Introduction

The olfactory nerve is the first and shortest cranial nerve (CN) of the Central Nervous System (CNS). We can refer to the olfactory nerve as Cranial Nerve I (CN I). CN I is a sensory nerve because it is made of sensory fibres. This nerve is responsible for your sense of smell. It carries information about odour molecules from your nostrils to the brain.

Our sense of smell helps us detect beautiful fragrances, harmful odours, and the smell of the food we eat and drink. Disorders of this nerve may lead to distorted, reduced, or complete loss of smell.¹

In this article, we will discuss the anatomy, function, and disorders of the olfactory nerve. We will learn the causes that can affect our olfactory nerve and how to maintain a healthy sense of smell.

Anatomy and structure of the olfactory nerve

The olfactory nerve is a paired nerve and part of our central nervous system (CNS). It contains only sensory fibres that arise from the olfactory epithelium and reach the olfactory bulb. These fibres form an olfactory tract from the olfactory bulb. The olfactory bulb then carries information from our nostrils to our olfactory cortex.¹

CN I nerve bypasses the thalamus (part of the CNS) because the olfactory cortex is part of the forebrain. The information is then processed in our olfactory cortex, which interacts with limbic structures to integrate it into our memory. The nerve is directly exposed to our environment through its specialised fibres present in the nostrils. Hence, any damage to the nose can directly impact the olfactory nerve.¹

Functions of the olfactory nerve

The olfactory nerve plays a crucial role in processing and integrating the information regarding different scents, aromas, and smells in our brain. It detects the odour when inhaled through our nostrils. It coordinates with the limbic system and cerebral cortex, that helps develop our flavour perception of different foods. This impacts our appetite. This is why when you smell something delicious, you feel hungry and want to eat.²

Olfaction serves a unique role in shaping our social interactions with others. Our brain saves our memory of smell and associates it with emotions and different experiences, due to our communication with the limbic system. The sense of smell in humans also helps in detecting dangerous odours in the environment, e.g, gas leaks, chemical leaks, fire, and spoiled food.²

What conditions can affect the function of your olfactory nerve?

Several conditions can affect your sense of smell. Your ability to detect smells tends to decrease as you grow older.

Other causes may include:^3,4

• Cold or nasal allergies
• Allergic rhinitis
• Upper respiratory tract infections URIs
• Viral infection such as COVID-19
• Cigarette smoke
• Medicines such as amphetamines, estrogen, or long-term use of nasal decongestants
• Nasal tumors
• Sinusitis 
• Infections in the throat
• Head injury
• Traumatic facial injury or fractures
• Endocrine disorders such as hypothyroidism, Addison's disease, and Cushing's syndrome
• Dementia
• Alzheimer’s disease
• Parkinson’s disease

Disorders of the olfactory nerve

The normal ability/sensitivity to smell is also called normosmia. The 2 main olfactory dysfunctions that occur include:⁴ 

• Conductive loss
• Sensory or neural loss

Conductive loss

It is due to a blockage in the path of odour molecules to reach the specialised epithelium in the nostrils. For example, nasal polyps and allergic rhinitis, etc.

Sensory or neural loss

Occurs when there’s damage to the olfactory nerve itself or along the olfactory pathway to the cerebral cortex. These can be seen in conditions such as head trauma, upper respiratory tract infections (URIs), alzheimer’s disease, multiple sclerosis, etc.

There are 4 kinds of disorders associated with the olfactory nerve (CN I):

• Anosmia
• Hyposmia
• Parosmia/Troposmia
• Dysosmia/Phantosmia

Anosmia

Anosmia is the complete loss of the sense of smell. Anosmia affects your quality of life because you're unable to detect, process, or perceive smell. It can also cause an altered taste of different foods. Anosmia can be temporary (also known as conductive) anosmia or permanent (also known as sensory or neural) anosmia.

Causes of conductive anosmia loss include:

• Cold
• COVID-19
• Nasal polyps
• Nasal congestion
• Hay fever
• Deviated nasal septum

Sensory or neural anosmia can be associated with:

• Alzheimer’s disease
• Parkinson’s disease
• Multiple sclerosis
• Sjogren’s syndrome
• Brain tumors
• Traumatic brain injury TBI
• Kallmann syndrome
• Antibiotics such as amoxicillin and azithromycin

Hyposmia

Hyposmia is your nose’s reduced ability to detect smell. It is an inability to differentiate between certain smells. It can be both temporary and permanent.

Causes of hyposmia include:

• Common cold
• Bacterial infections, such as chronic sinus infections and nasal vestibulitis
• COVID-19
• Alzheimer’s disease
• Parkinson’s disease
• Type 1 diabetes
• Tooth decay
• Gum diseases
• Long-term exposure to chemicals such as heavy metals
• Radiation therapies
• Antidepressants

Parosmia or Troposmia

Parosmia is a distorted sense of smell, for example, things smell burnt or rotten. It is also known as troposmia. It causes a mismatch between the actual smell and the smell that your brain is perceiving.⁵ Parosmia may follow post-COVID-19 anosmia.

Parosmia is further divided into two types:⁶

• Cacosmia: detecting unpleasant smells
• Euosmia: detecting pleasant smells

Other causes of parosmia include:^5,6

• Upper respiratory tract infections 
• Smoking
• Common cold
• Sinonasal disorders such as acute sinusitis and nasal polyps
• Traumatic brain injury
• Psychiatric disorders such as severe depression
• Exposure to toxic chemicals
• Chemotherapy in cancer patients
• After surgery 
• Present after a person has healed from hyposmia and anosmia

Dysosmia or Phantosmia

Dysosmia or phantosmia is a condition in which an affected individual senses odours without any source of the odour present. It is also called olfactory hallucinations. The difference between phantosmia and parosmia is that phantosmia is smelling something when it is not present, whereas parosmia is a distorted smell of a present substance. The origin of idiopathic phantosmia is unknown, despite several diseases are associated with phantosmia. 

These include:⁶

• Schizophrenia
• Parkinson’s disease
• Traumatic brain injury
• Rhinosinusitis
• Radiation therapy
• Viral infections
• Nose surgery

Diagnosis of olfaction disorders

An otolaryngologist or an ENT doctor diagnoses olfactory disorders. The doctor takes a detailed medical history and utilises other diagnostic tools to aid in making a diagnosis.⁷

These include:

• Smell test: Consists of a scratch-and-sniff booklet in which you have to identify different smells
• Nasal endoscopy: Allows the ENT physician to visually examine the nose
• CT scans and MRI: Imaging tests to look at your head and nose and detect any disease present

Treatment of olfaction disorders

Treating the underlying cause is the main solution. A surgery is performed when a nasal obstruction, such as nasal polyps, is present. If there's a viral infection or psychiatric disorder, medications are prescribed.⁸

Topical corticosteroids are given to treat sinonasal dysfunction disorders. A person with post-viral, idiopathic phantosmia and traumatic injury is advised to deliberately smell different odours at different times of the day. This has been shown to improve symptoms related to olfactory dysfunction.⁸

Everyday tips for maintaining a healthy sense of smell

Some tips advised by doctors are:⁹

• People with olfactory disorders should smell essential oils such as lemon, eucalyptus, and rose
• Avoid smoking
• Maintain good nasal hygiene
• Treat viral and sinus infections
• Use protective gear in toxic environments, such as chemical industries
• Regular checkups if the smell changes persist

FAQs

Can COVID-19 cause permanent loss of smell?

Yes, in some cases it can cause anosmia. Most recover in weeks to months, but some have long-term or permanent anosmia.

Which vitamin can help me treat my loss of smell?

The active form of Vitamin D in a nasal spray helps to treat loss of smell in some cases of inflammation.

Is smell loss always neurological?

Not always. It can be caused by nasal blockages, infections, or trauma, aside from neurological issues.

Can smell training help restore function?

Yes, especially in post-viral smell loss, daily smell training with essential oils such as rose, lemon, oranges, and eucalyptus may aid recovery.

When should you see a doctor for your smell loss?

If smell loss lasts more than 2 weeks, especially after a head injury or infection, consult an ENT specialist.

Summary

Our olfactory nerve (CN I) is responsible for our sense of smell. The olfaction centres process smells and saves them in our memory. Our sense of smell helps us remember our favourite scents and foods and is also associated with positive emotions. A healthy sense of smell plays a crucial role in maintaining overall well-being. A good sense of smell plays an important role, as it can help detect dangerous odours which can be harmful.. 

Smell disorders cause a distorted and reduced sense of smell. They are associated with viral infections, URIs, traumatic brain and facial injuries, psychiatric disorders, and tumours. You can maintain a healthy sense of smell by being hydrated, smelling essential oils, and treating disorders that cause them. If you suffer from any olfactory disorders, consult an otolaryngologist.