When James Parkinson first described the shaking palsy now known as Parkinson’s disease in 1817, he stated that there was “the absence of injury to the senses”. Of course we now know that this illness affects many body systems, not our movement alone. It was in 1975 that a study was published that described olfactory dysfunction as being part of the symptoms that Parkinson’s patients experienced.
Olfactory dysfunction refers to a loss of smell and this phenomenon occurs in almost all cases of Parkinson’s (more than 95%). In fact it is more common than one of the hallmark symptoms of PD – resting tremor. Not to imply that all individuals with a lack of smell have Parkinson’s. There are many other causes of this problem including sinus problems, viral infection and trauma.
Compared to most other investigations, olfactory testing is relatively inexpensive and non-invasive. One of the most commonly used and reliable tests is The University of Pennsylvania Smell Identification Test (UPSIT) where subjects are given a 40-item test card that they scratch and smell. For each item they are given four possible choices to identify the odor. There are of course other methods of evaluation but the UPSIT is most widely used in research.
So why is this important? It goes beyond the annoyance of lack of smell and how it also blunts our sense of taste. It may provide some information from a clinical standpoint as well.
(1) It may prove to be a potential biomarker. Olfactory dysfunction may precede the onset of motor symptoms of Parkinson’s disease by several years. It is difficult for patients to retrospectively recognize a reduction in their sense of smell but there have been studies where relatives of Parkinson’s patients have been tested for their sense of smell and then followed for a number years. Poor olfactory performance was associated with an increased risk for developing Parkinson’s disease within 5 years. This may be important in the future to identify people at risk if we have medications that can prevent the progression of the disease (what we call disease modifying medications).
(2 )It can help distinguish between Parkinson’s disease and other types of Parkinsonism. Parkinsonismis a broad term that refers to a group of neurological conditions that present with combinations of motor problems including resting tremor, rigidity, flexed posture, “freezing”, loss of postural reflexes and slowness of movement. Other conditions that fit into this category include multi-system atrophy (MSA), progressive supranuclear palsy (PSP) and corticobasal degeneration (CBD). Compared to PD, sense of smell is less affected in MSA and is essentially preserved in PSP and CBD. Although it has the potential to distinguish Parkinson’s from other parkinsonian syndromes, it is not being used as a formal recommendation because there are not enough large scale studies to show that it is accurate enough.
(3) Olfactory dysfunction may be used as a marker for disease progression in the early stages of Parkinson’s. Previously it was thought that loss of smell was just that – loss of smell. But we now know that if looked at very closely and evaluated carefully, the decrease in smell progresses with time and may give information regarding disease duration and progression.
Duda, John E., and Matthew B. Stern. "Olfactory Dysfunction." Parkinson's Disease: Non-motor and Non-dopaminergic Features. By C. W. Olanow, F. Stocchi, and Anthony E. Lang. Chichester, West Sussex, UK: Wiley-Blackwell, 2011. 304-12. Print.
Haehner, Antje, Thomas Hummel, and Heinz Reichmann. "Olfactory Dysfunction as a Diagnostic Marker for Parkinson's Disease." Expert Review of Neurotherapeutics9.12 (2009): 1773-779. Web.