Patients with Parkinson's disease exhibit not only a reduced sense of smell but also specific changes in how they perceive odors. Unlike healthy individuals, they do not hold their breath when exposed to unpleasant smells and may inhale such scents for longer periods, according to the results of a recent study.

Features of Parkinson's Disease

Parkinson's disease is a progressive neurodegenerative disorder that affects the central nervous system. The main cause is believed to be the death of neurons that produce dopamine in certain areas of the brain. Key symptoms include tremors, slowed movements, increased muscle tone, stiffness, balance difficulties, as well as behavioral, cognitive, and psychological disturbances.

One of the early signs of the disease is anosmia—the loss of the sense of smell. A diminished ability to distinguish and identify odors can appear years or even decades before the onset of motor symptoms. For this reason, standard smell tests are sometimes used for early diagnosis.

Diagnostic Challenges

Standard tests assess overall olfactory function, such as the ability to detect or name a scent. However, loss of smell is common and can be related to aging, viral infections, or sinus problems. This makes it difficult to determine whether poor test results are due to Parkinson's disease or other causes.

Changes in Sensory Processing

Recent research has shown that Parkinson's disease affects not only sensory acuity but also the way sensory information is processed. For example, patients may experience altered perception of colors or rhythms. Researchers have suggested that similar changes occur in the sense of smell.

Study Design

The study involved 94 participants: 33 patients diagnosed with Parkinson's disease, 33 healthy individuals matched by age and gender (control group), and 28 participants with loss of smell due to causes unrelated to Parkinson's. The average age of participants ranged from 58 to 66 years.

Initially, all participants took a standard smell test, which revealed reduced olfactory function in both groups except the control group, but did not identify the cause. Next, participants were asked to smell containers with a lemon scent, a fecal odor, or no odor at all. They were required to rate the intensity and pleasantness of the smell on a scale from 1 to 100.

Results

Patients with Parkinson's disease rated the intensity of the lemon scent similarly to healthy participants. However, they rated the pleasantness of this aroma as low as those with non-Parkinsonian loss of smell. This indicates specific changes in the evaluation of odor pleasantness, rather than a general dulling of the senses.

The most noticeable differences emerged when analyzing involuntary physiological reactions to odors. Typically, people take deeper breaths when smelling pleasant scents and hold their breath or take shorter breaths with unpleasant ones. Participants from the control group and those with non-Parkinsonian loss of smell followed this pattern. However, patients with Parkinson's disease did not adjust their breathing based on the type of odor and often inhaled unpleasant smells more deeply and for longer than the lemon scent.

Significance of New Tests

The discovered anomaly may serve as an additional diagnostic marker for Parkinson's disease. A new test, based solely on reactions to odors, allowed researchers to distinguish patients with Parkinson's from other study participants with high accuracy.