What is Anosmia (Loss of smell)?
Smell is responsible for 95% to 99% of our ability to detect chemicals in our environment, with taste making up the remaining percent. A loss of smell, known as anosmia, can happen for many reasons. It might be temporary or permanent, and could either be something a person is born with or develops over time.
For instance, if something physically blocks smells from reaching the smell nerves in our nose, that can cause a loss of smell. This could be due to something causing swelling, like a simple infection that leads to mucus plugs or nasal polyps. It might also be related to the nerves that send smell signals to the brain, such as issues with the sensory nerves in the smell bulb or along the signal path to the brain.
To better explain, when a particle with a smell is in the air, it travels up through our nose to the nasal cavity, where it comes in contact with cells that can detect smells. These cells send a signal to the smell center of our brain. Each of our nasal cavities contains about 5 million of these smell-detecting cells, and each cell has 500 to 1000 different smell-binding proteins on its surface. Each cell only expresses one type of these proteins. These cells receive the chemical signals (particles in the air) and convert them into electrical signals (signals sensed by the smell nerves). This signal is then sent from the smell bulb to different parts of the brain like the piriform cortex, entorhinal cortex, amygdala, and hippocampus, where it’s processed and we perceive it as a smell. If there’s any blockage or damage along this smell signal path, it could result in anosmia, or a loss of smell.
What Causes Anosmia (Loss of smell)?
Anosmia, or the loss of the sense of smell, can happen for a variety of reasons. Basically, anything that interrupts the path to how we perceive smells can cause anosmia. This could be physical blockages or issues along the nerves that carry smell information to our brain.
In most cases, anosmia is caused by conditions that cause swelling or blockages in the nose and sinuses, like sinusitis, common colds, or nasal polyps. These cause anosmia by swelling the lining of the nose or directly blocking the path air takes.
Head injuries are another common reason for anosmia. If the nose or sinuses get damaged, it can block air flow. Damage can also happen to the nerves that carry smell information to our brain, the part of our brain that processes smell, or the part of our brain that connects smell to other information. Depending on where and how severe the injury is, someone might lose their sense of smell temporarily or permanently. Interestingly, the nerves that carry smell information to our brain are different from other nerves in the body – they can regrow, which is a subject of a lot of research currently.
The processes of aging and certain brain diseases can also lead to anosmia. As we get older, we normally lose some of our ability to smell because we lose cells in the area of the brain that’s responsible for smell and part of the nose where smell is detected. Also, people with certain brain diseases like Alzheimer’s, Parkinson’s, and Lewy Body dementia often have trouble with their sense of smell. Studies have even found that people who struggle to smell are more likely to develop these diseases.
Other rare causes of anosmia are birth conditions like Kallmann syndrome and Turner syndrome, COVID-19 infection, and conditions caused by harmful substances like tobacco, drugs, and fumes that can damage the smell system. Also, rare causes include facial injuries that change the shape of the nose or sinuses, tumors in the nose or brain that interrupt the smell signal pathway, and bleeding in the spaces surrounding the brain. One type of tumor, called olfactory groove meningioma, can slowly worsen your ability to smell.
Medical conditions like diabetes and hypothyroidism can sometimes lead to a decreased sense of smell or anosmia.
Lastly, some medications can cause difficulties in smelling, like beta-blockers, anti-thyroid drugs, dihydropyridine, ACE inhibitors, and nasal zinc. Even though this is a side effect, it’s not a highly common one.
Risk Factors and Frequency for Anosmia (Loss of smell)
In the United States, 3% of adults over the age of 40 suffer from anosmia, which is a loss of smell. The likelihood of experiencing issues with smell increases as people age. According to a 2016 survey by the National Health and Nutrition Examination Survey (NHANES), which involved 1818 participants:
- About 4% of people aged 40-49 have problems with smell.
- This percentage increases to 10% for those aged 50-59.
- It is 13% for those between 60 and 69 years old.
- For those between 70 and 79, it is 25%.
- And it is as high as 39% for people above 80.
- Moreover, anosmia affects 14% to 22% of people over 60 years old.
Signs and Symptoms of Anosmia (Loss of smell)
Anosmia is a condition marked by a loss of the sense of smell. Its causes can vary and understanding the patient’s history can shed light on why this might be happening. Immediate loss of smell can be a sign of a head injury or a viral infection. A progressive loss over time might suggest conditions like allergic rhinitis, nasal polyps, or tumors. If smell loss comes and goes, it could be due to allergies or the use of certain medicines.
Knowing the events leading up to the loss of smell is crucial as two common causes of anosmia, chronic rhinitis and head trauma. The patient’s age plays a role too. A very young patient with additional symptoms may lead doctors to look into conditions present from birth, such as Kallmann syndrome, which would necessitate checks of the reproductive glands and neurological tests. On the other hand, if the patient is older, doctors might explore if the smell loss is just a part of aging or a sign of a nerve disease like Parkinson’s.
The patient’s job matters too, as it gives clues to possible exposure to harmful substances or allergens that could lead to smell loss. Looking into their history of medication use is also key. Sometimes, stopping the use of a suspected medicine may reveal whether or not it is the cause.
It’s important to remember that anosmia is a symptom, not a disease in and of itself. Symptoms like headaches and behavioral changes might suggest issues with the central nervous system. During the physical exam, doctors should carefully inspect the nose and sinus areas. They might also carry out a neurological exam to see if there are other nerve deficits that could cause smell loss. An eye exam checking for signs of increased pressure inside the skull might set the stage for brain imaging tests. A skin test carried out by an allergy specialist could be helpful in determining if rhinitis – inflammation of the mucous membrane inside the nose – is of an allergic or non-allergic nature.
Testing for Anosmia (Loss of smell)
At a basic level, your primary care doctor might perform a simple smell test using common items like coffee or chocolate. This initial test is quite subjective. If the doctor notices something unusual, they might send you to a specialised smell-testing clinic. Here, they’ll conduct more formal, detailed tests like chemosensory testing or a butanol threshold test. These provide a more accurate measure of “smell loss” as they determine the lowest concentration of a certain chemical you can detect, which is compared to what’s normal for your age group.
A common test used is the University of Pennsylvania Small Identification Test (UPSIT), which lasts about ten minutes.
Additional tests may be needed depending on what the doctor suspects could be causing your anosmia or smell loss. If head trauma, sinus disease, or a tumour is suspected, your doctor may order an MRI or CT scan. If they suspect allergies, skin testing may be done. In cases where inflammation is a concern, a sedimentation rate test could be helpful. Other tests could include blood tests, checks for thyroid and liver function, tests for autoimmune disorders, and checks for excessive exposure to heavy metals or other toxins.
However, it’s important to note that for those with unexplained smell loss, often even an MRI doesn’t reveal the cause. In one study of 839 patients with loss of smell, MRI was used more than half the time to try and uncover the cause, but it succeeded in finding an explanation just 0.8% of the time.
Treatment Options for Anosmia (Loss of smell)
Anosmia, or the loss of sense of smell, is not a disease itself but a symptom. Its treatment and management largely depend on what’s causing it.
The most common causes of anosmia are inflammatory and obstructive diseases such as nasal and sinus problems. In many of these cases, nasal sprays that contain corticosteroids can be used to manage the condition. Other medicines like antihistamines and systemic corticosteroids may also be helpful. If the anosmia is due to a bacterial sinus infection, antibiotics like ampicillin can be used.
In certain cases where medical treatments don’t work or the individual has chronic sinus problems or nasal polyps, surgery could be considered as an option.
In general, the approach to treating anosmia involves handling the underlying disease causing it, especially if this disease does not respond to normal medical treatment.
What else can Anosmia (Loss of smell) be?
- Hyposmia – This refers to a decrease in our ability to smell things.
- Parosmia – It’s when our perception changes concerning a particular smell, even after the source of the smell is gone.
- Phantosmia – It is smelling something even when there is no source of that smell; essentially, perceiving odors that aren’t really there.
What to expect with Anosmia (Loss of smell)
If the sense of smell is impaired due to damage to the smell neurons from an injury, there isn’t a specific treatment. However, it’s important to note that these smell neurons can repair themselves. The speed and extent of this self-repair depend on how severe the damage is and varies from person to person.
This regeneration process can take anywhere from a few days to several years, and it’s not guaranteed that a complete recovery will be achieved.
Possible Complications When Diagnosed with Anosmia (Loss of smell)
Anosmia, or the loss of sense of smell, doesn’t usually cause any problems on its own. However, it can pose some difficulties. For instance, not being able to sense harmful smells can pose a significant risk. Additionally, anosmia can also affect one’s ability to savor the flavor of food.
Preventing Anosmia (Loss of smell)
Loss of smell, also known as anosmia, among patients can potentially pose a safety risk. This is because individuals who can’t smell may not be able to detect important warning smells, such as smoke from a fire or the scent of a natural gas leak.
