Olfactory Disorders and Their Causes

Disorders of Smelling (Dysosmia) and What Lies Behind Them

Ear nose & throat, woman
BSIP / Getty Images

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?

What is Olfaction? - 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.

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 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 in 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)

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. Conditions related to the nose, such as nasal polyps, septal deviation, as well as surgery and injury to the nose (such as rhinoplasty) can disrupt the sense of smell.

Other possible causes include:

  • Head Trauma - Traumatic brain injury 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 or torn during the head injury. The trauma may also directly damage the olfactory bulb that detects the molecules we smell.
  • Brain tumors - Both malignant and benign brain tumors, especially those which involve the olfactory bulb or the temporal lobes, may associate with a change in the sense of smell. In some cases, a loss of the sense of smell may be the first symptom of a malignant or benign brain tumor.
  • Toxins in the environment - Tobacco products and smoke diminish the sense of smell. Toxins like ammonia, sulfuric acid, and formaldehyde can diminish olfaction as well.
  • Medications - Some medications, especially a few classes of medications used to control high blood pressure may interfere with the sense of smell. Examples include Procardia (nifedipine), Vasotec (enalapril), and Norvasc (amlodipine).
  • Head and neck radiation for cancer
  • Neurological disorders - More than 90 percent of people with Alzheimer’s disease have difficulty smelling, and dysosmia is also common in Parkinson’s disease.

  • Diabetes - Similar to the nerve damage that can lead to peripheral neuropathy and retinopathy in diabetes, damage to the nerves involved in olfaction may likewise occur.

  • Vitamin deficiencies - A lack of zinc or a lack of thiamine leading to Wernicke-Korsakoff syndrome are associated with a loss of the sense of smell.

The sense of smell is also commonly diminished through both natural aging, and in degenerative diseases like dementia. While the adult olfactory bulb in young adults has about 60,000 mitral neurons, both the number of mitral neurons and the diameter of their nuclei decrease dramatically with age.

In roughly one out of five people with olfactory disorders, the cause is "idiopathic," meaning no specific cause is found.

Diagnosis of Olfactory Disorders

The diagnosis of olfactory disorders often begins with a careful history and physical exam. A physical exam can look for evidence of a viral infection or nasal polyps. A careful history may reveal possible toxic exposures.

A test known as the University of Pennsylvania Smell Identification Test may be done do assess whether hyperosmia or anosmia is truly present. Since there are many possible causes ranging from brain disorders to nutritional causes, and more, further testing will depend on many factors.

Treatment and Coping with the Loss of a Sense of Smell

There are not any specific treatments which can reverse a change in the sense of smell. Sometimes, dysomia resolves on its own in time. Researchers have evaluated the use of high dose vitamin A and zinc supplementation, but as of yet, this does not appear to be effective. Olfactory training is currently being evaluated and appears to be promising in early studies.

Coping is thus the primary goal of treatment. For those who have no sense of smell, safety measures such as making sure you have a fire alarm are important. Nutritional counseling may be helpful since some foods and spices which are most likely to stimulate the receptors (trigeminal and olfactory chemoreceptors.)

Bottom Line on Olfaction and Disorders Which Affect Our Sense of Smell

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 (even by 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

Continue Reading