When someone is unable to see we call him blind; if someone is not able to hear sounds, we describe him as deaf. There is an analogous situation for the sense of smell: the complete absence of the sense of smell is called anosmia, whereas a reduced function of the sense of smell is called hyposmia.
Both anosmia and hyposmia are found relatively often. Researchers estimate that 5% of the population of western countries do not perceive any smells. This means that one person out of 20 is not able to smell; these persons are therefore considered anosmic. The numbers for hyposmia are even more impressive; up to 15% of the population have a reduced perception of smells. Therefore, one person in five has either a reduced sense of smell or does not smell anything at all. These numbers are considerably higher than those of blind or deaf persons – but of course anosmia and hyposmia have less devastating effects on the life of the affected persons.
But also persons with anosmia and hyposmia suffer from their condition. They have more depression symptoms and exhibit a reduced quality of life compared to people with a normal sense of smell. Amongst the typical problems people with anosmia or hyposmia describe to have is that they cannot appreciate food, they do not perceive the smell of a loved one, such as their partner or child, and others.
Qualitative olfactory dysfunctions are special forms of smell problems. Here people can perceive smells, but they perceive smells different from what they are supposed to smell like. For example, they can perceive flowers to have a burnt smell. This condition is called parosmia. In addition, some people perceive smells although there is no smell source, sometimes all the time. For example, some people constantly perceive a foul, rotten smell. This condition is called phantosmia. Both parosmia and phantosmia can be very disturbing and affect quality of life considerably.
Of course persons with a reduced sense of smell are running higher risks to be exposed to dangers, since the sense of smell has warning functions. For example, persons with a smell loss would not perceive if food is spoiled. Therefore it is very important for them to pay attention when storing food and to obey the expiration dates of food. If in doubt they should either ask someone else to smell the food or, in order not to run any risk, throw it away. People with anosmia should also consider that they do not perceive the smell of smoke or household gas. They have therefore to be especially careful when dealing with situations in which a fire may be burning or gas may be leaking.
There are different reasons why people can lose their sense of smell. The most important causes of a loss of smell function are different kinds of nasal problems such as nasal polyposis or chronic rhino-sinusitis. Although we do not know the exact mechanisms it is likely that the ongoing inflammation of the nasal mucosa in nasal polyposis and rhino-sinusitis affects the smell receptors in the upper part of the nasal cavity.
Next, there are two other, equally common, causes of a smell loss. The sense of smell may be lost as a consequence of a viral infection such as the flu; the specialists call this postviral anosmia/ hyposmia. This does not refer to a common cold; we all have experienced a common cold during which the nose is blocked, and with a blocked it is impossible to smell. However, in the case of a postviral anosmia/ hyposmia the loss of smell function persists even after the viral infection has healed. Interestingly, postviral anosmia/ hyposmia affect mainly women above the age of 50; we do not know why.
Smell function may also be lost as the consequence of an accident with traumatic brain injury (posttraumatic anosmia/ hyposmia). Although severe accidents are more likely to cause smell loss, even a concussion with no other long term effect may be enough to cause anosmia or hyposmia.
Other, less likely causes of smell loss include brain tumours or exposure to toxic substances. In many cases no apparent cause for the smell loss can be determined (idiopathic anosmia/ hyposmia). Up to 1% of all persons who can’t smell are born without a sense of smell and have never smelled in their life (congenital anosmia). Also, most of the persons suffering from neurodegenerative diseases such as Parkinson’s disease or Alzheimer’s disease have an impaired sense of smell.
There is hope for those who have lost their ability to smell. We know that the sense of smell can return, even after as long as several years. People with postviral anosmia/ hyposmia have been described to have a chance of 30% that their sense of smell will improve over one year. This percentage is lower, in the range of 10% for those who have lost their sense of smell after an accident.
IMPORTANT: The medical clarification of a smell loss has to be performed by a medical specialist such an ENT (ear-nose-throat) specialist or a neurologist. Diagnostic tools include a thorough medical history, nasal endoscopy and magnetic resonance imaging (MRI).
Dr. Frasnelli specialises in odor perception. He conducts research in the field of neurophysiology of smell and taste as well as therapy in loss of the chemical senses. Frasnelli is a graduate of the Medical Schools of the University of Vienna (Austria; 2001; Dr. med. univ.) and the Technical University of Dresden (Germany; 2009; Priv.-Doz.). Since 2006 he work in Montreal, first as an Academic Trainee at the Montreal Neurological Institute, since 2008 as a Postdoctoral Fellow at the Department of Psychology at the Université de Montréal. He currently hold a fellowship of the CIHR. Dr. Frasnelli research interest is the neurophysiology of smell and taste as well as therapy in loss of the chemical senses.
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