Functional Status of Rat Facial Muscles Based on Expression Rating Scales

Facial expressions result from the execution of motor commands and reflexes through the seventh cranial nerve (facial nerve). The purpose of this study was to develop a system for rating observable facial reflexes and to validate this subjective system against objective facial muscle contractile function in a rat model. The authors tested the hypothesis that compared to a control group, rats with a unilateral facial nerve lesion would show quantifiable changes in facial expression that directly reflect facial muscle contractile capacity. A second purpose was to determine the characteristics of long-term denervated rat facial muscles for use as a negative control group for reinnervation studies.

Criteria were established across a 3-point scale for facial expression including eye blink, vibrissae whisking, and facial symmetry tests. Fifteen male rats were pretested using the expression battery. Facial muscle denervation was performed by facial nerve-gap removal and tying nerve ends (denervated group, n = 9). Six unoperated rats served as the control group. Ratings on the expression battery were completed at weekly intervals for 2 months following the pretest/surgery week and again prior to facial muscle force testing. Maximal isometric forces for the orbicularis oris and superior orbicularis oculi facial muscles were measured in situ by direct stimulation. Expression test validity was determined by Pearson correlation on pairs of expression test scores matched with muscle force values for each rat.

Denervated group oculi (−65%) and oris (−52%) muscles produced less maximal force when compared with control group muscles. The denervated group scored > 60% fewer points on the expression rating scales. Each expression test correlated significantly with oculi and oris muscle force measurements (r > 0.72). All comparisons were significant at p < 0.05.

The rating scales for facial expression reflect muscle force capacity and therefore can be used to infer functional status of rat facial muscles. The ratings accurately discriminate between normal and denervated group muscles. Future work will examine the sensitivity of the ratings to partially innervated or reinnervated facial muscles.