Our fifth sense, often relegated to a mere mention at medical school has risen to prominence in the pandemic, ironically through its disappearance.

Anosmia has several causes, the most common of which are viral infections, such as colds or flu and now COVID-19. Any obstruction of the journey of scents to the roof of the nose, such as allergic rhinitis, nasal polyps, or tumors can cause this too. So can head trauma and anterior skull base fracture through the cribriform plate, exposure to pesticides and solvents, cocaine inhalation, smoking and poor air quality.

Of course, radiotherapy to the head and neck and normal ageing can cause anosmia too. It is important to exclude more serious etiologies such as brain tumors, Parkinsonism  and Alzheimer’s disease.

But for those with permanent loss of smell, the effects can be devastating. It can impact on their personal safety by being unable to detect a gas leak or a fire. There is extensive evidence linking anosmia to both depression and decreased quality of life—with people unable to appreciate flavours, enjoy their food, or detect if it is rancid. They may even miss a soiled nappy!
During the pandemic, restrictions and lockdowns often delayed thorough examinations and investigations that would exclude serious problems in patients.

The prime real estate area of concern is in the roof of the nose under the cribriform plate. That is where the olfactory fibers reside. This tiny area of mucosa, 5cm2, is where smell is primarily detected and vulnerable to attack. The mucosa is rich in ACE 2 receptors for which SARS-CoV-2 virus has an avid appetite, hence its ability to affect smell.

Up to 70 per cent of people with COVID-19 disease have developed anosmia. In many, this is their only symptom, and has led to the suggestion that anosmia could be a ‘biomarker’ for COVID-19 disease.

In a study of 1,420 European COVID-19 patients published in the J Int Medicine, Sept 2020, anosmia (70.2 per cent) was only marginally second to headache 

(70.3 per cent) as the commonest presenting symptoms in mild to moderate COVID-19 disease. A meta-analysis of 3,563 COVID-19 + patients published in Rhinology Oct 2020, the prevalence of smell and taste loss was 47 per cent. It was more common (67 per cent) in mild to moderate COVID-19 symptomatic patients than severe cases (31 per cent). Outcomes show smell recovery is either complete in a few days, or slowly over one to three months. The majority recover by six months.

Since Professor Theo Hummel of Dresden University, Germany, published his evidence-based seminal work on Smell Re-training Therapy (SRT) in 2009, there has been a growing acceptance of the brain’s neuroplasticity and the success of early intervention for anosmia. It is akin to physiotherapy for our smell centers— exercising our brain in the process. It calls on our brain cells’ memory to generate expectation of odour molecules.

Originally, Professor  Hummel used four basic smells, lime (fruity), cloves (spicy), eucalyptus (resinous) and rose (floral). Nowadays if four different and familiar odours are used, it doesn’t appear to decrease the effect. Essential or aromatherapy oils such as lemon, sandalwood, cinnamon, and peppermint are popular.

The two key aspects are to perform the re-training twice daily for about 20 seconds, for four months or until recovery, and to be mindful and focused during the task. This is done simply by opening each jar separately and taking gentle ‘bunny' sniffs for 20 seconds while concentrating on what you are trying to smell. Then continuing with the next fragrance.

Once serious pathologies causing anosmia have been excluded, SRT can empower patients. They can make their own home-made inexpensive kits and don’t need pricey commercial kits.

Dr Peter Friedland
Department of Otolaryngology-Head Neck Skull Base Surgery, Sir Charles Gairdner Hospital
Associate Professor in School of Medicine, University of Western Australia
Professor in School of Medicine, Notre Dame University
Chair, Royal Australian College of Surgeons WA ENT Training Scheme
Member of Australian Department Health Panel of Clinical Experts