What Is Anosmia?

Overview

Anosmia is the complete loss of sense of smell. It may be temporary, such as during a cold or allergies, or permanent, due to a more serious condition such as a tumour or head injury.

Hyposmia is the partial loss of smell. In rare cases, people are born with anosmia. Congenital anosmia affects about 1 in 10,000 people.

How do we smell? In humans, different odour molecules, such as lavender or vanilla, are detected by different types of smell receptors in the nose. These then create a chemical signal which is converted to electrical impulses by the olfactory nerves (neurons) in the nose, which then send a message to the brain about the type of smell detected.¹ There are more smells than there are smell receptors, so an odour molecule must stimulate a specific combination of smell receptors, which are then perceived as a unique smell by the brain. Any disruption in this pathway can therefore lead to a partial or complete loss of smell.

If you have lost your sense of smell, you won’t be able to taste your food. This can lead to weight loss and malnutrition. You also won’t be able to taste if food has spoiled or smell environmental dangers such as smoke, gas leaks, or dangerous chemicals.

While short-term anosmia is common in people of all ages, long-term anosmia is more common in people over the age of 60. Studies show that anomia affects 3.2% of US adults over the age of 40. This increases to nearly 40% for those over the age of 80.^2,3

While anosmia itself isn’t dangerous, it can have a significant impact on someone’s quality of life, and in extreme cases, it can even lead to depression.

Causes of anosmia

As mentioned earlier, any disruption to the pathway responsible for detecting smells (the pathway of olfactory conduction), whether structural or neurological, can lead to anosmia. It can be caused by issues with the nasal lining, nasal obstructions, or issues in the brain or nerves.

Irritation or inflammation in the nasal lining which leads to swelling and increased mucus in the nasal cavities can disrupt the olfactory pathway. It can be caused by the following:

• Viral infections such as the common cold, influenza, or COVID-19. Temporary anosmia can be an early symptom of COVID-19 infection, but in rare cases, long COVID can also lead to a long-term loss of smell and taste
• Acute or chronic sinusitis 
• Allergies, such as hay fever
• Chronic congestion (nonallergic rhinitis)
• Smoking or taking drugs

An obstruction that prevents odours from reaching the olfactory sensory neurons can be caused by:

• Nasal polyps
• A deviated septum
• Tumours
• Damage to the nose or sinuses due to surgery or injury

Brain conditions, brain injuries, or nerve damage can also lead to anosmia, as they can lead to damage to the olfactory sensory neurons, the olfactory bulb or the olfactory areas of the cerebral cortex which are responsible for smell. Causes include:

• Brain tumours
• Brain aneurysms
• Head trauma

Many other conditions have been linked to anosmia. These include:

• Congenital conditions such as Kallmann syndrome. Studies show that gene mutations associated with Kallmann syndrome affect olfactory neurons in the brain
• Age and neurodegenerative conditions. The sense of smell declines with age, as a person ages the number of cells in the olfactory bulb and the olfactory epithelium decrease, therefore decreasing the ability to smell. Alzheimer's disease, Parkinson’s disease, and Lewy body dementia have also been associated with anosmia⁴
• Diabetes
• Certain medications such as antibiotics and antihistamines

Signs and symptoms of anosmia

Anosmia can lead to a gradual or sudden loss of smell. Sometimes people notice a change in their sense of smell before it is lost completely. This may also lead to a loss of taste, which can lead to decreased appetite and decreased enjoyment of food. This can lead to weight loss, malnutrition, and depression in severe cases.

Other types of olfactory dysfunction include hyposmia (partial smell), parosmia (a change in how things smell), and phantosmia (perceiving smells that are not there).

Management and treatment for anosmia

Treatment for anosmia depends on the underlying cause. If it is caused by a runny or stuffy nose due to allergies, a cold, or a sinus infection, it will usually clear up on its own. If not, it is best to contact your doctor. Steroid tablets or nasal sprays/drops can be used to decrease the swelling of the nasal lining for people suffering from sinusitis or nasal polyps. Nasal polyps or a deviated septum may require surgery to alleviate the problem. However, sometimes the anosmia may be permanent.

Smell training has been shown to help with the recovery of smell. The ENT UK recommends this for anyone suffering symptoms for more than two weeks. The olfactory training repeatedly stimulates the olfactory nerves. You can use essential oils to help you do this.

Diagnosis

Diagnosis of anosmia will usually involve a doctor taking a medical history and a physical examination. They will look for any obvious causes such as sinusitis or nasal polyps. Depending on that, other diagnostic tests may need to be carried out. These could include a CT scan to rule out any serious condition or head trauma. You may also need a referral to an ear, nose and throat (ENT) specialist, who may examine your nose and carry out tests. 

Complications

When we taste food it is mainly due to retronasal olfactory perception, meaning that during eating, odours from food travel through the back of the throat into the nose and are then detected. Not being able to taste or smell food can therefore lead to a lack of appetite, weight loss, malnutrition, and even depression.⁵

Anosmia also puts people at greater risk as they won’t be able to detect environmental dangers such as a fire, gas leaks, or when food has spoiled. People suffering from anosmia should therefore take extra safety precautions. 

FAQs

How can I prevent anosmia?

Anosmia can’t always be prevented as it can be caused by many different conditions. You can, however, try to reduce your risk by not smoking or taking drugs and by taking extra precautions when playing dangerous sports or partaking in risky activities to avoid a serious head injury.

How common is anosmia?

Anosmia is quite common as it can be due to many different conditions. Temporary anosmia is common during the winter due to viral infections or in the spring and summer due to allergies. Nasal polyps, head injuries, and old age can also be causes of anosmia. Long-term anosmia is more common in older patients. According to studies, 3.2% of people over the age of 40 have been shown to suffer from anosmia.³

Who is at risk of anosmia?

Temporary anosmia affects people of all ages, but the risk of anosmia increases with age. Studies have shown that 14%-22% of patients over the age of 60 suffer from anosmia, increasing to nearly 40% after the age of 80.³

What can I expect if I have anosmia?

Someone suffering from anosmia will completely lose their sense of smell. Your sense of smell usually returns within a few weeks, but it may be permanent in some cases. If you lose your sense of smell you may also lose your appetite as you won’t be able to taste the flavours of food without it. Although, your tongue will still be able to differentiate between salty, bitter, sour, and sweet. You should also take extra safety precautions if you are unable to smell.

Does anosmia go away on its own?

Depending on the cause, such as an allergy, sinusitis, or colds, anosmia may go away on its own within a few days to weeks. If the causes are mechanical blockages such as nasal polyps, or more permanent due to a traumatic brain injury caused by head trauma, it won’t go away on its own as the underlying causes will first need treatment. 

When should I see a doctor?

Anosmia caused by allergies and viral infection usually goes away within a few weeks. If your sense of smell does not return to normal after a few weeks you should contact your healthcare provider.

Summary

Anosmia is the complete loss of the sense of smell. There are many different causes, as disruption anywhere along the olfactory pathway between the nose and the brain can lead to anosmia. The most common causes are inflammatory and obstructive disorders such as sinusitis, viral infections, and nasal polyps. It can also be a consequence of head trauma or neurological conditions. Old age also increases the risk of anosmia, as do neurodegenerative conditions such as Alzheimer’s.

Anosmia is usually not serious and can be treated with steroid tablets or nasal sprays. It mostly goes away after recovery from the cold or allergy. However, in some cases, it can be permanent.

If after several weeks you are still suffering from a loss of sense of smell, you should contact your healthcare provider for a medical evaluation to make sure there are no serious underlying issues. People suffering from anosmia should take extra care with fire and gas safety, and when eating food as they won’t be able to detect environmental dangers.