Introduction

Olfactory dysfunction refers to the disrupted ability of the sense of smell, which can come in many forms; a total loss of smell is referred to as anosmia, a reduced sense of smell is hyposmia, and distorted perception of smell is referred to as dysosmia.¹ The dysfunction can be subdivided into quantitative qualities by olfactory performance, for example, sensitivity to specific odours, inability to differentiate between odours, or identify odours.² The sense of smell is one of the five senses: sight, hearing, smell, touch and taste, allowing humans to interact with day-to-day activities. A dysfunction of smell can be caused normally with ageing as sensory ability declines, or it may be caused by an underlying health condition. It is important to be aware of why olfactory dysfunction is happening, as it can affect social interactions, reduce the ability of danger perception and affect overall quality of life.

Physiology of olfaction

The anatomy of the olfactory system is made up of the nasal cavity, the olfactory nerves, bulb and cortex.³ The physiological process of olfaction starts at the nasal cavity, where olfactory receptor neurons (ORNs)are located. These detect odour molecules and express an odour receptor.⁴ Therefore, as you detect an odour, the odour molecules bind to these receptors, which then trigger an electrical signal. These neurons send signals through the olfactory nerve to the olfactory bulb, which is located at the base of the brain, helping process the sense of smell. From there, the electrical signals travel to different areas of the brain involved in identifying smells, linking them to memories, and triggering emotional responses. Unlike most senses, smell signals go directly to the brain without passing through the thalamus. 

Normal age-related decline in olfaction

It is normal that olfactory function declines as you age, and it may become difficult to detect and identify odours. It is typical to lose sense of smell after age 65 to 80; however, this all depends on your geographical location, genetics and lifestyle. For example, a study in Japan portrayed that in the Japanese olfactory identification test (OSIT-J) identification ability of odours declined in the elderly from the age of 50.⁵ On the other hand, 21.6% of the population in Germany experiences olfactory dysfunction around the age of 75 based on a survey using olfactory identification tests.6 Therefore, in some countries, the decline of detecting and differentiating odours can start as early as 50 years of age, whereas in some it takes up to 75 years of age. It is important to know that this is a normal process of ageing and can be caused by various factors. 

The factors below may cause normal ageing-related loss, and they involve: changes to the neurons, nasal airflow, mucus and may be caused by limited neural processing.  

• Changes in neural receptors: There is a reduction in ORN, and the specificity of the response in ORN to the odour decreases with age. As people age, the expression levels of certain olfactory receptors (ORs) in the olfactory mucosa may decline, leading to reduced sensitivity to specific odours.⁷ This means that the way odours are perceived can change with age, as the combination of ORs activated by a given scent may be altered. Interestingly, this suggests that while the total number of ORNs decreases with age, the relative expression of each OR gene remains largely consistent between younger and older individuals
• Changes in nasal airflow or mucus: Changes are specifically seen in the volume of the nasal cavity, the nasal airflow and mucus deteriorate as you age, which increases intranasal volume, and this may be partly due to the thinning or weakening of the nasal lining.8 For example, studies have shown that with age, the bones around the sinuses can shrink, which may cause the walls of the nasal cavity to bow outwards, which can further increase the space inside the nose. These changes can cause olfactory dysfunction, nasal obstruction and dryness
• Diminished neural processing
  As you age, the brain's ability to process odours can decline even if the nose still detects odours. This is because the neural pathways that carry electrical signals from the nose to the brain become less efficient with age. Naturally, there is a loss of neurons and cells, which causes a slower or weaker processing of detecting odour. Therefore, the elderly can find it difficult to identify and distinguish specific odours, even though they can smell

Pathologic anosmia

Pathologic anosmia occurs as a result of an underlying health condition, trauma to the olfactory system or due to respiratory illnesses and is not caused by the normal ageing process. Research has shown that people who are affected with anosmia are likely to have neurodegenerative disorders such as Alzheimer's, Parkinson's and dementia. Imaging techniques show that brain scans in adults with smell disorders can have atrophy in parts of the brain that process odours, like the cortex.⁹ This supports the idea that dysfunction in these key parts of the brain plays a major role in odour detection loss in older adults. It can also be a sign of problems with cognitive decline or early brain changes linked to diseases like Alzheimer’s, especially in areas such as the hippocampus and temporal cortex. To help detect these conditions earlier, a tool called the “sniffing bead system” has recently been developed specifically to screen olfactory function in the elderly.⁹

Nevertheless, viral infections are also the number one pathological cause of olfactory dysfunction. Specifically, at the peak of the COVID-19 pandemic, olfactory dysfunction was the most common symptom of the SARS-CoV-2 virus. The occurrence of viral respiratory tract infection has been reported to induce the function of smell, which is likely due to viral-induced damage to the olfactory system.¹⁰ Other viruses, such as rhinovirus, influenza and Epstein-Barr virus (EBV) can similarly have the same effects by altering the function of detecting odours. Other health conditions, such as chronic sinus disease, which creates inflammation that affects the nasal cavities and sinuses, therefore, the olfactory system is impaired.¹¹ However, there are various treatment options available from anti-inflammatory medications to surgical interventions.

Distinguishing normal from pathological

Distinguishing between normal age-related and pathological olfactory dysfunction relies on a combination of medical assessment. This may include olfactory testing such as Sniffin’ Sticks and UPSIT, risk factors such as respiratory illnesses and consideration of medical history to see patterns of dysfunction. Medical history is taken during clinical assessment to see the pattern of loss. If this is a sudden loss of smell, then this can indicate an underlying health condition. However, if there is a gradual loss, then this can indicate age-related causes or can be associated with cognitive decline. 

The role of olfactory testing is very important as it can give a definitive diagnosis of this disorder. The University of Pennsylvania Smell Identification Test (UPSIT) is used to diagnose this condition. This test includes four small booklets that each contain 10 "scratch and sniff" strips with 40 different scents.¹² For each scent, the person is asked to choose the correct smell from four possible answers. The UPSIT results are then compared to the normal range based on the patients' age and sex. The result can verify if the patient is experiencing anosmia, hyposmia or dysosmia.

FAQs

Do olfactory receptors decline with age?

The receptors in the olfactory system normally decline due to ageing.

What is the commonest cause of olfactory dysfunction?

Most commonly during viral infections and ageing.

How to fix olfactory dysfunction?

Medication can be prescribed if you are experiencing viral infections which cause olfactory dysfunction.

Can you reverse anosmia?

Yes, if there is an underlying infection, but it is not always the case. 

Can a nasal spray help get the smell back?

Yes, nasal sprays are commonly used and can improve the condition temporarily. 

Summary 

Olfactory dysfunction can be caused by ageing as well as health conditions such as respiratory illnesses. Once you get treated with antibiotics or other suitable medication, then olfactory dysfunction should improve. However, this is not always the case with neurodegenerative disorders that cause olfactory dysfunction. As a part of ageing, these disorders like Alzheimer’s are likely to develop because cognitive function declines, which is associated with olfactory function. As a result, olfactory function declines, which can cause a reduced sense of smell or anosmia. It is important to remember that as soon as you experience any sort of olfactory impairment, it is crucial to visit a healthcare practitioner because early detection may be able to reverse and treat this condition.