A closer look at the paralyzed face; a narrative review of the neurobiological basis for functional and aesthetic appreciation between patients with a left and a right peripheral facial palsy

Background: The facial nerve or n. facialis (NVII) is the seventh cranial nerve and it is responsible for the innervation of the mimic muscles, the gustatory organ, and the secretomotor function to the salivary, lacrimal, nasal and palatine glands. Clinical presentation of Facial Palsy (FP) is characterized by unilateral facial asymmetry and may present with a change in taste, decreased saliva production, and dysarthria. A facial palsy has a notable effect on the facial appreciation by both the patient and the environment and also affects quality of life and emotional processing. There appear to be differences in the appreciation of people with a left and right facial palsy. Purpose of this review: The purpose of the review is to give an overview of the anatomy of the facial nerve, neuro-anatomy of face processing, and hemispheric specialization and lateralization. Further,an overview is given of the clinical studies that translated the neuro-anatomical and neurobiological basis of these concepts into clinical studies. What this review adds: This review emphasizes the neurobiological evidence of differences in face processing between the left and right cerebral hemisphere, wherein it seems that the right hemisphere is superior in emotional processing. Several theories are proposed; 1) a familiarity hypothesis and 2) a left-right hemispheric specialization hypothesis. In clinical studies, promising evidence might indicate that, in patients with FP, there is indeed a difference in how left and right FP are perceived. This might give differences in decreased quality of life and finally in occurrence of depression. Further research must aim to substantiate these findings and determine the need for altering the standard therapeutic advice given to patients.