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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.
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.
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.
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.