If you had to, would you rather become blind or loose your sense of smell? A classic question in the child’s game “would you rather?” The popular answer seems obvious: smell. Often the most overlooked sense, our ability to detect scent, or olfaction, is actually more important than you might think. Researchers have found that loosing your sense of smell is a powerful indicator of cognitive dysfunction and actually can predict neurodegenerative disorders like Alzheimer’s and Parkinson’s Disease.
Our ability to smell and closely related ability to taste, are regarded as our oldest and most evolutionarily important senses. It has been hypothesized that they originated billions of years ago in the earliest kinds of cells, which needed a mechanism to respond to harmful and helpful chemical signatures in their environment. Additionally, olfactory sensation occurs in the most primitive part of the brain – the limbic system – and unlike all of our other senses, does not rely on higher brain processing through a more advanced structure, the thalamus.
We often don’t think about our sense of smell as being particularly important, just associating it with the annoying fragrance of a sweaty gym locker room or the pleasant aroma of a flower garden. However poorly we might think of it, loosing one’s sense of smell, a condition called anosmia, is actually quite dangerous. Determining if food has gone bad or detecting invisible smoke in the air are very important for alerting the body to danger. Research has also examined the possible role that olfaction plays in sexual and social elements of human interaction both consciously and subconsciously.
However, up to now there has been limited clinical use of our sense of smell, but the body of research linking olfaction to brain dysfunction is growing. A study found that around 90% of people with Parkinson’s have trouble detecting smell, and specific anosmias may turn out to be a better predictor of developing Alzheimer’s disease than poor performance on memory or cognitive tests. Although preliminary results seem promising, large scale trials needed before the link between disease and loss of smell can be confirmed.
First off, we all have different baselines for smell, so the extent of a friend’s inability to detect a stinky tuna sandwich won’t be meaningful until an objective measure of smell is made. Additionally, smell tests are not standardized and often problems are only noticed when they become quite severe. A study found that about 1/3 patients do not note a problem before being tested. Currently, Japan is attempting to implement a standard test called the UPSIT Series 4-item Pocket Smell Test which can diagnose olfaction problems in in 3 minutes. Patients scratch scented strips and attempt to identify the smell. Standardizing olfactory tests in annual check ups might help alert doctors to the onset of neurological problems. Of course, deficits in smell alone are not a universal marker for cognitive problems.
Anosmia is a common disorder that occurs most frequently from viral and bacterial infections (even the common cold), but is also linked with smoking, being male and aging. Thus problems in olfaction alone cannot be relied upon to predict the onset of neurodegenerative disorders, but difficulties with smell coupled with other biological markers (like the presence of certain genes or build up of certain proteins in the brain) can be predictive. For example, individuals with anosmia and at least 1 genetic risk factor for Alzheimer’s were 5 times more likely to experience cognitive decline than those with could not smell but had no other risk factors.
Olfaction is a complicated but interesting area of research, not just for early diagnosis, but also for understanding brain aging and cognitive deterioration. Just don’t go diagnosing yourself with early Alzheimer’s just because you can’t smell the difference between rose and rosemary.