Anosmia is a condition which means you have a reduced or absent sense of smell. For example, if you’re cooking something fragrant, like onion or garlic, and notice that it doesn’t smell as strong as usual or doesn’t smell of anything, then you may be symptomatic of anosmia. This could happen very suddenly or over a period of time.

Usually, it’s only temporary, and most people often experience it as a side effect of:¹

• Having a cold, flu or COVID-19
• Sinus infections 
• Allergies like hay fever
• Growths in your nose (nasal polyps)

Typically, anosmia goes away on its own as your illness goes away, where the underlying illness or cause itself is temporary.³ However, there are occasions when anosmia can become permanent, and in order to understand why that may happen, we need to understand how the sense of smell works.

How does our sense of smell work?

Our sense of smell is crucial to our evolutionary programming, from detecting harmful odours in our surroundings to assessing the safety of substances, to playing a huge role in many pleasurable experiences, such as eating an enjoyable meal or walking through nature. So when someone is diagnosed with anosmia, it can affect their well-being and their quality of life.⁴

Smell and taste are part of something called the chemosensory system, meaning that the sensory cells responsible for sensing taste and smell are sensitive to chemical signals.⁵ The chemosensory system has many nerve endings, especially in places in the body that are moist, like your eyes, nose, mouth, and throat. These nerve endings help you to feel irritating substances, as they send signals to the brain, which is why when you eat very spicy food, your eyes, nose, AND mouth tend to sting. Smell and taste work together to determine whether a food should or should not be eaten. 

Olfactory sensory neurons

We are able to smell due to sensory cells known as olfactory sensory neurons. These cells are found in a small area of tissue high inside your nose and are directly connected to your brain (so don’t use your finger to find them).

The neurons work by detecting smells through receptors, which receive information and then generate signals to the brain. For example, when you’re cooking onions, tiny molecules are released from the onions, which, when inhaled, will stick to these receptors, and the signals sent to your brain will allow you to recognise the smell. Substances all around us release these microscopic molecules, like freshly cut grass or freshly washed laundry. 

Each neuron has one single receptor; however, there are so many more smells around us than there are receptors. To allow us to smell multiple things at once, a single smelly molecule can activate multiple receptors. This creates a unique pattern that your brain interprets as particular smells.

How are smell and taste related?

Smells can reach the olfactory sensory neurons by two methods: one is by travelling through your nostrils when you breathe in. The other is through a channel that connects the roof of your throat to your nose, called the nasopharynx. When you chew food, some of the aroma escapes through this second pathway. 

When you have a cold or flu, your nose can be clogged, which blocks this channel, so the smells can’t reach the neurons. This makes it harder to enjoy foods and flavours, as we know that smell and taste work very closely together.

Without the olfactory sensory neurons, it would be hard to tell the difference between two different flavours, like chocolate or oranges. Without the sense of smell, food would have very little or even no flavour, as smell accounts for 95%-99% of chemosensations, with taste making up the rest.⁶ When people who go to the doctor’s because they think they’ve lost their ability to taste, often they find that they’ve actually lost their sense of smell instead.⁷

What is happening when your sense of smell isn’t working properly?

Any condition which causes a disruption to any part of the olfactory chemosensory pathways can cause you to lose your sense of smell. This may occur as a physical blockage to the smell molecules from entering your nose or mouth, or as a blockage to the signals from reaching your brain. 

Conditions which obstruct the pathways through which smell molecules travel to the olfactory sensory neurons are the most common causes of a temporary loss of smell. For example, when you have allergies, a sinus infection, a cold, flu or COVID-19, the mucous membrane (mucosa), which lines and protects your nose, mouth and throat by keeping them moist, becomes irritated and inflamed, making more mucus to fight the infection.⁸ This wonderful substance is what makes you have a runny or clogged nose and gets stuck in your throat sometimes. As the mucosa gets thicker, smell particles are unable to reach the olfactory sensory neurons, as they are being blocked by the mucous membrane, which causes a reduced or loss of sense of smell.

When the inflammation/infection has cleared, the mucosa returns to normal, so the pathways are no longer blocked and your sense of smell returns to normal.

Non-cancerous growths like nasal polyps and hemangioma are generally harmless, painless tumours which can form in the mucosa and cause an obstruction to the pathway in the same way as a cold or allergies. They cause the mucosa to become irritated and swollen, which can reduce your sense of smell, but they are unlikely to go away on their own and may require treatment.

Cancerous tumours can also develop within your nose, within your nasal cavity or sinuses, which can block smell molecules from entering the nose and reaching olfactory sensory neurons, causing permanent anosmia unless treated.⁹ Squamous cell carcinoma (SCC) is an example of this, which is a common form of skin cancer. It can form in mucous membranes inside the mouth or nose, which can lead to anosmia. 

Central causes of anosmia

There are also some more complex causes of anosmia, and these are ones that affect the transmission of the sensory signals from your neurons to your brain. For example, the smell molecules are able to reach the sensory neurons without issue, but there is then a problem with transmitting the signal from the neurons to the brain. These are typically referred to as “central” causes, because they affect the central nervous system, and include things such as specific cancers, head traumas, neurodegenerative disorders and congenital conditions. 

Cancerous conditions

Neuroblastoma

Neuroblastoma is a childhood cancer which can develop in nerve tissue and can spread to the olfactory sensory neurons, disrupting the pathway at this point, which can result in a loss of sense of smell.

Adenoid cystic carcinoma

Adenoid cystic carcinoma (ACC) is an uncommon cancer that can develop in the salivary glands and other areas of the head and neck. Whilst it is rare, this cancer can spread to the sinuses, which can develop into anosmia.

Sarcoma

Sarcoma is a rare cancer that can develop within nerve structures such as olfactory sensory neurons, preventing receptors from detecting smells.

Head trauma

Head injuries are another common cause of loss of the sense of smell because a blow to the head can cause damage at multiple points along the pathway. For example, the trauma may cause physical damage to the nose or sinuses, leading to an obstruction or blockage. 

A head injury can also lead to a loss of the sense of smell by damaging or destroying the nerve fibres that are responsible for detecting smells, which are located at the thin bone at the base of the skull, or by harming the smell-related parts of the brain, like the olfactory bulb or other areas that interpret smells.

Damage to the central nervous system from a head trauma can result in a temporary or permanent loss of smell, depending on how severe and where the injury occurs. Luckily, olfactory neurons are able to repair themselves, unlike other parts of the central nervous system.¹¹

Ageing and neurodegenerative processes

As people get older, their ability to smell naturally decreases. This happens because the number of cells in the part of the brain responsible for smell, called the olfactory bulb, and the area of the tissue that detects smells, called the olfactory epithelium, decreases in function as people get older. 

Having a reduced sense of smell has been associated with a higher chance of developing neurodegenerative conditions later on in life. The most common link is between anosmia and the later development of diseases involving abnormal protein build-up, such as certain brain disorders like Alzheimer’s disease, Parkinson’s disease, and Lewy body dementia.¹²

Congenital conditions 

There are several conditions that are present from birth, called congenital conditions, that are associated with anosmia, including:

Kallmann syndrome 

Kallmann syndrome is a genetic disorder which prevents someone from starting or completing puberty, which is also characterised by a lack or reduced sense of smell. This is suggested to be because changes in certain genes linked to Kallmann syndrome interfere with the movement of smell-related nerve cells, as well as reproductive hormone-producing nerve cells, during brain development.¹³

Turner syndrome

Turner syndrome is a genetic disorder that only affects those assigned female at birth and is caused by a missing or partially missing X chromosome. There is a wide range of symptoms that affect their development, one of which includes anosmia, likely associated with disruption to the development of nerve cells in a similar way to Kallmann syndrome.¹⁴

Long-covid

There have been instances where people contract COVID-19, and their symptoms of illness stay for months or years, even if the COVID-19 infection has cleared. This has been termed long-COVID. Symptoms that may remain include anosmia. It has been shown that COVID-19 can be found by nasal brushing and olfactory mucosa biopsies, demonstrating that the virus itself affects the olfactory sensory neurons.¹⁵ 

Studies have shown that a technique called functional septorhinoplasty, which is used to fix deviated septa, can be used to increase the nasal cavities to boost airflow into the olfactory regions.¹⁶

Summary

There are many causes of anosmia, which affect your sense of smell by different mechanisms depending on the part of the pathway they impact. Where conditions are able to be treated or resolved, like the resolution of a cold or removal of a tumour causing a blockage at your nose, your sense of smell tends to return. 

Central causes of anosmia usually refer to more complex conditions that affect the nerve pathways between the sensory neurons and your brain. For this reason, treatment to return your sense of smell to normal can be more difficult. 

If you believe you may have anosmia, ask your healthcare provider whether it could be due to a cold, flu or benign growth before considering a potentially more serious medical issue.