A new study describes a coronavirus patient's 'sudden and complete' loss of smell - further evidence that mild cases can involve that symptom but few others

Crystal Cox/Business Insider

A man shops for groceries in New York City, March 27, 2020.

• A sudden loss of smell may be a symptom of the coronavirus, even when patients experience no other symptoms.
• A new case study found that a 40-year-old woman's primary COVID-19 symptom was a "sudden and complete" loss of smell.
• That aligns with evidence from South Korea, China, and Italy that suggests many asymptomatic patients experience a loss of smell.
• Visit Business Insider's homepage for more stories.

Most people who get the coronavirus exhibit a dry cough, fever, and difficulty breathing. Between 25% and 50% of infected people might not show no symptoms at all, while other coronavirus patients show less common symptoms like diarrhea or a loss of their senses of smell and taste.

A case study published Wednesday in the Journal of the American Medical Association describes a coronavirus patient in her 40s whose main COVID-19 symptom was a "sudden and complete" loss of smell. This is also known as anosmia. The woman had previously had a dry cough, but no fever.

The study authors tested the woman's ability to smell five different scents: caramel, goat cheese, fruit, manure, and a rose. She was unable to smell or identify any of them.

The research adds to a growing body of evidence that suggests individuals whose main symptom is a loss of smell could be infected with the new coronavirus, even if they lack other typical symptoms like cough or fever.

If a patient has all three symptoms - smell loss, cough, and fever - that "should alert the clinician" to suspect a coronavirus infection, the study said.

A swollen nasal cavity

An MRI and CT scan revealed why the patient in the case study was unable to detect smells.

The patient's olfactory cleft - a narrow space at the back of the nose into which air that carries scents flows - was swollen. The inflammation prevented odors in the inhaled air from reaching her olfactory epithelium, a mucous membrane inside the nose that detects smells.

Further examination showed that the woman's olfactory bulb (the berry-sized extension of the brain that sits above the nose and processes odor information) was normal and unharmed.

Roberto Pfeil/AP

A man sneezes into a napkin.

The woman did not lose her sense of taste, the researchers reported, nor did she have a fever or a runny nose, which can sometimes cause loss of smell.

A symptom in 'hidden carriers' who spread the illness without becoming sick

Other research supports this finding.

Scientists at King's College London used a tracking app to collect self-reported COVID-19 symptom data from 1.5 million users between March 24 and March 29. About 60% of patients who tested positive for the coronavirus had reported losing their sense of smell and taste, the data showed. The research, which has yet to be peer-reviewed, revealed that the loss of smell and taste were stronger predictors of a COVID-19 diagnosis than self-reported fever.

Lucy Nicholson/Reuters

An assistant store manager rings up groceries from behind a plexiglass barrier in Los Angeles, California, March 31, 2020.

In South Korea, China, and Italy, about a third of patients who have tested positive for COVID-19 also reported a loss of smell, according to a statement from ear, nose, and throat experts with The British Association of Otorhinolaryngology.

"In South Korea, where testing has been more widespread, 30% of patients testing positive have had anosmia as their major presenting symptom in otherwise mild cases," they said.

The experts also reported that "there have been a rapidly growing number of reports of a significant increase in the number of patients presenting with anosmia in the absence of other symptoms."

They added: "These patients may be some of the hitherto hidden carriers that have facilitated the rapid spread of COVID-19."

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