The whistle-smile reflex in Parkinson's disease: an exploratory case-control study

Background: Parkinson's disease (PD) is a well-known neurodegenerative disease characterized by tremors, rigidity, and bradykinesia. In 1943, Frederic Hanes described the whistle-smile reflex (WSR), noting its absence in PD patients due to reduced facial movements. Objective: To assess the WSR's diagnostic performance. Methods: In the current cross-sectional study, we selected PD patients and healthy controls. They were filmed while asked to whistle, and a blinded physician assessed if the individuals smiled or not. Absence of smiling after whistling was considered a positive test. We stratified the smiling intensity into 4 groups. We calculated the odds ratio (OR) of the positive WSR in PD and asymptomatic patients, which was reported with CIs, sensitivity (SE), specificity (SP), positive predictive value (PPV), negative predictive value (NPV), and we plotted a receiver operating characteristic (ROC) curve. Results: We identified 43 patients with PD (mean age = 63 ± 10 years; 30% of female subjects) and 44 asymptomatic controls (mean age = 64 ± 10 years; 51% of female subjects). The WSR test was positive in 35/43 PD cases versus 7/44 in the asymptomatic group (OR = 23.1; 95%CI = 7.6-70.5; SE = 81.4%; SP = 84.1%; PPV = 83.3%; NPV = 82.2%). When stratified considering the group with the least intense smile as a positive test, the test was positive in 42/43 PD cases versus 23/44 in the asymptomatic group (OR = 38.3; 95%CI = 4.8-303.7; SE = 97.7%; SP = 47.7%; PPV = 64.6%; NPV = 95.4%). Conclusion: The WSR test showed a good performance in identifying PD patients; it may complement the physical examination, and its result may be a useful finding during the assessment of a patient with PD.