Abstract
Olfactory impairment is a central non-motor symptom in Parkinson's disease (PD). Previous studies have demonstrated that olfactory dysfunction is associated with mental illness and impaired cognition. The frequently investigated olfactory functions are odor detection, discrimination, and identification. However, few studies have focused on odor recognition memory (ORM). ORM tasks involves episodic memory which therefore can facilitate the detection of dementia among patients with PD and consequently adjust their treatment. Thus, the aim of this systematic review is to summarize the existing research on ORM in PD. Databases and reference lists were used for data collection. Studies were included in the review if they met the eligibility criteria derived from the PICOS-framework. Quality evaluation of the studies was based on the STROBE-statement. Six studies with small samples were included in the analysis which demonstrated the scarce research on the subject. The studies targeting ORM were heterogenous and involved two main tasks: odor recognition and odor matching. The synthesis of the data demonstrated that PD patients performed significantly lower than controls on both tasks, especially on odor matching task. Only the odor recognition task exhibited a difference between patients with PD vs. Alzheimer's disease (AD). PD patients performed significantly better than AD patients. The findings based on the available limited data support the notion that odor recognition task can be of importance in identifying Parkinson's disease dementia (PDD). To investigate this hypothesis, future research needs to include larger samples of PD, PDD and AD patients executing the same odor recognition task.
Highlights
Parkinson’s disease (PD) is primarily identified by the evident debilitating motor symptoms: tremor, bradykinesia, rigidity, and postural instability. In 2015 ∼6 million people were reported to suffer from the disease worldwide, a number estimated to increase rapidly to 12 million by 2040 due to the increased elderly population (Dorsey et al, 2018)
To determine whether olfactory functions are correlated with disease progression in Alzheimer’s disease (AD) and PD To determine if asymmetry in olfactory function could be found on odor identification and odor recognition in patients with early-stage unilateral PD
The PD group’s best performance was not significantly differed from that of the control group Patients with PD performed significantly better in identification task and significantly worse in the matching task compared with controls
Summary
Parkinson’s disease (PD) is primarily identified by the evident debilitating motor symptoms: tremor, bradykinesia, rigidity, and postural instability. In 2015 ∼6 million people were reported to suffer from the disease worldwide, a number estimated to increase rapidly to 12 million by 2040 due to the increased elderly population (Dorsey et al, 2018). The prevalence of the olfactory dysfunction in PD is estimated to be 50–90% and one of the most studied NMS of the disease (Fullard et al, 2017). Patients with PD show an extensive alpha-synuclein pathology in anterior olfactory nucleus (AON), cortical nucleus of the amygdala, piriform cortex, olfactory tubercle, and entorhinal cortex. Another possible explanation for this dysfunction is the distorted levels of typically affected neurotransmitters in PD such as dopamine, acetylcholine, and serotonin (Fullard et al, 2017)
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