Olfactory dysfunction and dementia in newly diagnosed patients with Parkinson's disease

Studies report that up to 90% of patients with idiopathic Parkinson's disease (PD) have olfactory dysfunction (hyposmia). Hyposmia has also been connected to cognitive impairment and dementia in PD, but no studies of newly diagnosed patients followed longer than three years exists. The present study investigates the prevalence of olfactory dysfunction at PD diagnosis, how it evolves over time and whether hyposmia increases the risk of dementia in Parkinson's disease.

Olfactory function was assessed with Brief Smell Identification Test (B-SIT) in 125 newly diagnosed patients with PD. They were followed for a maximum of 10 years (median six years) with extensive investigations at baseline, 12, 36, 60 and 96 months. Patients with B-SIT<9 were considered hyposmic.

Hyposmia was found in 73% of the patients at diagnosis. During the follow up period of ten years 42 (46%) patients with hyposmia at baseline developed dementia compared to seven (21%) of the normosmic patients. Cox proportional hazards model showed that hyposmia at baseline (controlled for age, gender, UPDRS III and Mild Cognitive Impairment) increased the risk of developing dementia (hazard ratio (95%CI): 3.29 (1.44-7.52), p = 0.005). Only one of 22 patients with normal cognition and normal olfaction at baseline developed dementia.

Olfactory dysfunction was common at the time of PD diagnosis and increased the risk of dementia up to ten years after PD diagnosis regardless of baseline cognitive function. Normal olfaction together with normal cognition at baseline predicted a benign cognitive course up to ten years after diagnosis.