Mathematical Validation of the Modified Sunnybrook Facial Grading System Using Four-dimensional Imaging

Despite the limited validity of the Sunnybrook grading index, it is routinely used for the clinical evaluation of facial palsy. This study aimed to assess the dynamic asymmetry in unilateral facial palsy and mathematically validate a modified version of the Sunnybrook facial grading system.

The Sunnybrook facial grading system was modified to provide more descriptions of the measured parameters of the distorted facial expression in unilateral facial paralysis. This correlation study was conducted on 16 patients with unilateral facial palsy and a matched control group. Three-dimensional video recordings of six facial expressions - rest, maximum smile, cheek puff, lip purse, eyebrow raising, and eye closure - were used for each case in the analysis. Advanced geometric morphometrics were applied to quantify facial asymmetry and morphology throughout the course of each expression. Seven professional assessors graded facial asymmetry for the 16 cases, twice, using the modified Sunnybrook index. Cross-correlations between the objective mathematical measurements and the subjective clinical grades were calculated.

The inter- and intra-observer reproducibility of the modified Sunnybrook index was high (r = -0.8). Significant positive correlations were detected between the clinical grading of facial palsy and the mathematical measurements at rest, maximum smile, lip purse, and raising of eyebrows. The correlations between the modified Sunnybrook index and mathematical measurements were poor for cheek puff and forceful eye closure.

The modified Sunnybrook grading index proved reproducible and mathematically valid for the grading of unilateral facial paralysis in most facial expressions, except for cheek puff and forceful eye closure.