Olfactory Nerve and Smell Disorders

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Brain & Nervous System
Neurological Symptoms & Diagnosis
Overview and Causes of Olfactory Disorders
By Peter Pressman, MD
Medically reviewed by Shaheen Lakhan, MD, PhD
on January 04, 2020

What are olfactory disorders? What are the causes of dysosmia (a distortion in the sense of smell) and why do they occur? Why is this so important?
Ear nose & throat, woman
BSIP / Getty Images
Olfaction and the Importance of Our Sense of Smell

It would be hard to imagine living life without experiencing the smell of a rose, or the taste of morning coffee. Olfaction, our sense of smell, plays a significant role in almost everyone's life.

Not only does olfaction help us detect fragrances in the air around us, but it's also important in helping us enjoy the flavors of foods. You may have heard that "taste" primarily smells, and that is true to a large degree. A damaged sense of olfaction is severely disrupting: the joy of eating and drinking may be lost, and depression may result. Furthermore, there are dangers associated with the loss of smell, including the inability to detect leaking gas or spoiled food.

More than 2.7 million people in the United States have an olfactory disorder, and this is likely an underestimate. Some people have suggested that about half of people over the age of 60 have a decreased sense of smell.
The Anatomy and Physiology of Olfaction (The Sense of Smell)

In the upper and middle part of the nose, there's a small cell area called the olfactory mucosa. This area secretes several protective substances, like immunoglobulins (antibodies that bind with foreign matter such as microorganisms), in order to prevent pathogens from entering the head. There are also a large number of proteins, called receptors, that capture chemicals in the environment, or odorants. Each receptor is thought to have a special shape that fits these odorants like a key fits a lock.

The olfactory receptors live on an estimated six to ten million olfactory chemoreceptor cells in each nasal cavity.

While the olfactory bulb is commonly considered the “first cranial nerve,” it is technically not a nerve, but part of the brain itself. Signals transmitted from the olfactory bulb travel to special parts of the cerebral cortex and even the amygdala, which is a part of the brain involved with emotion. From the primary olfactory cortex, signals are relayed to other parts of the brain, including the thalamus and hypothalamus.
Smell Disorders

There are several terms that are used to describe the different types of smell disorders. These include:

Dysomia: A distortion of the sense of smell. Dysosmia is in turn broken down into two separate types of smell disorders. Parosmia refers to a change in the perception of an odor. Phantosmia, in contrast, refers to the perception of a smell which is not present. With parosmia, and odor may smell different than it did to you in the past, or you may now find an odor repulsive which you previously enjoyed. With phantosmia, you may, for example, smell a campfire when there is not a campfire present.
Hyposmia: A decreased ability to sense smells
Anosmia: A total inability to smell smells

Causes of Dysomia (A Distortion in the Sense of Smell)

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