London - Sense of smell plays a crucial role in our health — saving us from out-of-date food that looks fine to eat, even guiding our sexual behaviour through the subtle odour of pheromones (chemicals given off in sweat).
The perception of different odours is triggered when microscopic particles are breathed in and dissolve in mucous membranes in the nose.
Receptor cells in the nose then send signals via the olfactory nerve to receptors in the brain, to an area known as the olfactory bulb.
This also sends information to be processed further in other brain areas associated with such things as emotion, memory and learning — which is why some smells can remind us of certain memories or trigger specific feelings.
Women have more olfactory cells than men — up to 50 percent, according to one study — which might explain why they consistently outperform men when distinguishing between different smells.
“Our ability to smell is based on a complex process which can easily be disrupted at different stages,” says Michael Papesch, an ear, nose and throat consultant surgeon at Whipps Cross University Hospital in London.
“If any part of the chain is broken or damaged, then smell is not registered by the brain and all scents would be the same: sniffing a rose would be like sniffing cardboard.”
People who have lost their sense of smell completely — as a result of trauma or infection damaging olfactory receptors — can still perceive such strong scents as ammonia or menthol.
This is because these stimulate pain or temperature-sensitive nerves in a process known as chemesthesis, where signals bypass the olfactory bulb and are passed directly to the higher centre of the brain to trigger a sensation.
We may take it for granted, but a loss of sense of smell (hyposmia if it is partial, anosmia if total) can affect your health.
For instance, our sense of taste is closely connected with smell because the same nerves and parts of the brain are involved in both.
So losing our sense of smell can make us add sugar or salt to food to make it “taste” better, indirectly contributing to such problems as obesity and high blood pressure.
Here are some reasons why your sense of smell could be diminished...
GETTING OLD
“Our sense of smell declines as we get older probably because the number of olfactory sensors reduces considerably,” says Martin Rossor, a neurologist at the National Hospital for Neurology and Neurosurgery in London.
This is because cells die faster than they can be replaced.
Smell that declines with age is known as presbyosmia and is not preventable.
However, catching a cold can make things worse so make sure you wash hands frequently and avoid people who are infected.
TAKING MEDICINE
Antibiotics may interfere with the sense of smell, as can some antidepressants, anti-inflammatories and heart medications.
“Quite a few common medicines can interfere with your sense of smell, possibly by directly affecting sensory receptors in the nose,” says Dr Colin Cable, assistant chief scientist at the Royal Pharmaceutical Society.
“The effect usually wears off soon after stopping the medication. Generally, an alternative medicine can be found.”
Antibiotics that can cause smell loss include ampicillin, used to treat urinary tract infections and types of meningitis, and azithromycin, used to treat some ear infections as well as pneumonia.
Antidepressants linked to loss of smell include doxepin and amitriptyline (also used to treat chronic pain caused by nerve damage). Some ACE inhibitors for high blood pressure, including captopril, can alter smell, as can angina drug glyceryl trinitrate.
Statins, the most commonly prescribed medications in the UK, may interfere with taste and smell.
There are a number of ways these medications affect smell, including blocking nerve pathways and altering the mucous layers of the mouth so odour molecules can’t dissolve easily in mucous membranes surrounding the olfactory cells.
NASAL GROWTHS
Nasal polyps are soft, non-cancerous growths in the nasal passages, usually affecting adults with chronic inflammation in the nose.
This can be through chronic sinusitis or other respiratory conditions (between 20 and 40 percent of people with nasal polyps also have asthma, according to the NHS). They are thought to form when the lining of the nose becomes increasingly swollen, eventually hanging down in teardrop shapes.
They affect up to four percent of the population and are twice as common in men as in women. “They’re common and many cause no symptoms,” says Mr Papesch. “Polyps cause obstruction which prevents odour molecules entering.”
Corticosteroid treatment may help to shrink them but some patients have them removed under general anaesthetic. This should also reinstate the sense of smell.
HAVING A VIRUS
Colds and flu are the most common causes of hyposmia because upper respiratory infections block the passage of odour molecules.
“Rarely, a viral infection can permanently damage the olfactory nerves so that smells cannot be processed in the brain,” says Professor Rossor.
A blocked nose mutes smells and makes food taste bland. Tastebuds can detect only sweet, sour, bitter, salt and umami (savoury); all other “tastes” are combinations or variations of these and are interpreted by the nose’s olfactory cells.
“This is why people with anosmia often still enjoy sweet puddings, salty pickled vegetables, spicy foods and sour fruit like lemons.”
IS IT DEPRESSION?
Many people who are depressed report finding it difficult to distinguish odours. When they are treated successfully, their sense of smell improves. A 2014 study of brain scans, published in the Journal of Affective Disorders, found depressed women showed reduced activation in areas of the brain associated with smell.
One theory is that depression is accompanied by changes to the limbic network of the brain that underpins emotions, long-term memory and sense of smell.
DEMENTIA LINK
Alzheimer’s can reduce ability to smell. This is due to amyloid plaques (tangles of proteins) which kill brain cells, causing memory loss and impaired speech and cognition.
They can also damage brain cells which pass signals from the olfactory bulb, says Dr Ian Le Guillou, of the Alzheimer’s Society’s research and development team.
Studies show that people with Alzheimer’s can smell peanut butter from a given distance, more accurately through their right nostril than through their left.
As a great deal of research shows that Alzheimer’s-related brain shrinkage starts on the left side of the brain, this could be a significant marker for the disease to aid early diagnosis.
CANCER DRUGS
Chemotherapy often causes partial or complete loss of smell, says Dr Marina Parton, a consultant oncologist at the Royal Marsden Hospital in London.
“I treat lots of women with breast cancer and many tell me their senses of taste and smell are greatly reduced. They tend to prefer sharp and sweet or sour flavours.”
It is thought the drugs damage the mouth’s mucous layers.
“Some women also experience odd smells — smells that only they can sense, but not others, known as parosmia,” says Dr Parton. “This is likely to be an effect from the chemotherapy being excreted in body fluids (such as sweat or saliva) in very small amounts which only the patient senses.”
Dr Parton says most people should regain their senses of smell and taste three to six months after treatment has stopped.
Daily Mail