of it.  The huge role that the sense plays in our lives often only becomes apparent to people, unfortunately, when they lose it.

And according to Yale Medicine yalemedicine.org:

“Two of the great joys in people’s lives are the sensations of smell and taste,” says R. Peter Manes, MD, an ear, nose, and throat specialist at Yale Medicine. “When these senses are altered or absent, people lose that pleasure and can feel isolated from those around them who are not afflicted.”

The Mechanisms of Smell

Smells are carried by scent molecules in two basic ways: Either through the air we breathe in through our nose or through the back of our nose from our mouth.

Scent molecules entering through the front of our nose are drawn to the olfactory cleft at the top of the nose and trapped in a layer of mucus membrane called the olfactory epithelium. The air entering our nose is also warmed, humidified and filtered by the turbinates which are bony cushions inside the nose. Once the scent molecules are dissolved in the mucus they are carried by special proteins to the cilia. Cilia are hair-like structures which are attached to the olfactory receptor cells. Humans have around 12 million receptor cells capable of detecting 10,000 scents.

Once the scent molecule has an attached to a receptor cell, the cell sends a message to the brain. The signal is then passed by nerve fibers called axons through perforations in the cribriform plate which is a layer of bone at the use of the skull. Thousands of these axons make up the olfactory nerve.

This highway of axons that make up the olfactory nerve converge into the olfactory bulb located on the frontal lobe of the brain. Each nostril leads to its own olfactory bulb on each side of the frontal lobe. This information is then passed on to other parts of the brain for further processing.

Head Injury Can Cause Loss of Smell

Loss of smell due to a brain injury is a condition known as Anosmia from head injury. This can occur as a result of mild, moderate or severe brain injury. This can occur as a result of skull or facial fractures, concussions, diffuse axonal injury or shearing injuries caused by the shifting of different layers of the brain. These types of occurrences are more likely to impact the brain’s ability to receive the olfactory information due to damage to the axons themselves. In essence information would not be transmitted to the olfactory bulbs for dissemination to other areas of the brain.

Loss of smell due to head trauma is very common, occurring in up to 30% of all cases according to an article from PMC, US National Library of Medicine, National Institute of Health. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2767158/ Olfactory loss usually occurs within a two year period of time following a head injury but is more likely to occur in a shorter period of time following a traumatic event. Approximately 10% of those cases resolve fairly quickly though olfactory loss can persist for years or indefinitely in some cases. In a second article from NIH, entitled Head Trauma and olfactory function, it states:

 “Deficits in smell can be conductive or neurosensory, contingent on location of injury. The former may be amenable to medical or surgical treatment, whereas the majority of patients with neurosensory deficits will not recover. Many patients will not seek treatment for such deficits until days, weeks, or even months after the traumatic event due to focus on more pressing injuries.”

And further:

“Olfactory loss is often discounted as an annoyance, rather than a major health concern by both patients and many healthcare providers. Patients with olfactory impairment have diminished quality of life, decreased satisfaction with life, and increased risk for personal injury.”

An increased risk of physical injury is an inherent danger of loss of smell. People who have suffered a loss of smell are less likely to be able to detect whether food is safe to eat or too rancid for consumption. They are unable to detect toxins in the air such as gas leaks or toxic chemicals. And they are unable to identify dangerous liquids. Think about the numerous ways your nose warns you of dangers ahead; the fumes in a car which warn of imminent breakdown, the smell that warns you if your fridge is set at the right temperature, or the smell of something burning coming from your kitchen. We sniff the carton of milk to check its freshness. We have all had that experience of noting that something smells or tastes “off” and decided not to risk the consequences.

There are also health risks involved. I shared my experience with taste and smell disturbances from chemotherapy and radiation and confessed that those issues have been ongoing. I eat differently than I did before treatment. My food is saltier, spicier, and more sugary than previously. This is a common occurrence with people who have lost or experienced a warping in their smell or taste experiences. These changes in diet to counteract deficiencies in our enjoyment of food can lead to other health issues and weight problems. One’s weight can increase due to a search for that prior experience of food or to weight problems from a disinterest in eating altogether. This disinterest in food is common in those who have experienced olfactory disturbances or loss due to dementia or Alzheimer’s disease, other common causes of olfactory damage.

In the case of Covid-19, it is believed that the damage occurs in the support cells around the receptors. The mechanism for damage to the olfactory senses is unique to SARS-CoV-2 viruses which differ from other viruses in that they do not cause congestion. And unlike a straight attack on the receptors related directly to taste, Covid seems to attack other cells related to other sensations like our pain response to a super-hot chili or the refreshing bite of mint; smells which bring a physical response with them on a chemical level.

Loss of smell is most often the first symptom of a Covid infection, which is unlike other viruses where congestion comes into play later on and can cause a temporary loss of smell. Many report Parosmia, in which certain foods become unpleasant. And early studies are showing that Parosmia can last well beyond the virus and even after smell and taste have returned. This is probably similar to the aftereffects of chemotherapy in that the brain is overwhelmed by the processing of olfactory information which is returning and the process of cataloging these sensations becomes chaotic as a result.

There may be other mechanisms involved in the loss of olfactory senses due to Covid. A research team in Italy found that inflammation may also have a role in the loss of smell. They discovered inflammation and leaky blood vessels in the olfactory bulbs themselves in postmortem studies.

But let us take a closer look at the support cells which are thought to be affected by Covid. These support cells, called sustentacular cells, help maintain the balance of salt ions in the nasal mucus. They also provide structure and support for the neurons. According to GAVI, the Vaccine Alliance:

 “A recent study in hamsters suggested that it was these support cells that SARS-CoV-2 invaded, rather than the olfactory neurons, and that this prompted a massive infiltration of immune cells, followed by a disruption to the normal organization of the nasal lining, including the loss of the hair-like projections that the neurons use to detect odor molecules.”

Although researchers are not positive about the mechanisms which cause a loss of smell, and thus taste, in even those with mild cases of Covid, there is promise in potential treatments to relearn these skills for Covid survivors. Survivors are more likely to complain specifically over a loss of smell than a loss of taste, so most of the statistics are based solely on reported loss of olfactory senses. Statistically recovery is likely by six months.

One recent study, which tracked the health of 2,428 individuals who claimed to have lost their sense of smell and/or taste as a result of COVID-19, found that 40% of them had completely regained their sense of smell six months later, while only 2% reported no improvement at all. A separate survey suggested that recovery may be faster than this for many individuals, with 71.8% reporting a return to “very good” or “good” smell after one month, and 84.2% reporting a return to “very good” or “good” taste.”

Smell Training

Smell training has also proven to be beneficial for recovery for Covid survivors. It can be as simple as at home treatment utilizing scents in different basic categories or as extensive as smell testing and ongoing therapy. Smell testing is often used for those who have had damage to the olfactory senses by other means such as head injury. The tests involve a broad range of scents and pinpointing the exact areas of damage. Some test the ability to identify different scents while others test the level at which an odor is detected. However, like any therapy or treatment which involves the brain, there is no “one size fits all” cure for olfactory disturbances. It depends on the area of damage.

That brings us back to the original New York Times article which covers the struggle of a celebrated Italian epicure who enlisted the help of sensorial analysis experts to regain his former level of olfactory proficiency. His group contends that most smell appreciation is contained in the hypothalamus which is crucial in controlling emotions. At this point in time there is no definitive research suggesting that scent training can rebuild the neural pathways but there is some support to the belief that regular scent training can speed up natural recovery. There have been studies that show that those who have higher olfactory skills often utilize less brain effort to identify scents due to their proficiency and familiarity with scents and their ability to memorize scents or “visualize” scents. However, we can only applaud efforts to bring some resolution to the countless Covid survivors around the world.

While scientifically we are in the earliest stages of treatment options for olfactory disturbances, we do know that a loss of smell and taste can be devastating to the individual. It reduces their appreciation of food and drink, an important part of our sensory experience. It is likely to make people feel more isolated as they are unable to enjoy social activities as others do. Almost half of those with olfactory loss experience depression and over half say it affects their relationships. The majority are fearful that their personal safety is at risk as a result of not being able to detect dangers in food or their surroundings.

Just imagine a world in which we cannot  enjoy the scent of a partner, the smells in a garden, or the scent of cookies coming out of the oven.  Our world is an incredible source of stimuli for a brain built to receive it. So it is ultimately a terribly serious condition that goes beyond the words “loss of smell”. We often store and retrieve memories with olfactory cues. Scents tie us to people, places and experiences. They are every bit as important as our other senses and should be treated that way by medical professionals. Unfortunately, it is not until we lose our olfactory senses that we truly understand how important they are.

 

 

 

 

 

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