Rodent facial nerve recovery after selected lesions and repair techniques

Abstract

Background: Measuring rodent facial movements is a reliable method for studying recovery from facial nerve manipulation and for examining the behavioral correlates of aberrant regeneration. The authors quantitatively compared recovery of vibrissal and ocular function following three types of clinically relevant nerve injury.

Methods: One hundred seventy-eight adult rats underwent facial nerve manipulation and testing. In the experimental groups, the left facial nerve was either crushed, transected, and repaired epineurially, or transected and the stumps suture-secured into a tube with a 2-mm gap between them. Facial recovery was measured for the ensuing 1 to 4 months. Data were analyzed for whisking recovery. Previously developed markers of co-contraction of the upper and midfacial zones (possible synkinesis markers) were also examined.

Results: Animals in the crush groups recovered nearly normal whisking parameters within 25 days. The distal branch crush group showed improved recovery over the main trunk crush group for several days during early recovery. By week 9, the transection/repair groups showed evidence of recovery that trended further upward throughout the study period. The entubulation groups followed a similar recovery pattern, although they did not maintain significant recovery levels by the study conclusion. Markers of potential synkinesis increased in selected groups following facial nerve injury.

Conclusions: Rodent vibrissal function recovers in a predictable fashion following manipulation. Generalized co-contraction of the upper and midfacial zones emerges following facial nerve manipulation, possibly related to aberrant regeneration, polyterminal axons, or hypersensitivity of the rodent to sensory stimuli following nerve manipulation.

Figure 1

Microanatomy of the facial nerve. Note the emergence of fascicular architecture in the more distal aspects.

Figure 2

Examples of flaccidity and hypertonicity in facial paralysis. Both patients demonstrate left facial paralysis. A. The flaccid state. Note pan-facial ptosis and a widened palpebral fissure. B. The hypertonic state. Note pan-facial contracture and a narrowed palpebral fissure.

Figure 3

Facial nerve manipulations. Top panel: typical rodent facial nerve anatomy. Middle panels: experimental crush, transection/repair, and entubulation groups at either main trunk or distal to the trifurcation. Bottom panel: Denervation control groups, at either main trunk (left) or distal branches (right).

Figure 4

Photograph of rat in head-fixed position, with C1 whiskers entubulated to permit tracking via the laser micrometers.

Figure 5

Output generated from the facial movement tracking program of Bermejo et al (22). Red and blue tracings indicate right and left C1 whisker position, respectively. Gray tracings correspond to degree of right (top) and left (bottom) eye closure. A. Vigorous whisking induced by the delivery of scented air. B. Blinking induced by ocular air puff delivery.

Figure 6

Graph illustrating the recovery of whisking amplitude over time, following crush injury. Black line represents main trunk crush, and gray line represents peripheral branch crush. Large squares represent manipulated side, small squares represent unmanipulated side. Error bars represent standard deviation. Note the slightly earlier recovery in the distally-lesioned group.

Figure 7

Recovery of whisking amplitude following transection and repair. A. Epineurial repair. Black line represents main trunk injury/repair, and gray line represents peripheral branch injury/repair. Large squares represent manipulated side, small squares represent unmanipulated side. Error bars represent standard deviation. B. Entubulation repair; Black line represents main trunk injury/repair, and gray line represents peripheral branch injury/repair. Large squares represent manipulated side, small squares represent unmanipulated side. Error bars represent standard deviation.