What is persistent anosmia?

The COVID-19 pandemic had detrimental effects on millions of people worldwide. As a result, many individuals are left with lingering symptoms years after the viral outbreak. Anosmia is a medical term describing smell blindness and has been observed in people affected by COVID-19. While anosmia is not a COVID-19 symptom that everyone experiences, the sudden loss of smell and taste is a hallmark manifestation that is distinctive of the viral infection. While an altered sense of smell and taste occurs in a large number of individuals who are actively carrying the virus, symptoms subside after the infection period is over.

In the case of persistent anosmia, the inability to smell is maintained for months to years following infection.¹ Losing your sense of smell can affect your mood and mental health, as scents are a gateway to enjoying food, moments, and reconnecting with memories. Most importantly, being able to detect alarming smells, such as gas leaks and smoke, is essential for one’s safety. This article will explore the biology behind persistent anosmia post-COVID-19, factors influencing recovery, recovery patterns, and possible treatment options.

What causes persistent anosmia after COVID-19 infection?

The SARS-CoV-2 virus, responsible for COVID-19, is known to interact with the olfactory epithelium.² The olfactory epithelium is the specialised tissue that lines the roof of the inside of our nose, and is responsible for smell detection. While the virus does not directly infect the nerve cells responsible for detecting smells and sending signals to the brain, it targets a specialised cell type, called sustentacular cells, whose purpose is to support these nerve cells.³

Mechanisms involved in COVID-related anosmia

The biological mechanisms that are likely involved in persistent anomia involve:^4,5

• Chronic inflammation of the nasal epithelium, possibly as a result of the persistent presence of the virus
• Damage to the receptors in the nasal cavity that are responsible for smell detection
• Inability or failure to regenerate olfactory cells 

Prognosis: what is the likely outcome?

The prognosis of a condition describes the profile of the illness and what the recovery journey may look like.⁶ More specifically, it explains how the infection will progress, the chances of recovery, the duration of recovery, as well as the potential complications and permanent symptoms. Understanding your prognosis is essential for managing expectations, becoming informed about what recovery may look like and exploring treatment options. 

Most individuals affected by COVID-19 recover from symptoms such as loss of smell within the first few weeks after the infection. 5-10% of people who have had COVID-19 experience persistent olfactory dysfunction, which can last for months, years or even permanently.⁶ 

Management and treatment options

The following management strategies and treatment options have shown improvement in some individuals with COVID-19 persistent anosmia:

• Olfactory training: Training the nose with specific smells is an effective and safe intervention that can lead to faster recovery. With olfactory training, you are typically required to sniff odourants such as essential oils, lemon, and cloves, for several seconds daily. This may be done for months, or until the sense has improved. Research indicates a significant improvement in patients with complete or partial loss of smell following COVID-19 infection⁷
• Corticosteroids: Oral and topical (nasal spray) corticosteroid treatments may also be used to help with persistent anosmia. Steroids act by reducing inflammation, which may be beneficial when chronic inflammation of the olfactory cells causes ongoing olfactory dysfunction. Corticosteroids must be used with caution and according to instructions given by healthcare providers to avoid complications associated with improper use⁸ 

Emerging therapeutic approaches, at experimental stages, include:

• PRP therapy: Platelet-rich plasma (PRP) therapy involves the injection of the individual's platelet-rich plasma, which promotes healing across different areas. This therapeutic approach has shown promising early results in restoring the olfactory mucous membrane⁸
• Stem cell therapy: In this case, stem cells generally replace damaged cells of the nasal epithelium, promoting the regeneration of olfactory neurones. This approach has the potential to be greatly effective in “fixing” olfactory receptor neurones that have been affected by COVID-19⁸ 

Factors influencing recovery 

Age 

The olfactory epithelium of older individuals heals at a slower rate, as the stem cells responsible for regeneration become less active with age. Additionally, older individuals express higher levels of a specific receptor that SARS-CoV-2 uses as an entry point to our body.⁸

Severity of the initial smell loss level

Severe loss of smell has been linked to worse recovery outcomes. Serious damage to the olfactory epithelium or the neurones associated with smell detection may cause more lasting damage. Interestingly, you may be more likely to develop persistent anosmia if you are asymptomatic or if you have mild COVID-19 symptoms.⁹ 

Vaccination status

Vaccination against COVID-19 is known to shorten symptoms and provide a higher chance of recovery. For persistent anosmia, vaccination before infection has been shown to provide a faster recovery time, potentially by modulating an immune response once the virus enters the body.¹⁰ 

Olfactory training consistency 

Being consistent with your olfactory training is crucial for achieving the desired results. Consider this training as rehabilitation for your brain, particularly for the neurons responsible for smell detection. Just like physical therapy requires consistent care and can provide slow but effective relief to the body, olfactory training exposes the damaged neurones to triggers (scents) and retrains them to pick up on signals that detect and process scents.⁷ 

Patterns of recovery

Recovery patterns for persistent anosmia vary significantly and are complicated to assess. When testing your sense of smell, you may be asked to report on your ability to smell, or you may be examined with specialised tests like Sniffin' Sticks. Studies using subjective self-reports may underestimate olfactory dysfunction after COVID-19, as objective specialised tests show higher rates of persistent anosmia.⁹

Self-reported recovery rates are significantly high, with most individuals regaining their sense of smell within weeks, and the vast majority being fully recovered by six months after the infection.¹¹ However, studies comparing self-reports and objective Sniffin’ Sticks tests show a disparity between the results across the two methods.¹² This further highlights the complicated nature of olfactory dysfunctions; individuals may become comfortable with their level of smell detection over time and believe that they have recovered, only for an objective test to disregard that progress. Nevertheless, the vast majority of anosmia cases resolve after 2 years.¹³

A symptom that may sometimes manifest in individuals undergoing anosmia recovery is other smell distortions.¹⁰ Although not common, parosmia has been reported as an unwanted symptom that occurs a few months after the onset of anosmia. Parosmia occurs when familiar scents become distorted, such as food or coffee smelling like sewage or smoke. Although distressing, this symptom may be a good sign of neural regeneration that may ultimately point to full recovery.⁷ 

Living with persistent anosmia

Impact

Losing your sense of smell can significantly affect your life. Smells allow us to enjoy food, to experience the world around us and to reminisce on memories associated with specific scents. 

Not being able to smell may affect appetite, while altered olfactory function, such as parosmia, may cause food aversions and lead to deficiencies and weight loss. Most importantly, being able to detect certain smells, like gas, smoke, or when something has spoiled, is crucial to avoid threatening situations. 

Confidence and social life may be negatively influenced by persistent anosmia. Feelings of depression from the uncertainty of when you are able to regain the ability to smell, and anxiety surrounding your hygiene due to impaired smell, may arise. Given how personal your sense of smell is, not feeling like you can fully convey the extent of your problem to your loved ones can also lead to frustration and isolation. 

Coping strategies

Besides the management and treatment options discussed above, several coping strategies focusing on mental health, enjoyment and safety can be implemented to alleviate distress:¹⁰

• Mental health support: Support groups and therapy can help with feeling isolated and alone. Journaling your thoughts and keeping track of progress can also help process the emotional impact of the condition and boost confidence
• Prioritising enjoyment with food: Focusing on texture, experimenting with spices and trying new foods that enhance your eating experience may bring back enjoyment after losing your appetite
• Safety: Being mindful of your safety and taking extra precautions is important to ensure you stay alert. Installing gas and smoke detectors, while also paying attention to expiration dates and storing food properly, is essential

Summary

Persistent anosmia after COVID-19 infection affects a notable number of individuals worldwide. Not being able to smell directly affects one's safety, mental health and overall quality of life. While the viral outbreak is still relatively recent and the scientific world is actively uncovering new information, research shows positive signs regarding recovery from persistent anosmia. Raising awareness for the prognostic outlook of the condition and the patterns of recovery is important for managing expectations, being informed about available treatment, and helping with feelings of uncertainty. Current therapeutic approaches offer promising results with consistency and patience, while emerging treatments, like PRP and stem cell therapy, have great potential.