Since the outbreak of the COVID-19 pandemic throughout the world, there has been increased media focus on early symptoms or signs which could act as a clue that a person may have contracted the virus, so as to trigger early isolation, and hopefully limit the spread of this devastating virus.

One of the symptoms which has garnered particular attention is a loss of smell, with some patients experiencing this early on in their illness.

Dr Brendan Perry, an Ear, Nose and Throat Surgeon at Brisbane Private Hospital, explained that current data suggests that the sudden loss of smell and/or taste may be present in up to 30% of COVID positive cases.

"Disorders of olfaction include a complete loss of smell, known as anosmia, and hyposmia which is a reduction in the smell sense. These symptoms usually also affect taste, as it relies on retronasal olfaction.

It is worthwhile asking about both of these senses when assessing someone for COVID 19, as the presence of one or both may be an important trigger for testing and isolation."

Many theories have been proposed for how SARS-CoV-2 causes an altered sense of smell.

Initially it was hypothesised that the virus may directly infect the olfactory bulbs of the first cranial nerves and potentially the central nervous system.

A recent international collaboration has shown that the ACE2 gene, which is the entry point for the virus into a cell, is not expressed on these olfactory nerves.

Single cell sequencing has shown that ACE2 is expressed on the sustentacular, or support, cells in the olfactory cleft as well as in the perivascular cells.

Inflammation of these support cells is now believed to be the primary cause of olfactory dysfunction from SARS-CoV-2.

"While this evidence continues to evolve, it is important to reflect upon the other etiological causes of these symptoms, for cases where patients are reporting deficits in taste and smell to primary care and emergency staff, but are ultimately found to test negative to COVID 19."

"The assessment is analogous to the assessment of hearing loss, and can be roughly divided by the site of dysfunction - so, conductive, sensory, or neural in origin."

Conductive disorders of smell include any obstructive cause that reduces the airflow to the upper part of the nose, known as :he olfactory cleft. Common causes include nasal polyps, a nasal septum deviation, chronic rhinosinusitis, and other inflammatory conditions of the nose.

Sensory olfactory disorders are those that affect the receptors in the upper nose responsible for detecting smells, with certain viral infections directly affecting these cells responsible for one third of anosmia presentations.

Anosmia and hyposmia can also arise from neural causes, arising along the olfactory nerves or intracranially. This may be due to trauma affecting the nerve or brain, or rarely from intracranial lesions, which can often only be detected with MRls.

Cigarette smoking and certain medications can also cause dysfunctions of olfaction.

"Some of the conditions which impact olfaction and taste can cause significant morbidity for the patient. Thankfully we can fairly efficiently clarify the cause of a patient's symptoms through a combination of careful history and examination, an outpatient nasendoscopy, and sometimes radiological imaging. Appropriate management can then be recommended which may include lifestyle, medical, and/or surgical interventions."

"No matter the cause, it is essential to discuss safety issues arising from anosmia which the patients may not have considered, including not being able to smell smoke, gas or other noxious substances. Patients should be informed to install fire and gas detectors, consider changing to electrical stovetops instead of gas, and adhere to food expiry dates to reduce the risk of food poisoning."

In the age of COVID 19, asking about loss of taste and smell is becoming an important sentinel set of symptoms to ask about when assessing patients for this virus.

In cases where patients are not COVID positive, Ear, Nose and Throat Surgeons can be a resource to primary care and emergency colleagues, to assist in further investigating and managing these patients.