Anosmia is the complete absence of the sense of smell. Without your sense of smell, food doesn’t taste the same, you can’t smell a flower, and you could find yourself in a dangerous situation without knowing it. For example, you would not be able to detect smells like gas leaks, smoke from a fire, or curdled milk without the ability to detect odors.

The basics of smell

A person’s sense of smell is influenced by certain processes. First, a molecule released by a substance (such as the scent of a flower) must stimulate special nerve cells known as olfactory cells that are located in the nose. 

Nerve cells then send information to the brain, where the specific smell is identified. Anything that interferes with these processes, such as nasal congestion or nasal blockage, can lead to loss of smell. Damage to the nerve cells themselves can also lead to loss of smell.

The ability to smell also affects our ability to taste. Without a sense of smell, our taste buds are only able to detect a few flavors, which can impact your quality of life.

Causes of Anosmia

Nasal congestion caused by a cold, allergy, sinus infection, or poor air quality is the most common cause of anosmia. Other causes of anosmia include:

  • Nasal polyps – small, non-cancerous growths in the nose and sinuses that block the nasal passage.
  • Nose injury and nerve odour from surgery or head trauma.
  • Exposure to toxic chemicals, such as pesticides or solvents.
  • Certain medications, including antibiotics,  antidepressants, anti-inflammatories, and heart medications.
  • Cocaine abuse.
  • Old age. Like vision and hearing, your sense of smell can weaken as you age. In fact, the sense of smell is most pronounced between the ages of 30 and 60 and begins to decline after age 60.
  • Certain neurological diseases, such as Alzheimer’s disease, Parkinson’s disease, multiple sclerosis, nutritional deficiencies, congenital conditions, and hormonal disorders.
  • Radiological treatment of head and neck cancers.

The effect of aging on olfaction

The olfactory functions begin a decline beginning around the age of 60. The decline appears earlier in men than it does in women. Around the age of 80, more than half of the elderly have olfactory disorders.

The causes that lead to a poorer perception of odours are multiple: slow regeneration of the tissues of the olfactory system, alteration of the nasal mucosa which produces mucus-producing glands, etc.

Anosmia and other smell disorders

The doctor takes note of the complaints of the patient who smells badly or who no longer smells odours. Hyposmia (loss of sensitivity to odours) is the most common smell disorder, while anosmia is a smell disorder frequently caused by damage to the olfactory nerve. 

Parosmias are characterized by a diminished ability to perceive smells, while dysosmias (total loss of smell) are even more rare.

These complaints may be due to head trauma or “whiplash” or sinusitis (transmission anosmia), the latter being treated with corticosteroids.

Anosmia and Alzheimer’s disease

Disorders of smell can occur before the first cognitive symptoms appear. Researchers have actually noticed that there is a link between memory complaints reported by an individual and the presence of olfactory disorders detected using specific tests.

These observations are confirmed by a team of researchers from the University of Montreal. It would seem that 80% of people with Alzheimer’s disease first present with smell disorders. In particular, people would have difficulty distinguishing and memorizing an odour.

In a study published in 2009, it was shown that olfactory disorders are often present at the onset of  Alzheimer’s disease, and even before the first cognitive symptoms appear.

From a cohort of 144 people aged 50 to 86, the authors of this longitudinal study observed a correlation between subjective memory complaints (i.e. not validated by tests) and the presence of mental disorders. Olfaction was detected using qualitative discrimination and identification tests.

These results suggest that olfaction can be a good indicator for identifying subjects at risk of memory disorders.

Source:  Sohrabi HR et coll. Olfactory dysfunction is associated with subjective memory complaints in community-dwelling elderly individuals. J Alzheimers Dis. 2009;17(1):135-42.

Parkinson disease

People with Parkinson’s disease also have olfaction disorders, which are characterized by a loss of sensitivity in the perception of odors. The presence of olfactory disorders predicts cognitive impairment in the disease.

Researchers hope to diagnose Parkinson’s disease based on body odor.

How can these observations be explained?

Neurons send information via the olfactory nerve to the olfactory bulb, which in turn sends it to the olfactory cortex. Most regions of the olfactory cortex are part of the limbic system, responsible for processing emotions and memory (hippocampus, entorhinal cortex, amygdala). This explains why the memory of a smell is all the more pregnant as it arouses emotion.

The olfactory bulb is notably modulated by neurons which produce a neurotransmitter called acetylcholine. This neurotransmitter is involved in memory and learning. These neurons project to regions of the structural system of the limbic system (hippocampus and amygdala) involved in the perception of odors.

In Alzheimer’s and Parkinson’s diseases, these neurons are damaged early by lesions characteristic of these diseases, namely neurofibrillary tangles and alpha-synuclein deposits respectively.