Olfactory Disorders and Their Causes

The Importance of Sensing Scents

Woman smelling flowers in countryside.
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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 all of our lives.

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. 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 of Olfaction

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, 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 lock fits a key.

These receptors dwell on between six to ten million olfactory chemoreceptor cells in each nasal cavity.

New receptors are formed throughout adulthood—one of the few examples of how the brain can form new nerve cells throughout life. When there’s a fit between an environmental chemical and a receptor on an olfactory cell, the nerve cell fires a signal directly to the brain on the olfactory bulb.

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 involved with emotion. From the primary olfactory cortex, signals are relayed to other parts of the brain, including the thalamus and hypothalamus. When there is a distortion in the sense of small, doctors use the term dysosmia to describe it.

Causes of Dysomia 

There are a variety of factors and conditions that can cause dysosmia. The most common causes are nasal and sinus diseases: by clogging the nasal passages and inflaming the tissues that receive olfactory molecules, viral infections and allergies have an effect on our sense of smell that almost everyone has experienced at some time or another.

Head trauma can impact olfaction in many different ways: the nose may be damaged, or the nerve fibers that send information from the nose to the brain may be cut during the head injury. The trauma may also directly damage the olfactory bulb that detects the molecules we smell.

Some toxins or medications can also cause olfactory changes, including amlodipine, enalapril, lovastatin, and nifedipine. Tobacco products and smoke also diminish the sense of smell. Toxins like ammonia, sulfuric acid and formaldehyde can diminish olfaction as well.

The sense of smell is also commonly diminished through both natural aging, and in degenerative diseases like dementia. While the adult olfactory bulb has about 60,000 mitral neurons at age 25, there may only be about 14,500 at the age of 95. More than 90 percent of people with Alzheimer’s disease have difficulty smelling, and dysosmia is also common in Parkinson’s disease.

The importance of the sense of smell and taste is generally underappreciated in everyday society. While the olfactory nerve is labeled cranial nerve number one, suggesting the importance of olfaction, in practice the sense of smell is rarely tested by physicians in the office, including neurologists. While it’s true that the olfactory system in humans is relatively small compared to other mammals, smell both helps us enjoy life, and protects us from toxins in the environment. 

Sources

Bhatnagar KP, Kennedy RC, Baron G, Greenberg RA. Number of mitral cells and the bulb volume in the aging human olfactory bulb: a quantitative morphological study. Anat Rec 1987; 218:73.

Blumenfeld H, Neuroanatomy through Clinical Cases. Sunderland: Sinauer Associates Publishers 2002

Morrison EE, Costanzo RM. Morphology of the human olfactory epithelium. J Comp Neurol 1990; 297:1.

Mott AE, Leopold DA. Disorders in taste and smell. Med Clin North Am 1991; 75:1321.

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