Loss of smell

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Loss of smell
Other names: Anosmia, smell blindness,[1] odor blindness
A side view anatomical drawing of the nasal sinuses depicting inflamed mucosa.
Inflamed nasal mucosa causing anosmia
Pronunciation
SpecialtyOtorhinolaryngology
SymptomsLoss of the ability to detect one or more odors[1][2]
TypesPartial, total[2]
CausesBlockage of the noses, dysfunction of the neurons involved with smell[3]
Differential diagnosisHyposmia, parosmia, phantosmia[3]
TreatmentDepends on the underlying cause[3]
Frequency3 to 20% of those over 40 years[3][4]

Loss of smell, technically known as anosmia, is the loss of the ability to detect one or more odors.[1][2] It may be temporary or permanent.[3] Some cases are present at birth while others are acquired later in life.[3] The inability to identify harmful smell can be dangerous and enjoyment of food can be affected.[3]

About 50 to 70% of cases are due to blockage of the nose, such as from inflammation or nasal polyps.[3] Other cases are due to dysfunction of the neurons involved with smell as a result of head injury, tumors, aging, toxins, or certain genetic condition.[3] It may also be a symptom of COVID-19, particularly early in mild infection.[5] Determining the underlying cause involves taking into account other symptoms and examination.[3] Some centers have the ability to do more detailed analysis.[3] It differs from hyposmia, which is a decreased sensitivity to some or all smells.[2]

Treatment depends on the underlying cause.[3] Steroid nose sprays may help with a number of causes of blockage of the nose.[3] Surgery may be an option for some sinus problems.[3] There is no specific treatment for cases due to dysfunction of neurons, though some cases may improve over days to years.[3]

About 3 to 20% of people age over 40 are affected.[3][4] The condition becomes more common with age, affecting about 40% of those over the age of 80.[3] One of the earliest documented cases of loss of smell following head trauma, was a case reported by Hughling Jackson in 1864 in London.[6]

Signs and symptoms

Anosmia is the inability to smell.[1] It may be partial or total, and can be specific to certain smells.[2] Smell can be lost from one or both nostrils, and it may not always appear obvious.[7] Reduced sensitivity to some or all smells is hyposmia.[2]

Anosmia can have a number of harmful effects.[8] People with sudden onset anosmia may find food less appetizing and may be unaware of their own body odour.[9] Loss of smell can also be dangerous because it hinders the detection of gas leaks, fire, and spoiled food.[9] The common view of loss of smell as trivial can make it more difficult for a person to receive the same types of medical aid as someone who has lost other senses, such as hearing or sight.[9]

Losing an established and sentimental smell memory (e.g. the smell of grass, of the grandparents' attic, of a particular book, of loved ones, or of oneself) has been known to cause feelings of depression.[10][better source needed]

Loss of the ability to smell may lead to the loss of libido, though this usually does not apply to loss of smell present at birth.[10][better source needed]

Often people who have loss of smell at birth report that they pretended to be able to smell as children because they thought that smelling was something that older/mature people could do, or did not understand the concept of smelling but did not want to appear different from others. When children get older, they often realize and report to their parents that they do not actually possess a sense of smell, often to the surprise of their parents.[citation needed]

A study done on patients suffering from anosmia found that when testing both nostrils, there was no anosmia revealed; however, when testing each nostril individually, tests showed that the sense of smell was usually affected in only one of the nostrils as opposed to both. This demonstrated that unilateral anosmia is not uncommon in anosmia patients.[11]

Causes

Reduced ability to smell may occur as part of normal ageing.[12] However, loss of smell is common in conditions that cause a runny nose or inflammation in the nose.[12] The most common neurological cause is damage to the olfactory nerve from trauma to the head.[12] If the loss of smell is from one nostril only, a tumour may be suspected.[12] Some medications, recreational drugs, alcohol[13] and smoking can also affect the ability to smell.[12]

Some people may be born with the inability to smell, such as in Kallmann syndrome.[12] The occurrence within some families suggests that some congenital anosmia may follow an autosomal dominant pattern.[14] Anosmia may very occasionally be an early sign of a degenerative brain disease such as Parkinson's disease and Alzheimer's disease.[12] The association with depression is not clear.[15][16]

In 2020 during the COVID-19 pandemic, new onset loss of smell became a well-established symptom of COVID-19, particularly in the early stages of mild to moderate infection, where its prevalence reaches around 85%, and where it is frequently the first sign or only sign of the disease.[5][17] Loss of smell has also been found to be more predictive of COVID-19 than all other symptoms, including fever, cough or fatigue, based on a survey of 2 million participants in the UK and US.[18]

Prolonged use of some nasal sprays can cause loss of smell,[9] particularly those that cause vasoconstriction in the nose.[19] Anosmia can also be caused by nasal polyps.[15]

It can be caused by chronic meningitis and neurosyphilis that would increase intracranial pressure over a long period of time.[20] Loss of smell has been reported in a case in which a 66-year-old male was treated with amiodarone for ventricular tachycardia.[21] Olfactory neuroblastoma, an exceedingly rare cancer that originates in or near the olfactory nerve can present with loss of smell and may accompany chronic sinusitis for several years before diagnosis.[22]

Causes and contributory factors for dysfunction or loss of smell include:

Congenital

Nerve and brain

Drugs and toxins

Sinus and respiratory

Infection

Trauma

Surgical complications

Deficiencies

Other

Diagnosis

Both nostrils are tested separately to determine whether one or both nostrils are affected.[11]

Anosmia can be diagnosed by doctors by using acetylcysteine tests. Doctors will begin with a detailed elicitation of history. Then the doctor will ask for any related injuries in relation to anosmia which could include upper respiratory infections or head injury. Psychophysical Assessment of order and taste identification can be used to identify anosmia. A nervous system examination is performed to see if the cranial nerves are damaged.[46] The diagnosis, as well as the degree of impairment, can now be tested much more efficiently and effectively than ever before thanks to "smell testing kits" that have been made available as well as screening tests which use materials that most clinics would readily have.[47] Occasionally, after accidents, there is a change in a patient's sense of smell. Particular smells that were present before are no longer present. On occasion, after head traumas, there are patients who have unilateral anosmia. The sense of smell should be tested individually in each nostril.[11]

Many cases of congenital anosmia remain unreported and undiagnosed. Since the disorder is present from birth the individual may have little or no understanding of the sense of smell, hence is unaware of the deficit.[48] It may also lead to reduction of appetite.[49]

Treatment

Though anosmia caused by brain damage cannot be treated, anosmia caused by inflammatory changes in the mucosa may be treated with glucocorticoids. Reduction of inflammation through the use of oral glucocorticoids such as prednisone, followed by long term topical glucocorticoid nasal spray, would easily and safely treat the anosmia. A prednisone regimen is adjusted based on the degree of the thickness of mucosa, the discharge of oedema and the presence or absence of nasal polyps.[50] However, the treatment is not permanent and may have to be repeated after a short while.[50] Together with medication, pressure of the upper area of the nose must be mitigated through aeration and drainage.[51]

Anosmia caused by a nasal polyp may be treated by steroidal treatment or removal of the polyp.[52]

Although very early in development, gene therapy has restored a sense of smell in mice with congenital anosmia when caused by ciliopathy. In this case, a genetic condition had affected cilia in their bodies which normally enabled them to detect air-borne chemicals, and an adenovirus was used to implant a working version of the IFT88 gene into defective cells in the nose, which restored the cilia and allowed a sense of smell.[53][54]

Epidemiology

In the United States 3-20% of people age over 40 are affected by disturbance in smell.[3][4]

In 2012 smell was assessed in persons aged 40 years and older with rates of anosmia/severe hyposmia was 0.3% at age 40–49 rising to 14.1% at age 80+. Rates of hyposmia was much higher: 3.7% at age 40–49 and 25.9% at 80+.[55]

History

An early observation of loss of smell following trauma was documented in 1864 by Hughling Jackson in volume 1 of the London hospital reports, in which he recorded that a 50-year-old man who had fallen off his horse and suffered concussion and subsequently complained of an inability to smell.[6][56] Others shortly reported similar findings including reports with simultaneous loss of taste.[6]

Society and culture

Google searches for "smell", "loss of smell", "anosmia", and other similar terms increased since the early months of the COVID-19 pandemic in 2020, and strongly correlated with increases in daily cases and deaths.[57] Research into the mechanisms underlying these symptoms are currently ongoing.[58] While many countries list anosmia as an official COVID-19 symptom, some have developed "smell tests" as potential screening tools.[59][60] In 2020, the Global Consortium for Chemosensory Research, a collaborative research organization of international smell and taste researchers, formed to investigate loss of smell and related chemosensory symptoms.[61]

Terminology

The term derives from the New Latin anosmia, based on Ancient Greek ἀν- (an-) + ὀσμή (osmḗ, "smell"; another related term, hyperosmia, refers to an increased ability to smell). Some people may be anosmic for one particular odor, a condition known as "specific anosmia". The absence of the sense of smell from birth is known as congenital anosmia.

See also

References

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