Introduction

Anosmia refers to the complete loss of smell, which generally occurs as a side effect of medical conditions and can range from being temporary to permanent.¹ However, in some cases, there is no underlying cause of anosmia, which is referred to as idiopathic anosmia. This is when diagnostic testing and medical investigations do not determine the root cause of anosmia. This condition affects quality of life and has a psychological impact as loss of senses, such as smell, affects eating and can create difficulties in day-to-day social interactions.² Furthermore, this condition may also affect the safety of the person affected, as not being able to smell gas or smoke in a hazardous event can put their life in danger. This article will cover the current management and diagnosis of idiopathic anosmia and why it is important to treat the condition. 

How common is it and who is affected?

Although temporary anosmia is very common and affects most people in the population who experience common colds, idiopathic cases remain rare. Approximately 30% of anosmia cases are known to be idiopathic, and the rest are commonly caused by respiratory infections or trauma to the head.³ Anosmia affects people of all ages and genders. However, there is a trend seen in older-aged populations that have a higher risk of experiencing anosmia. In the United States, anosmia affects approximately 3% of the population who are aged 40 years and above.⁴ On the other hand, the impact of anosmia on daily life is very common, specifically emotional effects impact day-to-day life interactions and can turn into physiological conditions which need to be treated. Based on a study, women are more likely to experience possible depression or anxiety-related issues arising from idiopathic anosmia compared to men.⁵

How does it happen?

Anosmia can happen for several reasons, and one of the common causes is impairment in the olfactory system, like a blockage in the nose, such as from a cold, allergies, nasal polyps, or a deviated septum, which stops smell particles from reaching the smell receptors. It can also happen after a viral infection like the flu or COVID-19, which may damage the cells involved in detecting smells.⁶ In some cases, head injuries, brain diseases like Alzheimer's or Parkinson’s, can affect the nerves or brain areas responsible for smell. Nevertheless, as people age, their sense of smell can also naturally decrease, which is why older age is a risk factor for anosmia.

Some possible explanations behind idiopathic anosmia include genetic factors, including people born without a sense of smell, which is called congenital anosmia. Genetic modifications in gene CNGA2 affect the normal function of olfactory sensory neurons. This gene is important in helping transmit signals from the nose to the brain, and a study of this gene suggests that mutations in the gene specifically affect males and lead to loss of smell.⁷ Other factors that can cause anosmia are specific chemicals or overuse of nasal sprays, which can damage the signalling pathway of the nose to the brain which leading to a loss of smell. Essentially, anything that affects the nose, head and brain areas that process the senses of smell is likely to cause anosmia.

Diagnosis of idiopathic anosmia

Diagnosis of idiopathic anosmia can be difficult as it is a rare occurrence, and various factors can be the underlying cause of anosmia itself. A medical history is very important to track when loss of smell has started, and the related symptoms to this are important in identifying whether there is an associated root cause or whether it is idiopathic. If anosmia is present due to a recent head or sinus trauma, then medical imaging techniques such as magnetic resonance imaging (MRI) and computed tomography (CT) scans are done to visualise any injuries or conditions that may be causing anosmia.⁸ Nevertheless, a clinical examination of the nose and the sinuses is done by a medical professional to see if there are any blockages. Other diagnostic testing tools include smell tests such as Sniffin Sticks and UPSIT, which are orthonasal tests that measure how well you can smell.⁹ If there are no specific findings after these medical investigations and other potential conditions are excluded, then idiopathic anosmia is diagnosed. 

Management of idiopathic anosmia

Management of idiopathic anosmia involves first treating the root cause, if found, with specific medications, and if there are no medical findings, you can focus on supportive care, such as lifestyle support and olfactory training. 

• Medications and vitamins that can manage anosmia cases
  
  • Nasal steroid sprays can be prescribed for acute sinusitis or allergic rhinitis-related anosmia.
    
  • Oral steroids are suitable for COVID-19-related anosmia if symptoms are not developing. 
    
  • Dietary supplements like vitamin A, zinc, and omega-3 can provide supportive care for idiopathic anosmia.¹⁰

Olfactory training is one of the ways of managing idiopathic anosmia, since there is no underlying cause and no appropriate treatment method. This testing method involves exposure to specific scents for a period of time to help the brain relearn these scents. A study including 75 participants performed this test for 36 weeks in total using 12 different scents, which proved to be an effective treatment for anosmia.¹¹ Olfactory training was found to have a beneficial effect in improving the sense of smell in people who have been affected. 

What’s new in research

New research on idiopathic anosmia is ongoing, and studies looking for genetic markers are on the rise to explain if this could be the underlying cause of the condition.

Current concepts that have been investigated include platelet-rich plasma (PRP) therapy. A study based on 56 people with long-term smell loss from COVID-19 who received PRP injections in their nose, and later found that their ability to smell had improved significantly.¹² Most importantly, PRP caused no side effects, suggesting it may be a safe and promising treatment. Another recent study investigating whether the ingredients in supplements such as palmitoylethanolamide (PEA) and luteolin (LUT) can improve olfactory impairment showed promising results. Approximately 92% of people who took the supplement along with smell training showed better results, compared to only 42% of those who did smell training alone.¹³ These studies show that combining supplements with training could significantly improve the sense of smell.

Ongoing research on stem cell therapy in treating olfactory impairment is promising. Stem cells can replace damaged tissues that affect the sense of smell. Recent research has shown that cells deep inside the nose, called epithelial basal cells, act like stem cells. These special cells can grow into new smell-sensing nerve cells, which are essential for detecting scents. One of the most promising new approaches is finding ways to "wake up" these basal stem cells in the nose and encourage them to create new olfactory neurons.¹⁴ This kind of treatment could offer hope for people who have lost their sense of smell due to injury, illness, or ageing. Investigation for stem cell therapy is still ongoing, but the idea of using one’s own cells to restore olfactory function is innovative. 

FAQs 

How do you diagnose anosmia?

A medical professional will take diagnostic tests, like a CT scan, and clinically examine the nose and throat. 

What is the most common cause of anosmia?

The most common causes are flu-like illnesses and trauma to the head. 

What neurological conditions cause smell loss?

Neurodegenerative disorders such as alzheimers and parkinsons cause loss of smell.

What supplements help you smell again?

Vitamin A supplements are proven to be beneficial.

How do I get back my sense of smell?

Olfactory training, also known as smell training therapy, can show improvement. 

Summary 

Idiopathic anosmia is challenging to diagnose, as a definitive diagnosis can only be made if there is no underlying cause of anosmia. Therefore, patients have to undergo multiple diagnostic tests and clinical examinations, which can be a lengthy process, but new diagnostic tools and treatments are emerging. On the other hand, managing idiopathic anosmia requires taking non-medical approaches like vitamin and mineral supplements, emotional well-being support and olfactory training. As there is still ongoing research about ways of managing idiopathic anosmia, the future holds more effective ways of restoring the sense of smell.