Spring, 2010
A reduced sense of smell is often one of the earliest indicators of Parkinson’s disease. People with Parkinson’s disease are generally less able to identify odors or tell different odors apart, as well as being less sensitive to smells in general.
These symptoms may be related to a loss of nerve cells in olfactory or “smell” regions of the brain. Smell-related areas of the brain are some of the first places to show a clustering of protein known as alpha-synuclein inside nerve cells. Eventually, nearly all nerve cells that die in Parkinson’s disease show this clustering. However, scientists do not know for sure whether this protein clustering causes the nerve cells to die or whether it protects against damage that eventually overwhelms the cells.
Some investigators have even proposed that Parkinson’s disease is caused by something that we inhale through our noses – probably a virus. They contend that this would explain why smell-related nerve cells are affected so early in the disease. The theory of a virus that enters the body through the nose does not yet have much support. No one has yet identified a virus that causes Parkinson’s disease. However, involvement of the sense of smell has clearly stimulated a lot of thinking and research.
An interesting thing about the smell-related nerve cells that die in Parkinson’s disease is that they do not contain a chemical called dopamine. Scientists used to think that the only types of cells that died in Parkinson’s disease were those that contained dopamine. The loss of dopamine-containing nerve cells is responsible for the shaking that occurs in Parkinson’s disease, as well as the difficulty of beginning and maintaining movements. Scientists now know that dopamine-containing cells are not the only cells that die in Parkinson’s disease. The smell-related cells are an example of this. Instead of dopamine, these cells contain a chemical known as acetylcholine.
Although a reduced sense of smell is not disabling for patients, it is still a crucial symptom to study. Scientists may be able to determine important clues about the disease by comparing its effects on smell-related nerve cells with its effects on other types of cells. Greater problems with the sense of smell may tell us something about the progress of Parkinson’s disease. An evaluation of smell may even be helpful in making the diagnosis of Parkinson’s disease, particularly in earlier stages of the disease where motor problems are not as prominent.
Clearly, there is much to be learned about the sense of smell in Parkinson’s disease and the nerve damage that this may represent. It seems likely that future treatments for Parkinson’s disease will be able to help prevent its progression. Testing for a reduced sense of smell may be useful in diagnosing and treating the disease before it can cause movement problems.
This article originally appeared on page 5 of the Spring, 2010 PMDF newsletter.