Revisiting eye blink in Parkinson's disease

Abstract

Spontaneous blinking is a rapid and unconscious type of blinking that has been linked to several cognitive processes. Blink rate has been established as a reliable measure of cognitive processing and a predictor of dopamine-related cognitive function. Patients with Parkinson's disease (PD) have a reduced spontaneous blink rate. In this study, we propose an additional measure of blink characteristic, namely blink duration, and evaluate its alteration in a large cohort of 107 patients with PD. We also investigate the relationship between blink rate and blink duration and disease characteristics such as severity and dopaminergic neuronal loss. The results show that the blink rate is reduced, and blink duration is increased in patients compared to healthy controls. Blink rate is related to motor deficit severity and significantly correlated with dopamine depletion (dopamine transporter striatal binding ratio). Conversely, blink duration is related to non-motor aspects such as sleepiness. These findings highlight the potential of blink as a distinguishing feature of Parkinson's disease, underscoring the importance of incorporating blink assessments into standardized oculomotor testing protocols for PD.

Fig. 1

Distributions of blink features between HC and PD patients. (A) Blink rate comparison. The line at the center shows the mean. (B) Blink duration comparison. The x-axis presents two groups, and the y-axis is the blink duration in milliseconds. Each dot is the mean blink duration for each subject over the multiple blocks. The line at the center shows the mean. (C) Blink rate and blink duration correlation in PD. Each dot represents the value for a participant. The red robust regression line (β = − 0.37, P = 0.017) is significant. (D) Blink rate and blink duration correlation in controls. As in (C), each dot is data for an individual participant. The red robust regression line is not significant (β = − 0.02, P = 0.83).

Fig. 2

Correlations between blink features and disease severity scores. Each point is data for an individual participant. The red is the robust linear regression line (none of them are significant). (A) Blink rate versus UPDRS-I score: The x-axis is the UPDRS-I score, and the y-axis is the blink rate per minute. Each subject’s average blink rate over the blocks is shown using a filled black circular marker. The inset shows the Spearman correlation value (ρ) and P value. (B) Blink rate versus UPDRS-I score: The x-axis is UPDRS-II scores, and the y-axis is blink rate. The other figure conventions are the same as (A). (C) Blink rate versus UPDRS-III score: The x-axis is UPDRS-II scores, and the y-axis is blink rate. Each dot represents the value of a subject. Green markers are for dopamine medication-ON condition, and red markers are for dopamine medication-OFF condition. (D) Blink duration versus UPDRS-I score: The x-axis is UPDRS-I scores, and the y-axis is blink duration in milliseconds. The other figure conventions are the same as (A). (E) Blink duration versus UPDRS-II score: The x-axis is UPDRS-II scores, and the y-axis is blink duration. The other figure conventions are the same as (A). (F) Blink duration versus UPDRS-III score: The x-axis is UPDRS-III scores, and the y-axis is blink duration. Other figure conventions are the same as (C).

Fig. 3

Correlation of blink features with DaTScan SBR. Each black dot is the value for each participant. The linear regression line is shown in red (none of the regressions were significant). The inset shows the Spearman correlation value (ρ) and P value. The figure conventions are the same for all subplots. (A) Putamen SBR and blink rate. (B) Accumbens SBR and blink rate. (C) Caudate SBR and blink rate. (D) Putamen SBR and blink duration. (E) Accumbens SBR and blink duration. (F) Caudate SBR and blink duration.

Fig. 4

Blink features and ESS. (A) Blink rate and Epworth Score. Each data point represents the data of a participant in black circular markers. The unfilled markers are outliers (values that were outside three standard deviations of the data distribution) and were excluded from correlation calculations. The P value is defined as the values that were outside 3 standard deviations of the blink rate and blink duration distributions and the Spearman correlation is shown in the inset square box on the top right of each subplot. The red line shows a linear regression line on the data (none are significant). (B) Blink duration and Epworth Score. The conventions of the plot remain the same as (A).