How Does Our Sense of Smell Work?
The sense of smell is part of our chemical sensing system, or the chemosenses. Sensory cells in our nose, mouth and throat have a role in helping us interpret smells, as well as taste flavors. Microscopic molecules released by the substances around us (foods, flowers, etc.) stimulate these sensory cells. Once the cells detect the molecules they send messages to our brains, where we identify the smell. Olfactory, or smell nerve cells, are stimulated by the odors around us - the fragrance of a gardenia or the smell of bread baking. These nerve cells are found in a small patch of tissue high inside the nose, and they connect directly to the brain. Our sense of smell is also influenced by something called the common chemical sense. This sense involves nerve endings in our eyes, nose, mouth and throat, especially those on moist surfaces. Beyond smell and taste, these nerve endings help us sense the feelings stimulated by different substances, such as the eye-watering potency of an onion or the refreshing cool of peppermint. It's a surprise to many people to learn that flavors are recognized mainly through the sense of smell. Along with texture, temperature, and the sensations from the common chemical sense, the perception of flavor comes from a combination of odors and taste. Without the olfactory cells, familiar flavors like coffee or oranges would be harder to distinguish.
What Are the Smell Disorders?
People who experience smell disorders experience either a loss in their ability to smell or changes in the way they perceive odors. As for loss of the sense of smell, some people have hyposmia, which is when their ability to detect odor is reduced. Other people can't detect odor at all, which is called anosmia. As for changes in the perception of odors, some people notice that familiar odors become distorted. Or, an odor that usually smells pleasant instead smells foul. Still other people may perceive a smell that isn't present at all.
What Causes Smell Disorders?
Smell disorders have many causes, some clearer than others. Most people who develop a smell disorder have recently experienced an illness or an injury. Common triggers are upper respiratory infections and head injuries. Among other causes of smell disorders are polyps in the nasal cavities, sinus infections, hormonal disturbances, or dental problems. Exposure to certain chemicals, such as insecticides and solvents, and some medicines have also been associated with smell disorders. People with head and neck cancers who receive radiation treatment are also among those who experience problems with their sense of smell.
How Are Smell Disorders Diagnosed?
Doctors and scientists have developed tests to determine the extent and nature of a person's smell disorder. Tests are designed to measure the smallest amount of odor patients can detect as well as their accuracy in identifying different smells. In fact, an easily administered "scratch and sniff" test allows a person to scratch pieces of paper treated to release different odors, sniff them and try to identify each odor from a list of possibilities. In this way, doctors can easily determine whether patients have hyposmia, anosmia or another kind of smell disorder.
Are Smell Disorders Serious?
Yes. Like all of our senses, our sense of smell plays an important part in our lives. The sense of smell often serves as a first warning signal, alerting us to the smoke of a fire or the odor of a natural gas leak and dangerous fumes. Perhaps more important is that our chemosenses are sometimes a signal of serious health problems. Obesity, diabetes, hypertension, malnutrition, Parkinson's disease, Alzheimer's disease, multiple sclerosis, and Korsakoff's psychosis are all accompanied or signaled by chemosensory problems like smell disorders.
Can Smell Disorders Be Treated?
Yes. Some people experience relief from smell disorders. Since certain medications can cause a problem, adjusting or changing that medicine may ease its effect on the sense of smell. Others recover their ability to smell when the illness causing their olfactory problem resolves. For patients with nasal obstructions such as polyps, surgery can remove the obstructions and restore airflow. Not infrequently, people enjoy a spontaneous recovery because olfactory neurons may regenerate following damage.
What Research is Being Done?
The NIDCD supports basic and clinical investigations of chemosensory disorders at institutions across the nation. Some of these studies are conducted at several chemosensory research centers, where scientists are making advances that help them understand our olfactory system and may lead to new treatments for smell disorders. Some of the most recent research into our sense of smell is also the most exciting. Though a complete understanding of the uniquely sophisticated olfactory system is still in progress, recent studies on how receptors recognize odors, together with new technology, have revealed some long-hidden secrets to how the olfactory system manages to detect and discriminate between the many chemical compounds that form odors. Besides uncovering the physical mechanisms our bodies use to accomplish the act of identifying smell, these findings are helping scientists view the system as a model for other molecular sensory systems in the body. Further, scientists are confident that they are now laying the foundation to understanding the finest details about our sense of smell--research that may help them understand how smell affects and interacts with other physiological processes. Since scientists began studying the olfactory system, much has been discovered about how our chemosenses work, especially in how they are affected by aging. Like other senses in our bodies, our sense of smell can be greatly affected simply by our growing older. In fact, scientists have found that the sense of smell begins to decline after age 60. Women at all ages are generally more accurate than men in identifying odors, although smoking can adversely affect that ability in both men and women. Another area of discovery has been the olfactory system's reaction to different medications. Like our sense of taste, our sense of smell can be damaged by certain medicine. Surprisingly, other medications, especially those prescribed for allergies, have been associated with an improvement of the sense of smell. Scientists are working to find out why this is so and develop drugs that can be used specifically to help restore the sense of smell to patients who have lost it. Also, smell cells (along with taste cells) are the only sensory cells that are regularly replaced throughout the life span. Scientists are examining these phenomena, which may provide ways to replace these and other damaged sensory and nerve cells. The National Institute on Deafness and Other Communication Disorders' research program goals for chemosensory sciences include:
- Promoting the regeneration of sensory and nerve cells
- Appreciating the effects of the environment (such as gasoline fumes, chemicals, and extremes of relative humidity and temperature) on smell and taste
- Preventing the effects of aging
- Preventing infectious agents and toxins from reaching the brain through the olfactory nerve
- Developing new diagnostic tests
- Understanding associations between chemosensory disorders and altered food intake in aging as well as in various chronic illnesses.
- Improving treatment methods and rehabilitation strategies
What Can I Do to Help Myself?
The best thing you can do is see a doctor. Proper diagnosis by a trained professional, such as an otolaryngologist, is important. These physicians specialize in disorders of the head and neck, especially those related to the ear, nose and throat. Diagnosis may lead to an effective treatment of the underlying cause of your smell disorder. Many types of smell disorders are curable, and for those that are not, counseling is available to help patients cope.