Disorders of Smell & Taste
Jeremiah A. Alt, MD, PhD
Arthur Wu, MD
Zara M. Patel MD
Olfaction (the sense of smell) is an important function of the nasal cavity and skull base that has become increasingly investigated due to its role in behavior, cognition, disease severity, quality of life (QOL), and sinonasal disease. Smelling is critical and important to our daily lives. For instance, the sense of smell can alert us to the smoke of a fire or the odor of a natural gas leak, dangerous fumes, or rotten/spoiled food. Importantly, 80% of our taste is related to smell. Remember the smell of your spouse’s perfume or cologne or the taste of your mother’s cooking? Taste and smell unlock some of our deepest memories. They are critical to our perception of the outside world, factor into almost all important social interactions in our society, and the loss of taste and smell can negatively affect one’s QOL.
THE RELATIONSHIP BETWEEN TASTE AND SMELL
Most patients who complain of a loss of taste actually suffer from a loss of smell. The majority of a food’s flavor comes from our ability to smell it. The tongue is our taste organ, and can sense salty, sweet, sour, bitter, and umami (or savory). The rest of a food’s flavor is provided by our sense of smell. The difference between an apple or a pear or between chocolate and caramel are mostly differentiated by our ability to smell. This is why it is difficult to appreciate a food’s flavor when one has nasal obstruction from a cold, stuffy nose, or rhinosinusitis. Disorders of smell and taste are fairly common, affecting approximately 2 million people in the United States. However, true disorders of only taste are quite rare.
THE ANATOMY OF SMELL
The part of the nose that senses smell is a small area high up in the roof of the nose called the olfactory cleft. Each side of the nose has its own olfactory cleft, where special cells can sense the various odors carried in the air we breathe (Image 1). These cells send their signals to the brain via the olfactory nerve.
Causes of Loss of Smell
Loss of smell can result from a physical blockage of the nose or from damage to the olfactory cleft or nerve. Common obstructive causes of smell loss include a deviated septum, nasal allergies, and swelling of the mucosa caused by chronic rhinosinusitis (CRS) with or without nasal polyps. Those patients with structural abnormalities such as septal deviation and inferior turbinate hypertrophy (enlargement) can present with reduced smell, although they are more likely to have a normal sense of smell compared to those patients who have more of an inflammatory disease such as those with asthma, aspirin intolerance, and CRS. Those patients with CRS with nasal polyps have some of the most severe loss of smell secondary to both obstruction and underlying inflammation. Nasal polyps, are small non-cancerous, inflammatory growths in the nose, that can also block smells from entering the nose (Image 2). Inflammation from CRS can cause loss of smell by physically obstructing access to the smell nerves. However, the chronic inflammation from CRS can also permanently affect smell by damaging the special cells and nerves of the smell pathway. Evidence is accumulating that suggests that by controlling the underlying inflammation in the nose, we can improve the ability to smell.
The loss of smell may also be caused by benign or malignant tumors in the nose. Sometimes tumors can actually grow from the olfactory nerves themselves. A history of radiation treatment of sinonasal or other head and neck cancers can also cause the loss of smell as an unfortunate side effect.
The loss of smell can also present as a signal for other health problems including obesity, diabetes, hypertension, malnutrition, Parkinson's disease, Alzheimer's disease, multiple sclerosis, and Korsakoff's psychosis (a dementia caused by severe malnutrition or alcoholism).
The common cold
(also called an upper respiratory infection) often causes inflammation in the nose impairing smell via swelling and obstruction. Usually this loss of smell returns days to weeks after the initial illness has resolved, but a small number of patients experience a more prolonged or permanent loss of smell after a cold. This permanent loss is thought to be caused by direct injury and inflammation of the olfactory neurons by the viruses that cause the cold. It is assumed that patients without a clear reason for their loss of smell and taste likely have viral-related loss of smell but either do not remember the inciting incident, or the virus directly attacked the nerve cells themselves without causing significant nasal cavity inflammation.
There has been an association between zinc nasal sprays, previously used to prevent or shorten flu or colds, and sudden and permanent smell loss. Patients who have had this type of loss describe immediate burning sensation when using the spray followed by loss of smell. The specific brand of Zicam nasal spray has since been removed from shelves and is no longer available in the United States.
Trauma to the head, as may occur in a car accident, can also cause injury to the brain or the olfactory nerves causing temporary or permanent loss. It is also well described that the sense of smell diminishes with age in a similar fashion as vision and hearing.
There are many other reasons why people may lose their sense of smell, including metabolic, endocrinologic, infectious, congenital, developmental or drug-related reasons. Your otolaryngologist will know how to look for these other causes.
Testing for Loss of Smell
The physician examining a patient for loss of smell will need to perform an exam of the inside of the nose, usually with nasal endoscopy
. The physician will be looking for a deviated septum, polyps, inflammation, or other causes for physical blockage of the olfactory cleft and sources of smell loss.
The physician may also use a scratch-and-sniff or other smelling test to quantify the amount of smell loss a patient has. This enables the physician to quantify the amount of smell loss. We call a decreased sense of smell “hyposmia” and complete loss “anosmia”.
In the event that there is not an obvious reason for the loss of smell and there is a concern for further investigation into the loss, a physician may order an MRI to evaluate the olfactory system from the cleft to the nerve pathway and brain for any abnormalities.
Treatments for Loss of Smell
If the physician determines a patient’s loss of smell to be caused by allergies or other inflammatory diseases, he/she may recommend nasal or oral steroids and antihistamines to decrease the inflammation. Surgery
may be suggested if the patient is found to have nasal polyps, chronic sinusitis
, deviated septum
, or other surgically treatable disorders. However, our knowledge in recommending surgery to improve olfaction is limited, as there is no good evidence currently to suggest either surgery or medical management has a better outcome with regard to smelling ability. As mentioned before, despite medical or surgical treatments, certain inflammatory diseases like chronic rhinosinusitis can cause permanent loss, and return of smell to a totally normal state may not be possible. Although there is some controversy, many patients who have obstructive causes for their loss of smell usually do report significant improvements after both medical and surgical treatments. Patients with a permanent loss of smell after a viral illness, after trauma, or those without a clear reason for their loss of smell unfortunately have limited treatment options at this time. A short course of oral steroids (prednisone) may be beneficial in some patients. Recent research also has demonstrated that attempting to retrain one’s sense of smell after loss (a technique called “olfactory training”) can be beneficial. The idea behind this method is to try and stimulate the natural and unique regenerative capabilities of the olfactory system that have become damaged by the injury. Exposing oneself to strong and familiar odors several times a day may thus help to promote the recovery of smell. This may work to rewire the brain’s neural network as one might do during physical rehabilitation if recovering from a stroke. Research is ongoing to determine the exact causes and find additional treatments for this type of smell loss. Other medications that may have some benefit to improve the loss of smell include phentoxifylline, gabapentin, theophylline, anti-depressants, and long-term antibiotics. However, further studies need to be done if any evidence based clinical treatment is to be found.
Patients who suffer from the loss of smell or taste are encouraged to seek medical attention as soon as possible to determine the cause of their loss. While certain smell losses are permanent, others can be effectively treated medically or surgically by a nose and sinus specialist, especially if treated soon after the onset of loss.
Image 1: During a nasal endoscopy, an otolaryngologist can see the olfactory cleft between the nasal septum and the middle turbinate (MT) in a normal patient.
Image 2: A patient with a polyp (P) between the septum and middle turbinate (MT) that is affecting smell. The surgeon is pointing to the polyp with a metal suction instrument during a surgery to remove the patient’s polyps.
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