Anosmia

Anosmia is the loss of the sense of smell. Some people experience it temporarily after catching a cold or flu, but anosmia can be caused by a number of things, from damage to olfactory receptors or the olfactory nerve, to a brain injury or disease.

Our ability to smell comes from a small patch of tissue in the nasal cavity called the olfactory epithelium. This contains around 50 million nerve cells, or neurons, with microscopic hairs called cilia that extend into the mucus that lines the nasal passage. Receptors on the cilia bind to airborne molecules that reach the nostrils, setting off an electrical signal in the neuron which is then passed along the olfactory nerve to the brain.

Anosmia can arise through damage to many different parts of this olfactory system. Some people are born without the ability to smell due to genetic factors, but anosmia can also result from damage in the olfactory receptors in the nose, damage to the olfactory nerve, or from an injury which affects the brain region responsible for smell. Loss of smell can also be an early sign of conditions such as Alzheimer’s and Parkinson’s disease.

Many viruses that affect the respiratory system are known to interfere with smell receptors in the nose, and anosmia can be a symptom of covid-19.

Loss of smell is often confused with the loss of taste – known as ageusia. Taste is detected by taste buds on the tongue, and is limited to the sensation of bitter, sweet, sour, salt and the savoury “umami” flavour. Most of what we taste actually comes from odours from food in our nasal passage. When you bite into an apple, your tongue only picks up its sweetness and sourness, but the odours that rise into your nose tell you the aroma of the apple.

Estimates vary, but anosmia and hyposmia – the decreased ability to smell – seems to occur in 3-20 per cent of the population, with older people more affected. The main causes are head injuries and cold and flu viruses, as well as chronic sinus infections and nasal polyps.

Depending on the cause, anosmia can be temporary or permanent.

The loss of smell can have profound effects on daily living and mental health. Not only is food tasteless, but taste and smell act as a warning system for dangers such as leaking gas and rotten food. They also help us keep on top of personal hygiene. In addition, smell helps us to create and recall memories.