Olfactory hallucinations in Parkinson’s disease patients and the role of their evaluation in clinical practice.

We read with great interest the article by Mehanna and Colleagues [1] reporting on the frequency of olfactory hallucinations (OH) in patients with Parkinson’s disease (PD) This is a very remarkable topic, because only few previous studies have evaluated the prevalence of OH in PD patients [2–7], highlighting the need to properly identify OH in the daily clinical practice. We were really impressed about the 18.2% frequency of OH in the Mehanna’s sample, which was considerably higher than the 2–11% reported in other series [2–7]. Indeed, this frequency was surprisingly higher than visual hallucinations, commonly considered the most frequent hallucinations in PD [2,3,7], which were described in 23 patients (17%). However, we retain that the recently observed higher frequency of OH could not completely reflect the true frequency of the phenomenon in the population but could rather be related to several methodological issues. Firstly, OH detection was based on answering the standardized question “have you ever experienced in your day-to-day functioning any unusual smells or odors, which are not noticed by others around you?” The lack of any temporal limitation made the question open to recall bias and detection of false positives.