SINGAPORE - For several years, Mr Mohamed Patail relied on his wife's sense of smell when it came to buying hand creams. Then, last year, the 70-year-old consultant and trainer had surgery to remove nasal polyps. Now, he can detect scents for himself and says his favourite is lavender.

Anosmia, or the loss of the sense of smell, is often downplayed by those who have it, say ear, nose and throat (ENT) specialists. Its occurrence in Covid-19 patients is now focusing attention on the condition, which may signal other health issues.

About 40 per cent of all anosmia cases are caused by a viral infection, such as the common cold or the flu, says Dr Annabelle Leong, an ENT surgeon with clinics at Mount Elizabeth Novena Hospital and Gleneagles Hospital.

As a result of viral infection, "the mucous membranes which line the inside of the nose become irritated and swell up, preventing odour molecules from reaching the top of the nose where the smell nerves are located," she says. "Nasal allergies and sinus infections may also affect your sense of smell. Smoking also irritates the nasal linings due to the repeated exposure to toxins found in cigarettes."

The nose might also be blocked by non-cancerous growths known as polyps, or a tumour or structural deformity. All these can make the sense of smell worse, adds Dr Leong.

Smell loss can be complete (anosmia) or partial (hyposmia), says Dr Neville Teo, senior consultant at the Singapore General Hospital (SGH) department of otorhinolaryngology - head and neck surgery. Inflammation of the nerve fibres responsible for the sense of smell can lead to anosmia.

"Any simple cold or flu can result in loss of smell," he says. In rare cases, trauma from severe head injury could damage the nerves. "Sometimes, no specific cause can be found," he adds.

People tend to downplay smell loss, some specialists say.

Dr Dennis Chua, ENT specialist at Mount Elizabeth Medical Centre, says: "A lot of people don't realise they have lost the sense of smell. If one day you wake up and can't see, you would go to the emergency department. But smell loss might not be detected for weeks."

He sees about one case a week with a patient having lost the ability to smell. The most common cause is allergic rhinitis or nasal polyps.

Patients present the inability to taste well, he adds. They can taste whether something is salty, sweet, bitter or sour, but not the flavour of the food.

Dr Chua says that for patients who have lost this sense for four weeks or more, ENT specialists would conduct a nasal endoscopy and a brain scan to check for tumours on the smell fibres.

He adds that people lose their sense of smell at a slow and mild rate as they grow older, but one should be wary of an acute degeneration in the sense of smell as this can indicate more serious conditions.

Dr Chan Kwai Onn, ENT specialist at Novena Medical Centre, says: "Loss of smell can indicate something serious. Since only a tiny amount of odour needs to reach the nose for you to detect it, you have to have a really significant medical condition to have complete loss of smell."

He notes that permanent loss of smell is a disability. "Many people downplay their loss of smell because they think it's not important. But if one loses his sense of smell, one loses a connection to the world and the appreciation for the taste of food. It can also be dangerous as one can't detect gas leaks or spoilt food."

Dr Leong recalls a 40-year-old patient who caught Covid-19 in the Philippines earlier this year. The patient had a mild fever and cough, without any nasal congestion. Yet, she lost her sense of smell and could not smell durian, even when holding it close to her nose. "During that time, she felt rather depressed as she could not enjoy her favourite fruit anymore because 'it just tasted like heavy lumps of playdough.'"

Dr Leong says her patient was very worried that her sense of smell would not return because she worked for a food import company, where her main duty was to taste different delicacies and decide whether to order more in bulk quantity. "Fortunately, her sense of smell returned after a month, to her great relief. Otherwise, she would probably have lost her job."

Dr Teo from SGH also points out that loss of smell could affect the livelihoods of those who rely on it, such as chefs.

An SGH study shows that one is more likely to have smell and taste disturbance from Covid-19 (22.7 per cent) than other respiratory viruses (2.8 per cent), he says. However, he adds, various factors affect the chances of developing anosmia from Covid-19. Studies in Western countries have found a higher likelihood of Covid-19 patients developing anosmia, compared with Asian countries.

"This can be due to different strains of the virus, as well as genetic differences between ethnicities," says Dr Teo. He cites an SGH study of nearly 2,000 migrant workers in a community care facility, carried out from May to July last year. It found that 3 per cent of Covid-19 patients had anosmia, lower than in Western countries. The study also found that those of Chinese or Bangladeshi ethnicity had higher prevalence of smell and taste loss, compared with others. There have been no follow-up studies published on the rate at which the sense of smell is recovered.

Dr Chan from Novena Medical Centre says international studies show that up to 38 per cent of patients with Covid-19 have anosmia and it is also present in "long Covid". This is a condition in which patients who have otherwise recovered from the virus still show symptoms after the acute phase of infection.

He says: "I saw a couple who both had Covid-19 and recovered. While the wife recovered her sense of smell, the husband had not, two months after being discharged from the hospital. He also had other symptoms of memory fog and persistent fatigue."

Research has shown that the Covid-19 virus binds to the ACE2 receptor in the olfactory nerves as well as the cells that support these nerves. The damaged nerves and supporting cells may take six months or longer to recover.

"At the moment, we still do not have enough data to predict who will recover and who will not," adds Dr Chan.

There are concerns over what anosmia can mean for patients with long Covid. Researchers from Italy and the National Health Service in Britain have proposed long-term studies to examine whether patients with Covid-related anosmia have a higher chance of developing dementia because the virus impairs the nervous system.

Links have also been reported between anosmia and depression, says Dr Sarb Johal, a clinical psychologist from New Zealand who helped his government develop its response to Covid-19.

He tells The Straits Times: "Our sense of smell is closely linked with memory, probably more so than any of our other senses. We know that smells are handled by the olfactory bulb - a structure in the front of the brain that relays information to other areas for further processing. These include the amygdala, hippocampus and wider limbic system - brain regions related not only to emotion, but memory too."

Covid-related anosmia might lead to mood disturbances, though this link has not been adequately studied, according to Dr Johal.

Still, there is a remedy. Smell training - which is the repeat and deliberate sniffing of a set of odours (most commonly lemon, rose, clove and eucalyptus) for 20 seconds each, at least twice a day for at least three months - could be useful, he says.

It is not common in Singapore, though specialists have seen patients' quality of life improve with the return of their sense of smell.

Dr Chua from Mount Elizabeth Centre recalls a patient whose anosmia was cured by surgery to remove his nasal polyps. The patient was excited about getting to enjoy food again. "I remember this case very clearly because two weeks after surgery, the first thing he did was to get durians to eat. But now, he couldn't eat durians because the smell was too strong."

Mr Patail is also happier now that his sense of smell has improved. "My appetite has opened up. I enjoy food," he says. "Once you get back your sense of smell, you realise you've been missing all these fragrances - spices, lavender, even detergent."

Treating anosmia

Anosmia is caused by obstructions in the nose, such as nasal polyps or inflamed sinuses (sinusitis), or by damage to the olfactory nerves, or both. The common cold virus can cause temporary blockage of the nose and also damage the nerves.

Dr Chan Kwai Onn, ENT specialist at Novena Medical Centre, says that medications including antibiotics and steroids can be used to treat anosmia from sinusitis and nasal polyps. Endoscopic sinus surgery may be considered if these conditions do not improve despite medications. Steroids can also be given to those with damaged nerves.

He adds: "For people who have not recovered their sense of smell after three months, prognosis for a full recovery is guarded."

Dr Dennis Chua, ENT specialist at Mount Elizabeth Medical Centre, notes that damaged nerves can take six months to a year for full recovery. Apart from the treatments above, he recommends a saline rinse to clear the nose.

ENT surgeon Annabelle Leong, who has clinics at Mount Elizabeth Novena Hospital and Gleneagles Hospital, notes that steroids are often used to treat Covid-related anosmia. Steroids have potential side effects with long-term use, such as weight gain and high blood pressure.

She adds: "Research is being undertaken to develop an olfactory implant, a device embedded in the nose that would detect smell chemicals and send electrical signals to the brain. But this device is still a long way from being introduced into clinical practice."

She says that some European teams suggest smell training, or sniffing four different smells like rose, eucalyptus, lemon and clove twice daily for 20 seconds each time. "They claim an improvement of up to 45 per cent in sense of smell compared with just 25 per cent improvement in those who didn't undergo smell training," she says.

Smell training might help anosmia patients' overall well-being and mood, according to clinical psychologist Dr Sarb Johal from New Zealand, author of The Little Book Of Sleep and Steady: A Guide To Better Mental Health Through And Beyond the Coronavirus Pandemic, both of which are available online. He cites a 2016 review of research which found links between anosmia and depression. The greater the loss of smell, the more severe the depressive symptoms.

He says: "If you consider your nose like a muscle that can be strengthened, then giving it a workout through different odours and ways to sniff them can help bring back lost dimmed abilities and even add new vitality and sensitivity to our sense of smell that is different to what previously existed. We may find interesting benefits for our emotional experiences and memories too."