Asymmetry index of Blink Reflex Recovery Cycle differentiates Parkinson’s disease from atypical Parkinsonian syndromes

Background: Differential diagnosis between Parkinson’s disease (PD) and atypical Parkinsonian syndromes (APS), such as multiple system atrophy (MSA) and progressive supranuclear palsy (PSP), is often difficult because of overlap of common clinical features. We evaluated R2 Blink Reflex Recovery Cycle (R2BRRC) in drug-naive PD patients and in MSA and PSP patients to differentiate early PD from APS. Methods: We investigated 43 patients: 15 drug-naive PD patients, 16 MSA patients, and 12 PSP patients. R2BRRC was evaluated bilaterally at interstimulus intervals (ISIs) of 100, 150, 200, 300, 400, 500, and 750 ms. An asymmetry index (AI) of R2BRRC for each ISI was computed. Results: R2BRRC of PD patients showed an increased brainstem excitability for less affected side (LAS) stimulation at ISIs of 100, 150, 200 (p < 0.001), and 300 ms (p = 0.03) compared to more affected side (MAS) stimulation, whereas no differences between LAS and MAS stimulation were found in APS. AI of 0.87 at ISI of 100 ms differentiated PD from MSA with a sensitivity of 86.7% and a specificity of 100%, whereas AI of 0.78 at ISI of 100 ms permitted to discriminate PD from PSP with a sensitivity of 86.7% and a specificity of 91.7%. Conclusion: AI of R2BRRC may represent a reliable tool in differentiating PD from APS, especially at the early stage of the disease.