We are somewhat preoccupied with smell at PREDICT-PD; smell loss is one of the most sensitive risk factors for development of Parkinson's - although it is non specific and may be seen in a number of conditions. It is particularly interesting as it can be studied non-invasively and remotely. While self-reported smell loss is less reliable, objective tests such as the University of Pennsylvania Smell Idenfification Test (UPSIT) have a high degree of accuracy and can be completed at home by research participants.
This recognition sits well with the finding from post-mortem studies that there are early pathological changes in the olfactory bulb, the area just above the nose which receives smell inputs. The progressive brain changes in Parkinsons, defined by pathologist Heiko Braak, are now known as Braak stages, and the earliest stage, Braak stage 1 involves olfactory bulb involvement.
While smell loss is well characterised, its link with other features and particularly cognitive changes in in Parkinson's is less well explored. Smell loss is also associated with Alzheimer's so it wouldn't be surprising if it was also linked to cognitive changes in Parkinson's. The study below, from researchers in South Korea, looked at the relationship between these two features.
They took a group of people just diagnosed with Parkinson's and compared those with cognitive impairment to those without, showing poorer smell in all aspects of the smell tests in patients with cognitive impairment. In many ways, this goes against the 'Braak hypothesis' which posits that pathology in the higher cortical areas and the cognitive impairment which goes with it is a later phenomenon. It would be interesting to see if smell loss was particularly associated with specific domains of cognition and the areas more directly connected with the olfactory bulb.
As we have previously mentioned, cognitive impairment may be related to many underlying pathologies and it may be that the pathology described in Braak staging is not primarily responsible for cognitive impairment in Parkinson's. Certainly more work is needed in this area to tease out these relationships.
-Anna
Parkinsonism Relat Disord. 2018 Jan;46:69-73. doi: 10.1016/j.parkreldis.2017.11.334. Epub 2017 Nov 14.
Olfactory dysfunctions in drug-naïve Parkinson's disease with mild cognitive impairment.
Park JW, Kwon DY, Choi JH, Park MH, Yoon HK.
BACKGROUND:
Evaluation of olfactory function is valuable for the detection of pre-motor state of Parkinson's disease (PD). PD patients have an increased risk of associated dementia and one-third of PD patients have mild cognitive impairment (MCI) at the time of diagnosis. However, the characteristics of olfactory dysfunction in PD-MCI patients are unclear. This study examined the relationship between olfactory dysfunction and cognitive function in drug-naïve PD at the time of diagnosis with the patterns of olfactory function in PD-MCI patients using the Korean version of the Sniffin' stick test II (KVSS II). METHODS:
A total of 66 drug-naïve PD patients were enrolled. A neuropsychiatric assessment battery and KVSS II were performed. For the statistical analyses, univariate, multivariable linear regression and Student's t-test were used to determine the relationship between the variables and olfactory function. RESULTS:
Olfactory dysfunction was more prevalent in the PD-MCI group than in the PD-normal cognition (PD-CN) group. Each domains of odor threshold, discrimination, identification and total olfactory score were more impaired in the PD-MCI group than the PD-CN group. Whether cognitive impairment was single or multiple domain was not affected. CONCLUSION:
PD-MCI is more likely to be associated with severe olfactory impairment than PD-CN. There may be more extensive neurodegenerative processes affecting olfaction in PD-MCI patients. With further investigation and validation using neuropathological data, an objective olfactory function test could be used as a tool to evaluate disease progression. Further studies with prospective design investigating the prognostic value of olfactory dysfunction in PD-MCI patients are essential.
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