How do we smell?
Tiny odor molecules — of cookies baking in the oven, laundry that needs to be cleaned, or rosemary growing in the garden — travel in the air through the nose to the smell, or olfactory, nerve. These molecules bind to receptors on the olfactory nerve, which transmits information to the brain about what you’re smelling.
What causes smell loss?
Smell loss happens when any part of the pathway that enables smell (nasal passages, olfactory nerve and brain) is affected. Problems with smell can range from decreased (hyposmia) to complete loss (anosmia). Because the ability to taste is linked to smell, changes in taste (dysgeusia) often accompany changes in smell. In some people, taste loss can lead to decreased appetite and weight.
There are many possible causes of smell loss, including:
- Upper respiratory infection, such as the common cold
- Nasal problems, such as seasonal allergies or chronic sinus disease
- Head injury, if it damages the olfactory nerve or brain’s smell-processing centers
- Cigarette smoking
- Parkinson’s or other neurological diseases, such as Alzheimer’s
Does coronavirus cause smell loss?
Any type of viral infection (a cold is a good example) or condition that causes nasal congestion or inflammation (such as allergies) can cause a “stuffy nose” that affects smell. So if coronavirus disease (COVID-19) causes congestion, smell and taste changes would be expected. But doctors now are reporting that some people with COVID-19 develop the common fever, cough and shortness of breath after first experiencing only smell and taste loss for several days. Recognizing these various symptoms is important for diagnosis, treatment and prevention of disease spread as well as for gaining understanding of how the virus affects the body.
How is smell loss connected to Parkinson’s?
A significant number of people with Parkinson’s lose their sense of smell. (Some studies cite up to 80 percent.) For many, smell loss starts years, or even decades, before movement symptoms such as tremor, slowness and stiffness, which lead to diagnosis.
While scientists are not yet certain why smell loss happens in Parkinson’s, one theory is that the disease process begins in the olfactory nerve (as well as the gut). Some researchers believe that clumps of the protein alpha-synuclein, a hallmark of Parkinson’s disease, may first form in the smell nerve (and gut) and then move to the brain.
If you lose your sense of smell, will you get Parkinson’s?
Not necessarily. But smell loss for no known reason (not caused by chronic sinus disease, smoking or head injury, for example) is linked to a higher risk of Parkinson’s. Researchers are studying people with smell loss to understand who develops Parkinson’s and over what time period. This could lead to earlier diagnosis and even prediction of the disease. In The Michael J. Fox Foundation's landmark study, the Parkinson's Progression Markers Initiative (PPMI), researchers are following people with smell loss, some of whom have developed Parkinson’s. By looking at the brain scans and blood tests those volunteers contributed before their diagnosis, scientists can understand what is happening in the earliest stages of disease. That information could lead to diagnostic tests and potentially even treatments to slow or stop progression, perhaps before movement symptoms arise.
Some people wonder, “if I experience smell loss as a symptom of COVID-19, am I more likely to get Parkinson’s later on?” As information on COVID-19 is growing and evolving daily, we don’t yet have the data to directly answer this question. But researchers are actively studying why smell loss happens and gathering data on how this and other symptoms develop in both COVID-19 and Parkinson’s to understand disease pathways and progression. At this time, it is not clear how COVID-19 causes smell loss — if congestion blocks nasal passages or the virus directly affects the olfactory nerve, for example. It also is not known if the virus impacts other parts of the brain, which could lead to Parkinson’s or other neurological symptoms. (Read more on Parkinson’s and COVID-19.)
How is smell loss evaluated?
If you experience smell loss, you may want to start by talking with your primary care doctor, especially if you have other symptoms such as movement or memory changes. They can help direct treatment and, if necessary, referral to a specialist for further consultation. This may include an ear, nose and throat (ENT) doctor, also called otolaryngologist, who has expertise in evaluating and managing smell loss. These doctors typically perform brain or nasal imaging scans as well as smell tests to look for the cause of smell loss and target therapy.