Changes in a person’s sense of smell can sometimes offer insights into our overall health. Recent research indicates that alterations in smell may serve as an early indicator of neurological changes, including those associated with cognitive health. This connection has led to the development of smell tests as a way to assess brain function. These tests contribute to a broader understanding of cognitive well-being.
The Link Between Smell and Brain Health
The scientific basis for connecting smell loss to conditions like dementia lies in the intricate pathways of the brain. The olfactory nerve directly links cells that detect odors in the nose to the olfactory bulb at the base of the brain. This olfactory system is unique because it is the only cranial nerve exposed to the outside environment.
Brain regions involved in processing smell, such as the olfactory bulb and the entorhinal cortex, are among the first areas to show damage in conditions like Alzheimer’s disease. The plaques and tangles characteristic of Alzheimer’s often appear in these olfactory and memory-associated areas before developing in other parts of the brain. Consequently, changes in the sense of smell can sometimes precede the noticeable onset of other cognitive symptoms by several years. A rapid decline in the sense of smell during a period of normal cognition has been shown to predict features of Alzheimer’s disease and a higher risk of dementia.
How Smell Tests Work
Smell tests evaluate an individual’s ability to identify or detect specific odors in a standardized way. Common formats for these tests include “scratch-and-sniff” cards or devices similar to felt-tip pens that release specific scents.
Participants are presented with a series of odors, typically 5 to 40 different scents, and asked to identify them from a multiple-choice list. A common test might include odors like peppermint, fish, orange, rose, and leather. These tests are non-invasive and can even be administered at home.
Interpreting Test Results
A score on a smell test indicates an individual’s olfactory function. In a five-odor test, correctly identifying at least four out of five odors is generally considered a strong performance. Conversely, a low score, such as identifying fewer than four odors in this type of test, suggests impaired olfactory function.
A low score on a smell test does not by itself confirm a diagnosis of dementia. Instead, it serves as an indicator that further evaluation may be needed. Individuals who score poorly on these tests are at a higher risk of future cognitive decline or dementia compared to those with normal smell function. For example, participants unable to identify at least four out of five odors in one study were twice as likely to develop dementia five years later.
Role in Diagnosis and Limitations
Smell tests function as screening or assessment tools within a broader diagnostic process. They are integrated into a comprehensive neurological and cognitive evaluation, which typically includes cognitive assessments, brain imaging, and blood tests. While a low score can suggest an increased risk for memory decline, it does not specify the underlying cause, as various neurodegenerative diseases can affect the olfactory bulb.
Limitations of smell tests include other potential causes of smell loss that are unrelated to cognitive decline. These can range from head injuries and infections to general aging or the side effects of certain medications. There is natural variability in individual smell perception, which can influence test outcomes. For example, some studies indicate that only a percentage of individuals with low smell scores show specific brain changes associated with Alzheimer’s. A “positive” or low result on a smell test warrants further medical consultation to explore the cause and determine if there are underlying cognitive concerns.