Ask the MD: Sense of Smell Is a Window into Brain Health
Tuesday, 14th February 2023

Smell loss became a popular topic of conversation during the COVID pandemic. Many now are personally aware of the importance of sense of smell and the significant role it plays in how we move through life. And researchers and doctors now understand more about how smell works, why it might decrease, and how to develop better tools to define and measure smell loss.

Many people with Parkinson’s disease (PD) have long been familiar with this somewhat lesser-known and lesser-talked-about potential symptom. Smell loss can happen at any time in PD, from years before diagnosis through decades into life with the disease. (Join a research study looking at smell loss and brain disease.)

To learn more about smell loss, brain health and PD, we spoke with Alfred Marc Iloreta, MD, assistant professor of otolaryngology [an ear, nose and throat (ENT) doctor] at Mount Sinai Hospital in New York, New York.

The Michael J. Fox Foundation (MJFF): Let’s skip to the punch. What do you want people to know about sense of smell?

Alfred Marc Iloreta, MD (AMI): Most of us don’t think about our sense of smell until it’s gone. And if we do notice a change, we still might not think too much about it or see a doctor. If you lose your sense of smell, take it seriously. This can be an early indicator of allergies or, in the time we’re living in, COVID or another viral disease. It could even be a sign of brain disease, like Parkinson’s or Alzheimer’s. It’s important to seek the advice of a medical professional, have your sense of smell evaluated, and get the proper diagnosis and treatment.

MJFF: Ok, now let’s take a big step back. How, exactly, do we smell things?

AMI: As you inhale, air comes into your nose through your nostrils. Air contains millions of odor molecules, which attach to receptors on cells in the lining of the nose. These cells send signals to your brain, which processes them so that you recognize a specific scent or smell.

It’s a very complex process. And it’s different from person to person. Some people — those who are pregnant or expert wine tasters (sommeliers) — have a heightened sense of smell. (Read about the “super-smeller” who can detect a Parkinson’s scent.) And it’s all relative. What smells like pizza to me may or may not smell like pizza (or smell delicious) to you.

MJFF: Why is sense of smell important?

AMI: Smell is one of our most critical senses. First, it’s protective. It’s an early warning system that danger — from smoke, chemicals or even rotten food — is nearby. Second, it helps us enjoy life. And it’s closely linked with memory. When you smell something, it can elicit an emotional response. For example, when I smell freshly cut grass, I think about playing soccer as a kid. People who lose their sense of smell can feel a bit disconnected; studies show they often experience anxiety or depression. Lastly, sense of smell helps us taste food and stimulates appetite. Eating — which is an important way we can connect with others (good for brain health!) — may become less enjoyable. Some people even lose weight.

MJFF: What impacts the ability to smell?

AMI: The most common reason for smell loss is what we call inflammatory sinus disease. This is a broad category that includes allergies (to dust mites, pollen or pets, for example), chronic sinusitis, and other conditions that block air and smell pathways in the nose. The second most common is viral infection, like COVID or another respiratory illness, which temporarily impacts sense of smell. Other, less common reasons may include head trauma, medical conditions, or tumors that damage or otherwise impact smell pathways. Further down on the list of potential causes is neurodegenerative brain disease, like Parkinson’s or Alzheimer’s.

MJFF: Speaking of Parkinson’s, how might the disease affect sense of smell?

AMI: In some people, smell loss can be one of the earliest indicators of brain disease, like Parkinson’s. I’ve had a few patients with smell loss as their first sign of PD. When evaluating anyone with smell loss, I first look for allergies, sinus disease and other common causes. If none of these is present, I might refer them to a neurologist for additional evaluation, especially if they have movement changes or other Parkinson’s symptoms.

The more common scenario is that a person gets a diagnosis of Parkinson’s and then realizes they previously lost sense of smell or connects that symptom to the disease. Smell loss can happen at any time in Parkinson’s, too — from long before diagnosis to years into living with disease.

The smell loss in Parkinson’s happens in the brain, not in the nose, as it does with allergies or some viral infections. In Parkinson’s, there are changes in how the brain receives or interprets smell signals.

MJFF: How do you know if you’ve lost your sense of smell?

AMI: Smell is one of those things that you really don’t notice until it’s gone. During the height of COVID, when I was worried about losing my own sense of smell, I put my face in my fridge every morning to check whether and what I could smell. To quickly check your sense of smell, you can “stick your nose” close to something potent. That could be your child’s diaper, a bottle of perfume, or some other strong odor you know is there.

There are also different levels of smell loss. Anosmia is a complete loss of smell. Hyposmia is a decreased sense of smell. Parosmia is a distorted sense of smell in which normal or enjoyable smells become unpleasant.

MJFF: How do you measure smell loss?

AMI: Typically, we use a “scratch and sniff” method to check whether you can detect a certain scent. This type of test can help determine if you have smell loss and to what extent. It can’t, however, determine the cause. It can’t, for example, say, “if you can’t smell peppermint, this is what’s happening in your smell pathways or brain.”

Because of this, smell loss can be a hard symptom to carefully measure and research.

MJFF: Who should have their sense of smell tested?

AMI: Anyone who has any sort of smell loss. See your primary doctor or an ENT who can help test and treat you.

People who are getting older also may want to test their sense of smell to be proactive about brain health. We’re at an exciting turning point in terms of potential therapies for brain diseases, like Parkinson’s and Alzheimer’s. And we’re learning that early intervention — when brain symptoms (like movement or thinking changes) are mild or even before they come on — might lead to better outcomes. If you are at risk for brain disease, even if only because you’re getting older, you might want to consider testing your sense of smell.

Editor’s Note: MJFF’s Parkinson’s Progression Markers Initiative Study (PPMI) is asking people over age 60 without Parkinson’s to take a free smell test. Learn more.

MJFF: Can you treat smell loss?

AMI: Safety is the most important part of treatment. Install fire, smoke and natural gas alarms. (Or don’t use natural gas in the home.) Be diligent about monitoring “best by” dates on food. And let family and others know you’ve lost sense of smell so they can help.

There aren’t yet many proven treatments for smell loss. But there are some options that may help and don’t have many downsides. One of these is “smell retraining.” It’s like physical therapy but for your sense of smell. A doctor teaches you how to exercise your sense of smell, perhaps by sniffing essential oils and meditating on memories and emotional connections that arise. I also often recommend fish oil supplements as these may help decrease inflammation and build new brain connections. It’s important to stress that supplements aren’t regulated by the FDA, so ask your doctor which are the highest quality. I suggest supplements that are tested and certified by an independent lab, which confirms the supplement truly is what the label says it is.

You also can adapt your diet to include foods of different colors, textures, spices and more to enhance the experience of eating. (Read more in MJFF’s guide on diet and brain health.)

MJFF: Can you prevent smell loss?

AMI: Some people, such as those with Parkinson’s or Alzheimer’s, can’t prevent smell loss that might happen as part of that disease. But everyone can take steps to lessen risk and impact. You can, for example, try to avoid environmental exposures, like cigarette smoke, chemicals, and anything to which you’re allergic. Some people may also benefit from a regular nasal irrigation or saline spray, to keep the nose clean and healthy. I also suggest avoiding antibiotics, where possible, as these disrupt the natural bacteria that live in our nose and gut to keep our bodies and brains working properly. (Still, antibiotics are useful tools in fighting bacterial infections and can’t always be avoided!)

MJFF: Anything else?

AMI: Researchers are working to better understand smell loss, how to treat it, and how it’s connected to brain disease. This work can lead to treatments that could, one day, restore sense of smell and even stop brain disease from progressing or even happening in the first place.

Source: The Michael J. Fox Foundation for Parkinson’s Research