There is no single test available to diagnose Parkinson’s disease, and clinical diagnosis is usually based on motor symptoms such as tremor, slow movements, rigidity, and postural instability. Brain imaging is performed to confirm these motor symptoms long after the disease has developed. However, many non-motor symptoms, such as loss of smell, constipation, and sleep problems, may appear several years before the onset of the disease.

A smell test is a simple, sensitive but nonspecific test, which is still under development for the diagnosis of Parkinson’s prior to the appearance of motor symptoms.

Why a smell test is performed

Loss of smell (also called olfactory dysfunction or hyposmia) is one of the most common and best characterized non-motor symptoms of Parkinson’s disease. Studies suggest that patients may lose their sense of smell up to 10 years before other symptoms appear.

Smell function studies showed that about 96 percent of newly diagnosed Parkinson’s disease patients have lost some ability to smell. Recent studies also suggest that this could be used to identify at-risk patients earlier. Moreover, testing for loss of smell is inexpensive and easy, making it an attractive option for the diagnosis of Parkinson’s.

Smell tests may also serve as a differential diagnostic tool to distinguish between Parkinson’s disease and other conditions such as progressive supranuclear palsy, corticobasal degeneration, essential tremor, atypical parkinsonian syndromes, drug-induced parkinsonism, and vascular or other causes of parkinsonism.

In addition, smell tests could be used to predict whether a first-degree relative of someone with Parkinson’s disease may also develop the condition in the future.

How a smell test is performed

The vast majority of all olfactory studies in Parkinson’s disease patients have used the University of Pennsylvania smell identification test. This test, developed in the early 1980s, is a “scratch and sniff” test, made up of 40 microencapsulated odorants. The person being tested is required to choose among four descriptors for each odorant. This test has been adapted for use in many different languages and cultures.

Clinical studies

In an observational study (NCT00096876) published in the scientific journal Annals of Neurology, researchers studied the sense of smell in 361 first-degree relatives of Parkinson’s disease patients, none of whom exhibited any symptoms of the disease. The study found that relatives who had a poor sense of smell had about a 10 percent higher risk of developing Parkinson’s over the next five years. These relatives also exhibited significantly reduced levels of the dopamine transporter in parts of the brain that degenerate in the disease, as determined by brain imaging.

Another study recruited more than 2,500 people involved in the health, aging, and body composition study to determine the relationship between sense of smell and the risk of developing Parkinson’s disease during a 10-year follow-up period. The study used a simple scratch and sniff test in which participants had to identify common odors such as cinnamon, lemon, gasoline, and onion. Results from the study showed that those who had a poor sense of smell were nearly five times more likely to develop Parkinson’s than those with a good sense of smell.

The smell test is nonspecific

It is important to note that smell tests are nonspecific; about 15 to 25 percent of older people have a reduced sense of smell, but only a small number of them develop Parkinson’s. Since loss of smell can be caused by other factors such as blocked sinuses or head injury, smell tests cannot be used for the accurate diagnosis of Parkinson’s disease.

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