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Parkinson's disease (PD) patients can present deficits in all three main modalities of smell tests: odour detection threshold, discrimination and identification. Cross-sectional studies have shown no relationship between odour identification or detection threshold and disease severity, symptom duration or pharmacological treatment of PD, although odour discrimination might be worse in patients with more severe disease.1 Few longitudinal studies have addressed the antiparkinsonian treatment effect on olfaction. Apomorphine did not change the odour detection threshold when administered to PD patients.2 Quinn et al3 applied olfactory threshold tests to PD patients with motor fluctuations and noticed no change between ON and OFF states. Nevertheless, surgical treatment for PD using deep brain stimulation (DBS) of the subthalamic nucleus may improve olfaction in PD. Hummel and colleagues4 tested the odour detection threshold and discrimination in 11 PD patients with the DBS stimulator ON versus OFF, and noticed that discrimination improved while the stimulator was ON. Guo and colleagues5 tested PD patients before and after subthalamic nucleus DBS surgery, in addition to stimulator ON and OFF; they found no difference in olfactory measures before and after surgery but reported an improvement in odour identification threshold (but not detection threshold) …
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